Chapter 4. Advanced C#

In this chapter, we cover advanced C# topics that build on concepts explored in Chapters 2 and 3. You should read the first four sections sequentially; you can read the remaining sections in any order.

Delegates

A delegate is an object that knows how to call a method.

A delegate type defines the kind of method that delegate instances can call. Specifically, it defines the method’s return type and its parameter types. The following defines a delegate type called Transformer:

delegate int Transformer (int x);

Transformer is compatible with any method with an int return type and a single int parameter, such as this:

static int Square (int x) { return x * x; }

or more tersely:

static int Square (int x) => x * x;

Assigning a method to a delegate variable creates a delegate instance:

Transformer t = Square;

which can be invoked in the same way as a method:

int answer = t(3);    // answer is 9

Here’s a complete example:

delegate int Transformer (int x);

class Test
{
  static void Main()
  {
    Transformer t = Square;          // Create delegate instance
    int result = t(3);               // Invoke delegate
    Console.WriteLine (result);      // 9
  }
  static int Square (int x) => x * x;
}

A delegate instance literally acts as a delegate for the caller: the caller invokes the delegate, and then the delegate calls the target method. This indirection decouples the caller from the target method.

The statement:

Transformer t = Square;

is shorthand for:

Transformer t = new Transformer (Square);

Note

Technically, we are specifying a method group when we refer to Square without brackets or arguments. If the method is overloaded, C# will pick the correct overload based on the signature of the delegate to which it’s being assigned.

The expression:

t(3)

is shorthand for:

t.Invoke(3)

Note

A delegate is similar to a callback, a general term that captures constructs such as C function pointers.

Writing Plug-in Methods with Delegates

A delegate variable is assigned a method at runtime. This is useful for writing plug-in methods. In this example, we have a utility method named Transform that applies a transform to each element in an integer array. The Transform method has a delegate parameter, for specifying a plug-in transform.

public delegate int Transformer (int x);

class Util
{
  public static void Transform (int[] values, Transformer t)
  {
    for (int i = 0; i < values.Length; i++)
      values[i] = t (values[i]);
  }
}

class Test
{
  static void Main()
  {
    int[] values = { 1, 2, 3 };
    Util.Transform (values, Square);      // Hook in the Square method
    foreach (int i in values)
      Console.Write (i + "  ");           // 1   4   9
  }

  static int Square (int x) => x * x;
}

Multicast Delegates

All delegate instances have multicast capability. This means that a delegate instance can reference not just a single target method, but also a list of target methods. The + and += operators combine delegate instances. For example:

SomeDelegate d = SomeMethod1;
d += SomeMethod2;

The last line is functionally the same as:

d = d + SomeMethod2;

Invoking d will now call both SomeMethod1 and SomeMethod2. Delegates are invoked in the order they are added.

The - and -= operators remove the right delegate operand from the left delegate operand. For example:

d -= SomeMethod1;

Invoking d will now cause only SomeMethod2 to be invoked.

Calling + or += on a delegate variable with a null value works, and it is equivalent to assigning the variable to a new value:

SomeDelegate d = null;
d += SomeMethod1;       // Equivalent (when d is null) to d = SomeMethod1;

Similarly, calling -= on a delegate variable with a single target is equivalent to assigning null to that variable.

Note

Delegates are immutable, so when you call += or -=, you’re in fact creating a new delegate instance and assigning it to the existing variable.

If a multicast delegate has a nonvoid return type, the caller receives the return value from the last method to be invoked. The preceding methods are still called, but their return values are discarded. In most scenarios in which multicast delegates are used, they have void return types, so this subtlety does not arise.

Note

All delegate types implicitly derive from System.MulticastDelegate, which inherits from System.Delegate. C# compiles +, -, +=, and -= operations made on a delegate to the static Combine and Remove methods of the System.Delegate class.

Multicast delegate example

Suppose you wrote a method that took a long time to execute. That method could regularly report progress to its caller by invoking a delegate. In this example, the HardWork method has a ProgressReporter delegate parameter, which it invokes to indicate progress:

public delegate void ProgressReporter (int percentComplete);

public class Util
{
  public static void HardWork (ProgressReporter p)
  {
    for (int i = 0; i < 10; i++)
    {
      p (i * 10);                           // Invoke delegate
      System.Threading.Thread.Sleep (100);  // Simulate hard work
    }
  }
}

To monitor progress, the Main method creates a multicast delegate instance p, such that progress is monitored by two independent methods:

class Test
{
  static void Main()
  {
    ProgressReporter p = WriteProgressToConsole;
    p += WriteProgressToFile;
    Util.HardWork (p);
  }

  static void WriteProgressToConsole (int percentComplete)
    => Console.WriteLine (percentComplete);

  static void WriteProgressToFile (int percentComplete)
    => System.IO.File.WriteAllText ("progress.txt",
                                     percentComplete.ToString());
}

Instance Versus Static Method Targets

When an instance method is assigned to a delegate object, the latter must maintain a reference not only to the method, but also to the instance to which the method belongs. The System.Delegate class’s Target property represents this instance (and will be null for a delegate referencing a static method). For example:

public delegate void ProgressReporter (int percentComplete);

class Test
{
  static void Main()
  {
    X x = new X();
    ProgressReporter p = x.InstanceProgress;
    p(99);                                 // 99
    Console.WriteLine (p.Target == x);     // True
    Console.WriteLine (p.Method);          // Void InstanceProgress(Int32)
  }
}

class X
{
  public void InstanceProgress (int percentComplete)
    => Console.WriteLine (percentComplete);
}

Generic Delegate Types

A delegate type may contain generic type parameters. For example:

public delegate T Transformer<T> (T arg);

With this definition, we can write a generalized Transform utility method that works on any type:

public class Util
{
  public static void Transform<T> (T[] values, Transformer<T> t)
  {
    for (int i = 0; i < values.Length; i++)
      values[i] = t (values[i]);
  }
}

class Test
{
  static void Main()
  {
    int[] values = { 1, 2, 3 };
    Util.Transform (values, Square);      // Hook in Square
    foreach (int i in values)
      Console.Write (i + "  ");           // 1   4   9
  }
  static int Square (int x) => x * x;
}

The Func and Action Delegates

With generic delegates, it becomes possible to write a small set of delegate types that are so general they can work for methods of any return type and any (reasonable) number of arguments. These delegates are the Func and Action delegates, defined in the System namespace (the in and out annotations indicate variance, which we will cover shortly):

delegate TResult Func <out TResult>                ();
delegate TResult Func <in T, out TResult>          (T arg);
delegate TResult Func <in T1, in T2, out TResult>  (T1 arg1, T2 arg2);
... and so on, up to T16
delegate void Action                 ();
delegate void Action <in T>          (T arg);
delegate void Action <in T1, in T2>  (T1 arg1, T2 arg2);
... and so on, up to T16

These delegates are extremely general. The Transformer delegate in our previous example can be replaced with a Func delegate that takes a single argument of type T and returns a same-typed value:

public static void Transform<T> (T[] values, Func<T,T> transformer)
{
  for (int i = 0; i < values.Length; i++)
    values[i] = transformer (values[i]);
}

The only practical scenarios not covered by these delegates are ref/out and pointer parameters.

Note

Prior to Framework 2.0, the Func and Action delegates did not exist (because generics did not exist). It’s for this historical reason that much of the Framework uses custom delegate types rather than Func and Action.

Delegates Versus Interfaces

A problem that can be solved with a delegate can also be solved with an interface. For instance, we can rewrite our original example with an interface called ITransformer instead of a delegate:

public interface ITransformer
{
  int Transform (int x);
}

public class Util
{
 public static void TransformAll (int[] values, ITransformer t)
 {
   for (int i = 0; i < values.Length; i++)
     values[i] = t.Transform (values[i]);
 }
}

class Squarer : ITransformer
{
  public int Transform (int x) => x * x;
}
...

static void Main()
{
  int[] values = { 1, 2, 3 };
  Util.TransformAll (values, new Squarer());
  foreach (int i in values)
    Console.WriteLine (i);
}

A delegate design may be a better choice than an interface design if one or more of these conditions are true:

The interface defines only a single method.
Multicast capability is needed.
The subscriber needs to implement the interface multiple times.

In the ITransformer example, we don’t need to multicast. However, the interface defines only a single method. Furthermore, our subscriber may need to implement ITransformer multiple times, to support different transforms, such as square or cube. With interfaces, we’re forced into writing a separate type per transform, since Test can implement ITransformer only once. This is quite cumbersome:

class Squarer : ITransformer
{
  public int Transform (int x) => x * x;
}

class Cuber : ITransformer
{
  public int Transform (int x) => x * x * x;
}
...

static void Main()
{
  int[] values = { 1, 2, 3 };
  Util.TransformAll (values, new Cuber());
  foreach (int i in values)
    Console.WriteLine (i);
}

Delegate Compatibility

Type compatibility

Delegate types are all incompatible with one another, even if their signatures are the same:

delegate void D1();
delegate void D2();
...

D1 d1 = Method1;
D2 d2 = d1;                           // Compile-time error

Note

The following, however, is permitted:

D2 d2 = new D2 (d1);

Delegate instances are considered equal if they have the same method targets:

delegate void D();
...

D d1 = Method1;
D d2 = Method1;
Console.WriteLine (d1 == d2);         // True

Multicast delegates are considered equal if they reference the same methods in the same order.

Parameter compatibility

When you call a method, you can supply arguments that have more specific types than the parameters of that method. This is ordinary polymorphic behavior. For exactly the same reason, a delegate can have more specific parameter types than its method target. This is called contravariance.

Here’s an example:

delegate void StringAction (string s);

class Test
{
  static void Main()
  {
    StringAction sa = new StringAction (ActOnObject);
    sa ("hello");
  }

  static void ActOnObject (object o) => Console.WriteLine (o);   // hello
}

(As with type parameter variance, delegates are variant only for reference conversions.)

A delegate merely calls a method on someone else’s behalf. In this case, the StringAction is invoked with an argument of type string. When the argument is then relayed to the target method, the argument gets implicitly upcast to an object.

Note

The standard event pattern is designed to help you leverage contravariance through its use of the common EventArgs base class. For example, you can have a single method invoked by two different delegates, one passing a MouseEventArgs and the other passing a KeyEventArgs.

Return type compatibility

If you call a method, you may get back a type that is more specific than what you asked for. This is ordinary polymorphic behavior. For exactly the same reason, a delegate’s target method may return a more specific type than described by the delegate. This is called covariance. For example:

delegate object ObjectRetriever();

class Test
{
  static void Main()
  {
    ObjectRetriever o = new ObjectRetriever (RetrieveString);
    object result = o();
    Console.WriteLine (result);      // hello
  }
  static string RetrieveString() => "hello";
}

ObjectRetriever expects to get back an object, but an object subclass will also do: delegate return types are covariant.

Generic delegate type parameter variance

In Chapter 3, we saw how generic interfaces support covariant and contravariant type parameters. The same capability exists for delegates too (from C# 4.0 onward).

If you’re defining a generic delegate type, it’s good practice to:

Mark a type parameter used only on the return value as covariant (out).
Mark any type parameters used only on parameters as contravariant (in).

Doing so allows conversions to work naturally by respecting inheritance relationships between types.

The following delegate (defined in the System namespace) has a covariant TResult:

delegate TResult Func<out TResult>();

allowing:

Func<string> x = ...;
Func<object> y = x;

The following delegate (defined in the System namespace) has a contravariant T:

delegate void Action<in T> (T arg);

allowing:

Action<object> x = ...;
Action<string> y = x;

Events

When using delegates, two emergent roles commonly appear: broadcaster and subscriber.

The broadcaster is a type that contains a delegate field. The broadcaster decides when to broadcast by invoking the delegate.

The subscribers are the method target recipients. A subscriber decides when to start and stop listening by calling += and -= on the broadcaster’s delegate. A subscriber does not know about, or interfere with, other subscribers.

Events are a language feature that formalizes this pattern. An event is a construct that exposes just the subset of delegate features required for the broadcaster/subscriber model. The main purpose of events is to prevent subscribers from interfering with one another.

The easiest way to declare an event is to put the event keyword in front of a delegate member:

// Delegate definition
public delegate void PriceChangedHandler (decimal oldPrice,
                                          decimal newPrice);
public class Broadcaster
{
  // Event declaration
  public event PriceChangedHandler PriceChanged;
}

Code within the Broadcaster type has full access to PriceChanged and can treat it as a delegate. Code outside of Broadcaster can only perform += and -= operations on the PriceChanged event.

How Do Events Work on the Inside?

Three things happen under the covers when you declare an event as follows:

public class Broadcaster
{
  public event PriceChangedHandler PriceChanged;
}

First, the compiler translates the event declaration into something close to the following:

PriceChangedHandler priceChanged;   // private delegate
public event PriceChangedHandler PriceChanged
{
  add    { priceChanged += value; }
  remove { priceChanged -= value; }
}

The add and remove keywords denote explicit event accessors—which act rather like property accessors. We’ll describe how to write these later.

Second, the compiler looks within the Broadcaster class for references to PriceChanged that perform operations other than += or -= and redirects them to the underlying priceChanged delegate field.

Third, the compiler translates += and -= operations on the event to calls to the event’s add and remove accessors. Interestingly, this makes the behavior of += and -= unique when applied to events: unlike in other scenarios, it’s not simply a shortcut for + and - followed by an assignment.

Consider the following example. The Stock class fires its PriceChanged event every time the Price of the Stock changes:

public delegate void PriceChangedHandler (decimal oldPrice,
                                          decimal newPrice);
public class Stock
{
  string symbol;
  decimal price;

  public Stock (string symbol) { this.symbol = symbol; }

  public event PriceChangedHandler PriceChanged;

  public decimal Price
  {
    get { return price; }
    set
    {
      if (price == value) return;      // Exit if nothing has changed
      decimal oldPrice = price;
      price = value;
      if (PriceChanged != null)           // If invocation list not
        PriceChanged (oldPrice, price);   // empty, fire event.
    }
  }
}

If we remove the event keyword from our example so that PriceChanged becomes an ordinary delegate field, our example would give the same results. However, Stock would be less robust, in that subscribers could do the following things to interfere with each other:

Replace other subscribers by reassigning PriceChanged (instead of using the += operator).
Clear all subscribers (by setting PriceChanged to null).
Broadcast to other subscribers by invoking the delegate.

Note

WinRT events have slightly different semantics in that attaching to an event returns a token which is required to detach from the event. The compiler transparently bridges this gap (by maintaining an internal dictionary of tokens) so that you can consume WinRT events as though they were ordinary CLR events.

Standard Event Pattern

The .NET Framework defines a standard pattern for writing events. Its purpose is to provide consistency across both Framework and user code. At the core of the standard event pattern is System.EventArgs, a predefined Framework class with no members (other than the static Empty property). EventArgs is a base class for conveying information for an event. In our Stock example, we would subclass EventArgs to convey the old and new prices when a PriceChanged event is fired:

public class PriceChangedEventArgs : System.EventArgs
{
  public readonly decimal LastPrice;
  public readonly decimal NewPrice;

  public PriceChangedEventArgs (decimal lastPrice, decimal newPrice)
  {
    LastPrice = lastPrice;
    NewPrice = newPrice;
  }
}

For reusability, the EventArgs subclass is named according to the information it contains (rather than the event for which it will be used). It typically exposes data as properties or as read-only fields.

With an EventArgs subclass in place, the next step is to choose or define a delegate for the event. There are three rules:

It must have a void return type.
It must accept two arguments: the first of type object, and the second a subclass of EventArgs. The first argument indicates the event broadcaster, and the second argument contains the extra information to convey.
Its name must end with EventHandler.

The Framework defines a generic delegate called System.EventHandler<> that satisfies these rules:

public delegate void EventHandler<TEventArgs>
  (object source, TEventArgs e) where TEventArgs : EventArgs;

Note

Before generics existed in the language (prior to C# 2.0), we would have had to instead write a custom delegate as follows:

public delegate void PriceChangedHandler
  (object sender, PriceChangedEventArgs e);

For historical reasons, most events within the Framework use delegates defined in this way.

The next step is to define an event of the chosen delegate type. Here, we use the generic EventHandler delegate:

public class Stock
{
  ...
  public event EventHandler<PriceChangedEventArgs> PriceChanged;
}

Finally, the pattern requires that you write a protected virtual method that fires the event. The name must match the name of the event, prefixed with the word On, and then accept a single EventArgs argument:

public class Stock
{
  ...

  public event EventHandler<PriceChangedEventArgs> PriceChanged;

  protected virtual void OnPriceChanged (PriceChangedEventArgs e)
  {
    if (PriceChanged != null) PriceChanged (this, e);
  }
}

Note

In multithreaded scenarios (Chapter 14), you need to assign the delegate to a temporary variable before testing and invoking it to avoid a thread-safety error:

var temp = PriceChanged;
if (temp != null) temp (this, e);

We can achieve the same functionality without the temp variable from C# 6 with the null-conditional operator:

PriceChanged?.Invoke (this, e);

Being both thread-safe and succinct, this is now the best general way to invoke events.

This provides a central point from which subclasses can invoke or override the event (assuming the class is not sealed).

Here’s the complete example:

using System;

public class PriceChangedEventArgs : EventArgs
{
  public readonly decimal LastPrice;
  public readonly decimal NewPrice;

  public PriceChangedEventArgs (decimal lastPrice, decimal newPrice)
  {
    LastPrice = lastPrice; NewPrice = newPrice;
  }
}

public class Stock
{
  string symbol;
  decimal price;

  public Stock (string symbol) {this.symbol = symbol;}

  public event EventHandler<PriceChangedEventArgs> PriceChanged;

  protected virtual void OnPriceChanged (PriceChangedEventArgs e)
  {
    PriceChanged?.Invoke (this, e);
  }

  public decimal Price
  {
    get { return price; }
    set
    {
      if (price == value) return;
      decimal oldPrice = price;
      price = value;
      OnPriceChanged (new PriceChangedEventArgs (oldPrice, price));
    }
  }
}

class Test
{
  static void Main()
  {
    Stock stock = new Stock ("THPW");
    stock.Price = 27.10M;
    // Register with the PriceChanged event
    stock.PriceChanged += stock_PriceChanged;
    stock.Price = 31.59M;
  }

  static void stock_PriceChanged (object sender, PriceChangedEventArgs e)
  {
    if ((e.NewPrice - e.LastPrice) / e.LastPrice > 0.1M)
      Console.WriteLine ("Alert, 10% stock price increase!");
  }
}

The predefined nongeneric EventHandler delegate can be used when an event doesn’t carry extra information. In this example, we rewrite Stock such that the PriceChanged event is fired after the price changes, and no information about the event is necessary, other than it happened. We also make use of the EventArgs.Empty property in order to avoid unnecessarily instantiating an instance of EventArgs.

public class Stock
{
  string symbol;
  decimal price;

  public Stock (string symbol) { this.symbol = symbol; }

  public event EventHandler PriceChanged;

  protected virtual void OnPriceChanged (EventArgs e)
  {
    PriceChanged?.Invoke (this, e);
  }

  public decimal Price
  {
    get { return price; }
    set
    {
      if (price == value) return;
      price = value;
      OnPriceChanged (EventArgs.Empty);
    }
  }
}

Event Accessors

An event’s accessors are the implementations of its += and -= functions. By default, accessors are implemented implicitly by the compiler. Consider this event declaration:

public event EventHandler PriceChanged;

The compiler converts this to the following:

A private delegate field
A public pair of event accessor functions (add_PriceChanged and remove_PriceChanged), whose implementations forward the += and -= operations to the private delegate field

You can take over this process by defining explicit event accessors. Here’s a manual implementation of the PriceChanged event from our previous example:

private EventHandler priceChanged;         // Declare a private delegate

public event EventHandler PriceChanged
{
  add    { priceChanged += value; }
  remove { priceChanged -= value; }
}

This example is functionally identical to C#’s default accessor implementation (except that C# also ensures thread safety around updating the delegate via a lock-free compare-and-swap algorithm—see http://albahari.com/threading). By defining event accessors ourselves, we instruct C# not to generate default field and accessor logic.

With explicit event accessors, you can apply more complex strategies to the storage and access of the underlying delegate. There are three scenarios where this is useful:

When the event accessors are merely relays for another class that is broadcasting the event.
When the class exposes a large number of events, where most of the time very few subscribers exist, such as a Windows control. In such cases, it is better to store the subscriber’s delegate instances in a dictionary, since a dictionary will contain less storage overhead than dozens of null delegate field references.
When explicitly implementing an interface that declares an event.

Here is an example that illustrates the last point:

public interface IFoo { event EventHandler Ev; }

class Foo : IFoo
{
  private EventHandler ev;

  event EventHandler IFoo.Ev
  {
    add    { ev += value; }
    remove { ev -= value; }
  }
}

Note

The add and remove parts of an event are compiled to add_XXX and remove_XXX methods.

Event Modifiers

Like methods, events can be virtual, overridden, abstract, or sealed. Events can also be static:

public class Foo
{
  public static event EventHandler<EventArgs> StaticEvent;
  public virtual event EventHandler<EventArgs> VirtualEvent;
}

Lambda Expressions

A lambda expression is an unnamed method written in place of a delegate instance. The compiler immediately converts the lambda expression to either:

A delegate instance.
An expression tree, of type Expression<TDelegate>, representing the code inside the lambda expression in a traversable object model. This allows the lambda expression to be interpreted later at runtime (see “Building Query Expressions” in Chapter 8).

Given the following delegate type:

delegate int Transformer (int i);

we could assign and invoke the lambda expression x => x * x as follows:

Transformer sqr = x => x * x;
Console.WriteLine (sqr(3));    // 9

Note

Internally, the compiler resolves lambda expressions of this type by writing a private method, and moving the expression’s code into that method.

A lambda expression has the following form:

(parameters) => expression-or-statement-block

For convenience, you can omit the parentheses if and only if there is exactly one parameter of an inferable type.

In our example, there is a single parameter, x, and the expression is x * x:

x => x * x;

Each parameter of the lambda expression corresponds to a delegate parameter, and the type of the expression (which may be void) corresponds to the return type of the delegate.

In our example, x corresponds to parameter i, and the expression x * x corresponds to the return type int, therefore being compatible with the Transformer delegate:

delegate int Transformer (int i);

A lambda expression’s code can be a statement block instead of an expression. We can rewrite our example as follows:

x => { return x * x; };

Lambda expressions are used most commonly with the Func and Action delegates, so you will most often see our earlier expression written as follows:

Func<int,int> sqr = x => x * x;

Here’s an example of an expression that accepts two parameters:

Func<string,string,int> totalLength = (s1, s2) => s1.Length + s2.Length;
int total = totalLength ("hello", "world");   // total is 10;

Lambda expressions were introduced in C# 3.0.

Explicitly Specifying Lambda Parameter Types

The compiler can usually infer the type of lambda parameters. When this is not the case, you must specify the type of each parameter explicitly. Consider the following two methods:

void Foo<T> (T x)         {}
void Bar<T> (Action<T> a) {}

The following code will fail to compile because the compiler cannot infer the type of x:

Bar (x => Foo (x));     // What type is x?

We can fix this by explicitly specify x’s type as follows:

Bar ((int x) => Foo (x));

This particular example is simple enough that it can be fixed in two other ways:

Bar<int> (x => Foo (x));   // Specify type parameter for Bar
Bar<int> (Foo);            // As above, but with method group

Capturing Outer Variables

A lambda expression can reference the local variables and parameters of the method in which it’s defined (outer variables). For example:

static void Main()
{
  int factor = 2;
  Func<int, int> multiplier = n => n * factor;
  Console.WriteLine (multiplier (3));           // 6
}

Outer variables referenced by a lambda expression are called captured variables. A lambda expression that captures variables is called a closure.

Captured variables are evaluated when the delegate is actually invoked, not when the variables were captured:

int factor = 2;
Func<int, int> multiplier = n => n * factor;
factor = 10;
Console.WriteLine (multiplier (3));           // 30

Lambda expressions can themselves update captured variables:

int seed = 0;
Func<int> natural = () => seed++;
Console.WriteLine (natural());           // 0
Console.WriteLine (natural());           // 1
Console.WriteLine (seed);                // 2

Captured variables have their lifetimes extended to that of the delegate. In the following example, the local variable seed would ordinarily disappear from scope when Natural finished executing. But because seed has been captured, its lifetime is extended to that of the capturing delegate, natural:

static Func<int> Natural()
{
  int seed = 0;
  return () => seed++;      // Returns a closure
}

static void Main()
{
  Func<int> natural = Natural();
  Console.WriteLine (natural());      // 0
  Console.WriteLine (natural());      // 1
}

A local variable instantiated within a lambda expression is unique per invocation of the delegate instance. If we refactor our previous example to instantiate seed within the lambda expression, we get a different (in this case, undesirable) result:

static Func<int> Natural()
{
  return() => { int seed = 0; return seed++; };
}

static void Main()
{
  Func<int> natural = Natural();
  Console.WriteLine (natural());           // 0
  Console.WriteLine (natural());           // 0
}

Note

Capturing is internally implemented by “hoisting” the captured variables into fields of a private class. When the method is called, the class is instantiated and lifetime-bound to the delegate instance.

Capturing iteration variables

When you capture the iteration variable of a for loop, C# treats that variable as though it was declared outside the loop. This means that the same variable is captured in each iteration. The following program writes 333 instead of writing 012:

Action[] actions = new Action[3];

for (int i = 0; i < 3; i++)
  actions [i] = () => Console.Write (i);

foreach (Action a in actions) a();     // 333

Each closure (shown in boldface) captures the same variable, i. (This actually makes sense when you consider that i is a variable whose value persists between loop iterations; you can even explicitly change i within the loop body if you want.) The consequence is that when the delegates are later invoked, each delegate sees i’s value at the time of invocation—which is 3. We can illustrate this better by expanding the for loop as follows:

Action[] actions = new Action[3];
int i = 0;
actions[0] = () => Console.Write (i);
i = 1;
actions[1] = () => Console.Write (i);
i = 2;
actions[2] = () => Console.Write (i);
i = 3;
foreach (Action a in actions) a();    // 333

The solution, if we want to write 012, is to assign the iteration variable to a local variable that’s scoped inside the loop:

Action[] actions = new Action[3];
for (int i = 0; i < 3; i++)
{
  int loopScopedi = i;
  actions [i] = () => Console.Write (loopScopedi);
}
foreach (Action a in actions) a();     // 012

Because loopScopedi is freshly created on every iteration, each closure captures a different variable.

Note

Prior to C# 5.0, foreach loops worked in the same way:

Action[] actions = new Action[3];
int i = 0;

foreach (char c in "abc")
  actions [i++] = () => Console.Write (c);

foreach (Action a in actions) a();   // ccc in C# 4.0

This caused considerable confusion: unlike with a for loop, the iteration variable in a foreach loop is immutable, and so one would expect it to be treated as local to the loop body. The good news is that it’s been fixed since C# 5.0, and the example above now writes “abc.”

Warning

Technically, this is a breaking change because recompiling a C# 4.0 program in C# 5.0 could create a different result. In general, the C# team tries to avoid breaking changes; however in this case, a “break” would almost certainly indicate an undetected bug in the C# 4.0 program rather than intentional reliance on the old behavior.

Anonymous Methods

Anonymous methods are a C# 2.0 feature that has been mostly subsumed by C# 3.0 lambda expressions. An anonymous method is like a lambda expression, but it lacks the following features:

Implicitly typed parameters.
Expression syntax (an anonymous method must always be a statement block).
The ability to compile to an expression tree by assigning to Expression<T>.

To write an anonymous method, you include the delegate keyword followed (optionally) by a parameter declaration and then a method body. For example, given this delegate:

delegate int Transformer (int i);

we could write and call an anonymous method as follows:

Transformer sqr = delegate (int x) {return x * x;};
Console.WriteLine (sqr(3));                            // 9

The first line is semantically equivalent to the following lambda expression:

Transformer sqr =       (int x) => {return x * x;};

or simply:

Transformer sqr =            x  => x * x;

Anonymous methods capture outer variables in the same way lambda expressions do.

Note

A unique feature of anonymous methods is that you can omit the parameter declaration entirely—even if the delegate expects it. This can be useful in declaring events with a default empty handler:

public event EventHandler Clicked = delegate { };

This avoids the need for a null check before firing the event. The following is also legal:

// Notice that we omit the parameters:
Clicked += delegate { Console.WriteLine ("clicked"); };

try Statements and Exceptions

A try statement specifies a code block subject to error-handling or cleanup code. The try block must be followed by a catch block, a finally block, or both. The catch block executes when an error occurs in the try block. The finally block executes after execution leaves the try block (or if present, the catch block) to perform cleanup code, whether or not an error occurred.

A catch block has access to an Exception object that contains information about the error. You use a catch block to either compensate for the error or rethrow the exception. You rethrow an exception if you merely want to log the problem or if you want to rethrow a new, higher-level exception type.

A finally block adds determinism to your program: the CLR endeavors to always execute it. It’s useful for cleanup tasks such as closing network connections.

A try statement looks like this:

try
{
  ... // exception may get thrown within execution of this block
}
catch (ExceptionA ex)
{
  ... // handle exception of type ExceptionA
}
catch (ExceptionB ex)
{
  ... // handle exception of type ExceptionB
}
finally
{
  ... // cleanup code
}

Consider the following program:

class Test
{
  static int Calc (int x) => 10 / x;

  static void Main()
  {
    int y = Calc (0);
    Console.WriteLine (y);
  }
}

Because x is zero, the runtime throws a DivideByZeroException, and our program terminates. We can prevent this by catching the exception as follows:

class Test
{
  static int Calc (int x) => 10 / x;

  static void Main()
  {
    try
    {
      int y = Calc (0);
      Console.WriteLine (y);
    }
    catch (DivideByZeroException ex)
    {
      Console.WriteLine ("x cannot be zero");
    }
    Console.WriteLine ("program completed");
  }
}

OUTPUT:
x cannot be zero
program completed

Note

This is a simple example to illustrate exception handling. We could deal with this particular scenario better in practice by checking explicitly for the divisor being zero before calling Calc.

Checking for preventable errors is preferable to relying on try/catch blocks because exceptions are relatively expensive to handle, taking hundreds of clock cycles or more.

When an exception is thrown, the CLR performs a test: Is execution currently within a try statement that can catch the exception?

If so, execution is passed to the compatible catch block. If the catch block successfully finishes executing, execution moves to the next statement after the try statement (if present, executing the finally block first).
If not, execution jumps back to the caller of the function, and the test is repeated (after executing any finally blocks that wrap the statement).

If no function takes responsibility for the exception, an error dialog box is displayed to the user, and the program terminates.

The catch Clause

A catch clause specifies what type of exception to catch. This must either be System.Exception or a subclass of System.Exception.

Catching System.Exception catches all possible errors. This is useful when:

Your program can potentially recover regardless of the specific exception type.
You plan to rethrow the exception (perhaps after logging it).
Your error handler is the last resort, prior to termination of the program.

More typically, though, you catch specific exception types, in order to avoid having to deal with circumstances for which your handler wasn’t designed (e.g., an OutOfMemoryException).

You can handle multiple exception types with multiple catch clauses (again, this example could be written with explicit argument checking rather than exception handling):

class Test
{
  static void Main (string[] args)
  {
    try
    {
      byte b = byte.Parse (args[0]);
      Console.WriteLine (b);
    }
    catch (IndexOutOfRangeException ex)
    {
      Console.WriteLine ("Please provide at least one argument");
    }
    catch (FormatException ex)
    {
      Console.WriteLine ("That's not a number!");
    }
    catch (OverflowException ex)
    {
      Console.WriteLine ("You've given me more than a byte!");
    }
  }
}

Only one catch clause executes for a given exception. If you want to include a safety net to catch more general exceptions (such as System.Exception), you must put the more specific handlers first.

An exception can be caught without specifying a variable if you don’t need to access its properties:

catch (OverflowException)   // no variable
{
  ...
}

Furthermore, you can omit both the variable and the type (meaning that all exceptions will be caught):

catch { ... }

Exception filters (C# 6)

From C# 6.0, you can specify an exception filter in a catch clause by adding a when clause:

catch (WebException ex) when (ex.Status == WebExceptionStatus.Timeout)
{
  ...
}

If a WebException is thrown in this example, the Boolean expression following the when keyword is then evaluated. If the result is false, the catch block in question is ignored, and any subsequent catch clauses are considered. With exception filters, it can be meaningful to catch the same exception type again:

catch (WebException ex) when (ex.Status == WebExceptionStatus.Timeout)
{ ... }
catch (WebException ex) when (ex.Status == WebExceptionStatus.SendFailure)
{ ... }

The Boolean expression in the when clause can be side-effecting, such as a method that logs the exception for diagnostic purposes.

The finally Block

A finally block always executes—whether or not an exception is thrown and whether or not the try block runs to completion. finally blocks are typically used for cleanup code.

A finally block executes either:

After a catch block finishes
After control leaves the try block because of a jump statement (e.g., return or goto)
After the try block ends

The only things that can defeat a finally block are an infinite loop or the process ending abruptly.

A finally block helps add determinism to a program. In the following example, the file that we open always gets closed, regardless of whether:

The try block finishes normally
Execution returns early because the file is empty (EndOfStream)
An IOException is thrown while reading the file

static void ReadFile()
{
  StreamReader reader = null;    // In System.IO namespace
  try
  {
    reader = File.OpenText ("file.txt");
    if (reader.EndOfStream) return;
    Console.WriteLine (reader.ReadToEnd());
  }
  finally
  {
    if (reader != null) reader.Dispose();
  }
}

In this example, we closed the file by calling Dispose on the StreamReader. Calling Dispose on an object within a finally block is a standard convention throughout the .NET Framework and is supported explicitly in C# through the using statement.

The using statement

Many classes encapsulate unmanaged resources, such as file handles, graphics handles, or database connections. These classes implement System.IDisposable, which defines a single parameterless method named Dispose to clean up these resources. The using statement provides an elegant syntax for calling Dispose on an IDisposable object within a finally block.

The following:

using (StreamReader reader = File.OpenText ("file.txt"))
{
  ...
}

is precisely equivalent to:

{
  StreamReader reader = File.OpenText ("file.txt");
  try
  {
    ...
  }
  finally
  {
    if (reader != null)
      ((IDisposable)reader).Dispose();
  }
}

We cover the disposal pattern in more detail in Chapter 12.

Throwing Exceptions

Exceptions can be thrown either by the runtime or in user code. In this example, Display throws a System.ArgumentNullException:

class Test
{
  static void Display (string name)
  {
    if (name == null)
      throw new ArgumentNullException (nameof (name));

    Console.WriteLine (name);
  }

  static void Main()
  {
    try { Display (null); }
    catch (ArgumentNullException ex)
    {
      Console.WriteLine ("Caught the exception");
    }
  }
}

Rethrowing an exception

You can capture and rethrow an exception as follows:

try {  ...  }
catch (Exception ex)
{
  // Log error
  ...
  throw;          // Rethrow same exception
}

Note

If we replaced throw with throw ex, the example would still work, but the StackTrace property of the newly propagated exception would no longer reflect the original error.

Rethrowing in this manner lets you log an error without swallowing it. It also lets you back out of handling an exception should circumstances turn out to be outside what you expected:

using System.Net;       // (See Chapter 16)
...

string s = null;
using (WebClient wc = new WebClient())
  try { s = wc.DownloadString ("http://www.albahari.com/nutshell/");  }
  catch (WebException ex)
  {
    if (ex.Status == WebExceptionStatus.Timeout)
      Console.WriteLine ("Timeout");
    else
      throw;     // Can't handle other sorts of WebException, so rethrow
  }

From C# 6.0, this can be written more tersely with an exception filter:

catch (WebException ex) when (ex.Status == WebExceptionStatus.Timeout)
{
  Console.WriteLine ("Timeout");
}

The other common scenario is to rethrow a more specific exception type. For example:

try
{
  ... // Parse a DateTime from XML element data
}
catch (FormatException ex)
{
  throw new XmlException ("Invalid DateTime", ex);
}

Notice that when we constructed XmlException, we passed in the original exception, ex, as the second argument. This argument populates the InnerException property of the new exception and aids debugging. Nearly all types of exception offer a similar constructor.

Rethrowing a less specific exception is something you might do when crossing a trust boundary so as not to leak technical information to potential hackers.

Key Properties of System.Exception

The most important properties of System.Exception are the following:

StackTrace: A string representing all the methods that are called from the origin of the exception to the catch block.
Message: A string with a description of the error.
InnerException: The inner exception (if any) that caused the outer exception. This, itself, may have another InnerException.

Note

All exceptions in C# are runtime exceptions—there is no equivalent to Java’s compile-time checked exceptions.

Common Exception Types

The following exception types are used widely throughout the CLR and .NET Framework. You can throw these yourself or use them as base classes for deriving custom exception types.

System.ArgumentException: Thrown when a function is called with a bogus argument. This generally indicates a program bug.
System.ArgumentNullException: Subclass of ArgumentException that’s thrown when a function argument is (unexpectedly) null.
System.ArgumentOutOfRangeException: Subclass of ArgumentException that’s thrown when a (usually numeric) argument is too big or too small. For example, this is thrown when passing a negative number into a function that accepts only positive values.
System.InvalidOperationException: Thrown when the state of an object is unsuitable for a method to successfully execute, regardless of any particular argument values. Examples include reading an unopened file or getting the next element from an enumerator where the underlying list has been modified partway through the iteration.
System.NotSupportedException: Thrown to indicate that a particular functionality is not supported. A good example is calling the Add method on a collection for which IsReadOnly returns true.
System.NotImplementedException: Thrown to indicate that a function has not yet been implemented.
System.ObjectDisposedException: Thrown when the object upon which the function is called has been disposed.

Another commonly encountered exception type is NullReferenceException. The CLR throws this exception when you attempt to access a member of an object whose value is null (indicating a bug in your code). You can throw a NullReferenceException directly (for testing purposes) as follows:

throw null;

The TryXXX Method Pattern

When writing a method, you have a choice, when something goes wrong, to return some kind of failure code or throw an exception. In general, you throw an exception when the error is outside the normal workflow—or if you expect that the immediate caller won’t be able to cope with it. Occasionally, though, it can be best to offer both choices to the consumer. An example of this is the int type, which defines two versions of its Parse method:

public int Parse     (string input);
public bool TryParse (string input, out int returnValue);

If parsing fails, Parse throws an exception; TryParse returns false.

You can implement this pattern by having the XXX method call the TryXXX method as follows:

public return-type XXX (input-type input)
{
  return-type returnValue;
  if (!TryXXX (input, out returnValue))
    throw new YYYException (...)
  return returnValue;
}

Alternatives to Exceptions

As with int.TryParse, a function can communicate failure by sending an error code back to the calling function via a return type or parameter. Although this can work with simple and predictable failures, it becomes clumsy when extended to all errors, polluting method signatures and creating unnecessary complexity and clutter. It also cannot generalize to functions that are not methods, such as operators (e.g., the division operator) or properties. An alternative is to place the error in a common place where all functions in the call stack can see it (e.g., a static method that stores the current error per thread). This, though, requires each function to participate in an error-propagation pattern that is cumbersome and, ironically, itself error-prone.

Enumeration and Iterators

Enumeration

An enumerator is a read-only, forward-only cursor over a sequence of values. An enumerator is an object that implements either of the following interfaces:

System.Collections.IEnumerator
System.Collections.Generic.IEnumerator<T>

Note

Technically, any object that has a method named MoveNext and a property called Current is treated as an enumerator. This relaxation was introduced in C# 1.0 to avoid the boxing/unboxing overhead when enumerating value type elements but was made redundant when generics were introduced in C# 2.

The foreach statement iterates over an enumerable object. An enumerable object is the logical representation of a sequence. It is not itself a cursor, but an object that produces cursors over itself. An enumerable object either:

Implements IEnumerable or IEnumerable<T>
Has a method named GetEnumerator that returns an enumerator

Note

IEnumerator and IEnumerable are defined in System.Collections. IEnumerator<T> and IEnumerable<T> are defined in System.Collections.Generic.

The enumeration pattern is as follows:

class Enumerator   // Typically implements IEnumerator or IEnumerator<T>
{
  public IteratorVariableType Current { get {...} }
  public bool MoveNext() {...}
}

class Enumerable   // Typically implements IEnumerable or IEnumerable<T>
{
  public Enumerator GetEnumerator() {...}
}

Here is the high-level way of iterating through the characters in the word beer using a foreach statement:

foreach (char c in "beer")
  Console.WriteLine (c);

Here is the low-level way of iterating through the characters in beer without using a foreach statement:

using (var enumerator = "beer".GetEnumerator())
  while (enumerator.MoveNext())
  {
    var element = enumerator.Current;
    Console.WriteLine (element);
  }

If the enumerator implements IDisposable, the foreach statement also acts as a using statement, implicitly disposing the enumerator object.

Chapter 7 explains the enumeration interfaces in further detail.

Collection Initializers

You can instantiate and populate an enumerable object in a single step. For example:

using System.Collections.Generic;
...

List<int> list = new List<int> {1, 2, 3};

The compiler translates this to the following:

using System.Collections.Generic;
...

List<int> list = new List<int>();
list.Add (1);
list.Add (2);
list.Add (3);

This requires that the enumerable object implements the System.Collections.IEnumerable interface and that it has an Add method that has the appropriate number of parameters for the call. You can similarly initialize dictionaries (see “Dictionaries” in Chapter 4) as follows:

var dict = new Dictionary<int, string>()
{
  { 5, "five" },
  { 10, "ten" }
};

Or, as of C# 6:

var dict = new Dictionary<int, string>()
{
  [3] = "three",
  [10] = "ten"
};

The latter is valid not only with dictionaries, but with any type for which an indexer exists.

Iterators

Whereas a foreach statement is a consumer of an enumerator, an iterator is a producer of an enumerator. In this example, we use an iterator to return a sequence of Fibonacci numbers (where each number is the sum of the previous two):

using System;
using System.Collections.Generic;

class Test
{
  static void Main()
  {
    foreach (int fib in Fibs(6))
      Console.Write (fib + "  ");
  }

  static IEnumerable<int> Fibs (int fibCount)
  {
    for (int i = 0, prevFib = 1, curFib = 1; i < fibCount; i++)
    {
      yield return prevFib;
      int newFib = prevFib+curFib;
      prevFib = curFib;
      curFib = newFib;
    }
  }
}

OUTPUT: 1  1  2  3  5  8

Whereas a return statement expresses “Here’s the value you asked me to return from this method,” a yield return statement expresses “Here’s the next element you asked me to yield from this enumerator.” On each yield statement, control is returned to the caller, but the callee’s state is maintained so that the method can continue executing as soon as the caller enumerates the next element. The lifetime of this state is bound to the enumerator such that the state can be released when the caller has finished enumerating.

Note

The compiler converts iterator methods into private classes that implement IEnumerable<T> and/or IEnumerator<T>. The logic within the iterator block is “inverted” and spliced into the MoveNext method and Current property on the compiler-written enumerator class. This means that when you call an iterator method, all you’re doing is instantiating the compiler-written class; none of your code actually runs! Your code runs only when you start enumerating over the resultant sequence, typically with a foreach statement.

Iterator Semantics

An iterator is a method, property, or indexer that contains one or more yield statements. An iterator must return one of the following four interfaces (otherwise, the compiler will generate an error):

// Enumerable interfaces
System.Collections.IEnumerable
System.Collections.Generic.IEnumerable<T>

// Enumerator interfaces
System.Collections.IEnumerator
System.Collections.Generic.IEnumerator<T>

An iterator has different semantics, depending on whether it returns an enumerable interface or an enumerator interface. We describe this in Chapter 7.

Multiple yield statements are permitted. For example:

class Test
{
  static void Main()
  {
    foreach (string s in Foo())
      Console.WriteLine(s);         // Prints "One","Two","Three"
  }

  static IEnumerable<string> Foo()
  {
    yield return "One";
    yield return "Two";
    yield return "Three";
  }
}

yield break

The yield break statement indicates that the iterator block should exit early without returning more elements. We can modify Foo as follows to demonstrate:

static IEnumerable<string> Foo (bool breakEarly)
{
  yield return "One";
  yield return "Two";

  if (breakEarly)
    yield break;

  yield return "Three";
}

Note

A return statement is illegal in an iterator block—you must use a yield break instead.

Iterators and try/catch/finally blocks

A yield return statement cannot appear in a try block that has a catch clause:

IEnumerable<string> Foo()
{
  try { yield return "One"; }    // Illegal
  catch { ... }
}

Nor can yield return appear in a catch or finally block. These restrictions are due to the fact that the compiler must translate iterators into ordinary classes with MoveNext, Current, and Dispose members, and translating exception handling blocks would create excessive complexity.

You can, however, yield within a try block that has (only) a finally block:

IEnumerable<string> Foo()
{
  try { yield return "One"; }    // OK
  finally { ... }
}

The code in the finally block executes when the consuming enumerator reaches the end of the sequence or is disposed. A foreach statement implicitly disposes the enumerator if you break early, making this a safe way to consume enumerators. When working with enumerators explicitly, a trap is to abandon enumeration early without disposing it, circumventing the finally block. You can avoid this risk by wrapping explicit use of enumerators in a using statement:

string firstElement = null;
var sequence = Foo();
using (var enumerator = sequence.GetEnumerator())
  if (enumerator.MoveNext())
    firstElement = enumerator.Current;

Composing Sequences

Iterators are highly composable. We can extend our example, this time to output even Fibonacci numbers only:

using System;
using System.Collections.Generic;

class Test
{
  static void Main()
  {
    foreach (int fib in EvenNumbersOnly (Fibs(6)))
      Console.WriteLine (fib);
  }

  static IEnumerable<int> Fibs (int fibCount)
  {
    for (int i = 0, prevFib = 1, curFib = 1; i < fibCount; i++)
    {
      yield return prevFib;
      int newFib = prevFib+curFib;
      prevFib = curFib;
      curFib = newFib;
    }
  }

  static IEnumerable<int> EvenNumbersOnly (IEnumerable<int> sequence)
  {
    foreach (int x in sequence)
      if ((x % 2) == 0)
        yield return x;
  }
}

Each element is not calculated until the last moment—when requested by a MoveNext() operation. Figure 4-1 shows the data requests and data output over time.

The composability of the iterator pattern is extremely useful in LINQ; we discuss the subject again in Chapter 8.

Nullable Types

Reference types can represent a nonexistent value with a null reference. Value types, however, cannot ordinarily represent null values. For example:

string s = null;       // OK, Reference Type
int i = null;          // Compile Error, Value Type cannot be null

To represent null in a value type, you must use a special construct called a nullable type. A nullable type is denoted with a value type followed by the ? symbol:

int? i = null;                     // OK, Nullable Type
Console.WriteLine (i == null);     // True

Nullable<T> struct

T? translates into System.Nullable<T>, which is a lightweight immutable structure, having only two fields, to represent Value and HasValue. The essence of System.Nullable<T> is very simple:

public struct Nullable<T> where T : struct
{
  public T Value {get;}
  public bool HasValue {get;}
  public T GetValueOrDefault();
  public T GetValueOrDefault (T defaultValue);
  ...
}

The code:

int? i = null;
Console.WriteLine (i == null);              // True

translates to:

Nullable<int> i = new Nullable<int>();
Console.WriteLine (! i.HasValue);           // True

Attempting to retrieve Value when HasValue is false throws an InvalidOperationException. GetValueOrDefault() returns Value if HasValue is true; otherwise, it returns new T() or a specified custom default value.

The default value of T? is null.

Implicit and explicit nullable conversions

The conversion from T to T? is implicit, and from T? to T is explicit. For example:

int? x = 5;        // implicit
int y = (int)x;    // explicit

The explicit cast is directly equivalent to calling the nullable object’s Value property. Hence, an InvalidOperationException is thrown if HasValue is false.

Boxing and unboxing nullable values

When T? is boxed, the boxed value on the heap contains T, not T?. This optimization is possible because a boxed value is a reference type that can already express null.

C# also permits the unboxing of nullable types with the as operator. The result will be null if the cast fails:

object o = "string";
int? x = o as int?;
Console.WriteLine (x.HasValue);   // False

Operator Lifting

The Nullable<T> struct does not define operators such as <, >, or even ==. Despite this, the following code compiles and executes correctly:

int? x = 5;
int? y = 10;
bool b = x < y;      // true

This works because the compiler borrows, or “lifts,” the less-than operator from the underlying value type. Semantically, it translates the preceding comparison expression into this:

bool b = (x.HasValue && y.HasValue) ? (x.Value < y.Value) : false;

In other words, if both x and y have values, it compares via int’s less-than operator; otherwise, it returns false.

Operator lifting means you can implicitly use T’s operators on T?. You can define operators for T? in order to provide special-purpose null behavior, but in the vast majority of cases, it’s best to rely on the compiler automatically applying systematic nullable logic for you. Here are some examples:

int? x = 5;
int? y = null;

// Equality operator examples
Console.WriteLine (x == y);    // False
Console.WriteLine (x == null); // False
Console.WriteLine (x == 5);    // True
Console.WriteLine (y == null); // True
Console.WriteLine (y == 5);    // False
Console.WriteLine (y != 5);    // True

// Relational operator examples
Console.WriteLine (x < 6);     // True
Console.WriteLine (y < 6);     // False
Console.WriteLine (y > 6);     // False

// All other operator examples
Console.WriteLine (x + 5);     // 10
Console.WriteLine (x + y);     // null (prints empty line)

The compiler performs null logic differently depending on the category of operator. The following sections explain these different rules.

Equality operators (== and !=)

Lifted equality operators handle nulls just like reference types do. This means two null values are equal:

Console.WriteLine (       null ==        null);   // True
Console.WriteLine ((bool?)null == (bool?)null);   // True

Further:

If exactly one operand is null, the operands are unequal.
If both operands are non-null, their Values are compared.

Relational operators (<, <=, >=, >)

The relational operators work on the principle that it is meaningless to compare null operands. This means comparing a null value to either a null or a non-null value returns false:

bool b = x < y;    // Translation:

bool b = (x.HasValue && y.HasValue)
         ? (x.Value < y.Value)
         : false;

// b is false (assuming x is 5 and y is null)

All other operators (+, −, *, /, %, &, |, ^, <<, >>, +, ++, --, !, ~)

These operators return null when any of the operands are null. This pattern should be familiar to SQL users:

int? c = x + y;   // Translation:

int? c = (x.HasValue && y.HasValue)
         ? (int?) (x.Value + y.Value)
         : null;

// c is null (assuming x is 5 and y is null)

An exception is when the & and | operators are applied to bool?, which we will discuss shortly.

Mixing nullable and non-nullable operators

You can mix and match nullable and non-nullable types (this works because there is an implicit conversion from T to T?):

int? a = null;
int b = 2;
int? c = a + b;   // c is null - equivalent to a + (int?)b

bool? with & and | Operators

When supplied operands of type bool? the & and | operators treat null as an unknown value. So, null | true is true, because:

If the unknown value is false, the result would be true.
If the unknown value is true, the result would be true.

Similarly, null & false is false. This behavior would be familiar to SQL users. The following example enumerates other combinations:

bool? n = null;
bool? f = false;
bool? t = true;
Console.WriteLine (n | n);    // (null)
Console.WriteLine (n | f);    // (null)
Console.WriteLine (n | t);    // True
Console.WriteLine (n & n);    // (null)
Console.WriteLine (n & f);    // False
Console.WriteLine (n & t);    // (null)

Nullable Types & Null Operators

Nullable types work particularly well with the ?? operator (see “Null-Coalescing Operator”) in Chapter 2. For example:

int? x = null;
int y = x ?? 5;        // y is 5

int? a = null, b = 1, c = 2;
Console.WriteLine (a ?? b ?? c);  // 1 (first non-null value)

Using ?? on a nullable value type is equivalent to calling GetValueOrDefault with an explicit default value, except that the expression for the default value is never evaluated if the variable is not null.

Nullable types also work well with the null-conditional operator (see “Null-conditional operator (C# 6)” in Chapter 2). In the following example, length evaluates to null:

System.Text.StringBuilder sb = null;
int? length = sb?.ToString().Length;

We can combine this with the null-coalescing operator to evaluate to zero instead of null:

int length = sb?.ToString().Length ?? 0;  // Evaluates to 0 if sb is null

Scenarios for Nullable Types

One of the most common scenarios for nullable types is to represent unknown values. This frequently occurs in database programming, where a class is mapped to a table with nullable columns. If these columns are strings (e.g., an EmailAddress column on a Customer table), there is no problem, as string is a reference type in the CLR, which can be null. However, most other SQL column types map to CLR struct types, making nullable types very useful when mapping SQL to the CLR. For example:

// Maps to a Customer table in a database
public class Customer
{
  ...
  public decimal? AccountBalance;
}

A nullable type can also be used to represent the backing field of what’s sometimes called an ambient property. An ambient property, if null, returns the value of its parent. For example:

public class Row
{
  ...
  Grid parent;
  Color? color;

  public Color Color
  {
    get { return color ?? parent.Color; }
    set { color = value == parent.Color ? (Color?)null : value; }
  }
}

Alternatives to Nullable Types

Before nullable types were part of the C# language (i.e., before C# 2.0), there were many strategies to deal with nullable value types, examples of which still appear in the .NET Framework for historical reasons. One of these strategies is to designate a particular non-null value as the “null value”; an example is in the string and array classes. String.IndexOf returns the magic value of −1 when the character is not found:

int i = "Pink".IndexOf ('b');
Console.WriteLine (i);         // −1

However, Array.IndexOf returns −1 only if the index is 0-bounded. The more general formula is that IndexOf returns 1 less than the lower bound of the array. In the next example, IndexOf returns 0 when an element is not found:

// Create an array whose lower bound is 1 instead of 0:

Array a = Array.CreateInstance (typeof (string),
                                new int[] {2}, new int[] {1});
a.SetValue ("a", 1);
a.SetValue ("b", 2);
Console.WriteLine (Array.IndexOf (a, "c"));  // 0

Nominating a “magic value” is problematic for several reasons:

It means that each value type has a different representation of null. In contrast, nullable types provide one common pattern that works for all value types.
There may be no reasonable designated value. In the previous example, −1 could not always be used. The same is true for our earlier example representing an unknown account balance.
Forgetting to test for the magic value results in an incorrect value that may go unnoticed until later in execution—when it pulls an unintended magic trick. Forgetting to test HasValue on a null value, however, throws an InvalidOperationException on the spot.
The ability for a value to be null is not captured in the type. Types communicate the intention of a program, allow the compiler to check for correctness, and enable a consistent set of rules enforced by the compiler.

Operator Overloading

Operators can be overloaded to provide more natural syntax for custom types. Operator overloading is most appropriately used for implementing custom structs that represent fairly primitive data types. For example, a custom numeric type is an excellent candidate for operator overloading.

The following symbolic operators can be overloaded:

`+` (unary)	`-` (unary)	`!`	`~`	`++`
`--`	`+`	`-`	`*`	`/`
`%`	`&`	`\|`	`^`	`<<`
`>>`	`==`	`!=`	`>`	`<`
`>=`	`<=`

The following operators are also overloadable:

Implicit and explicit conversions (with the implicit and explicit keywords)
The true and false operators (not literals).

The following operators are indirectly overloaded:

The compound assignment operators (e.g., +=, /=) are implicitly overridden by overriding the noncompound operators (e.g., +, /).
The conditional operators && and || are implicitly overridden by overriding the bitwise operators & and |.

Operator Functions

An operator is overloaded by declaring an operator function. An operator function has the following rules:

The name of the function is specified with the operator keyword followed by an operator symbol.
The operator function must be marked static and public.
The parameters of the operator function represent the operands.
The return type of an operator function represents the result of an expression.
At least one of the operands must be the type in which the operator function is declared.

In the following example, we define a struct called Note representing a musical note and then overload the + operator:

public struct Note
{
  int value;
  public Note (int semitonesFromA) { value = semitonesFromA; }
  public static Note operator + (Note x, int semitones)
  {
    return new Note (x.value + semitones);
  }
}

This overload allows us to add an int to a Note:

Note B = new Note (2);
Note CSharp = B + 2;

Overloading an operator automatically overloads the corresponding compound assignment operator. In our example, since we overrode +, we can use += too:

CSharp += 2;

Just as with methods and properties, C# 6 allows operator functions comprising a single expression to be written more tersely with expression-bodied syntax:

public static Note operator + (Note x, int semitones)
                               => new Note (x.value + semitones);

Overloading Equality and Comparison Operators

Equality and comparison operators are sometimes overridden when writing structs and in rare cases, when writing classes. Special rules and obligations come with overloading the equality and comparison operators, which we explain in Chapter 6. A summary of these rules is as follows:

Pairing: The C# compiler enforces operators that are logical pairs to both be defined. These operators are (== !=), (< >), and (<= >=).
Equals and GetHashCode: In most cases, if you overload (==) and (!=), you will usually need to override the Equals and GetHashCode methods defined on object in order to get meaningful behavior. The C# compiler will give a warning if you do not do this. (See “Equality Comparison” in Chapter 6 for more details.)
IComparable and IComparable<T>: If you overload (< >) and (<= >=), you should implement IComparable and IComparable<T>.

Custom Implicit and Explicit Conversions

Implicit and explicit conversions are overloadable operators. These conversions are typically overloaded to make converting between strongly related types (such as numeric types) concise and natural.

To convert between weakly related types, the following strategies are more suitable:

Write a constructor that has a parameter of the type to convert from.
Write ToXXX and (static) FromXXX methods to convert between types.

As explained in the discussion on types, the rationale behind implicit conversions is that they are guaranteed to succeed and not lose information during the conversion. Conversely, an explicit conversion should be required either when runtime circumstances will determine whether the conversion will succeed or if information may be lost during the conversion.

In this example, we define conversions between our musical Note type and a double (which represents the frequency in hertz of that note):

...
// Convert to hertz
public static implicit operator double (Note x)
  => 440 * Math.Pow (2, (double) x.value / 12 );

// Convert from hertz (accurate to the nearest semitone)
public static explicit operator Note (double x)
  => new Note ((int) (0.5 + 12 * (Math.Log (x/440) / Math.Log(2) ) ));
...

Note n = (Note)554.37;  // explicit conversion
double x = n;           // implicit conversion

Note

Following our own guidelines, this example might be better implemented with a ToFrequency method (and a static FromFrequency method) instead of implicit and explicit operators.

Warning

Custom conversions are ignored by the as and is operators:

Console.WriteLine (554.37 is Note);   // False
Note n = 554.37 as Note;              // Error

Overloading true and false

The true and false operators are overloaded in the extremely rare case of types that are Boolean “in spirit” but do not have a conversion to bool. An example is a type that implements three-state logic: by overloading true and false, such a type can work seamlessly with conditional statements and operators—namely, if, do, while, for, &&, ||, and ?:. The System.Data.SqlTypes.SqlBoolean struct provides this functionality. For example:

SqlBoolean a = SqlBoolean.Null;
if (a)
  Console.WriteLine ("True");
else if (!a)
  Console.WriteLine ("False");
else
  Console.WriteLine ("Null");

OUTPUT:
Null

The following code is a reimplementation of the parts of SqlBoolean necessary to demonstrate the true and false operators:

public struct SqlBoolean
{
  public static bool operator true (SqlBoolean x)
    => x.m_value == True.m_value;

  public static bool operator false (SqlBoolean x)
    => x.m_value == False.m_value;

  public static SqlBoolean operator ! (SqlBoolean x)
  {
    if (x.m_value == Null.m_value)  return Null;
    if (x.m_value == False.m_value) return True;
    return False;
  }

  public static readonly SqlBoolean Null =  new SqlBoolean(0);
  public static readonly SqlBoolean False = new SqlBoolean(1);
  public static readonly SqlBoolean True =  new SqlBoolean(2);

  private SqlBoolean (byte value) { m_value = value; }
  private byte m_value;
}

Extension Methods

Extension methods allow an existing type to be extended with new methods without altering the definition of the original type. An extension method is a static method of a static class, where the this modifier is applied to the first parameter. The type of the first parameter will be the type that is extended. For example:

public static class StringHelper
{
  public static bool IsCapitalized (this string s)
  {
    if (string.IsNullOrEmpty(s)) return false;
    return char.IsUpper (s[0]);
  }
}

The IsCapitalized extension method can be called as though it were an instance method on a string, as follows:

Console.WriteLine ("Perth".IsCapitalized());

An extension method call, when compiled, is translated back into an ordinary static method call:

Console.WriteLine (StringHelper.IsCapitalized ("Perth"));

The translation works as follows:

arg0.Method (arg1, arg2, ...);              // Extension method call
StaticClass.Method (arg0, arg1, arg2, ...); // Static method call

Interfaces can be extended, too:

public static T First<T> (this IEnumerable<T> sequence)
{
  foreach (T element in sequence)
    return element;

  throw new InvalidOperationException ("No elements!");
}
...
Console.WriteLine ("Seattle".First());   // S

Extension methods were added in C# 3.0.

Extension Method Chaining

Extension methods, like instance methods, provide a tidy way to chain functions. Consider the following two functions:

public static class StringHelper
{
  public static string Pluralize (this string s) {...}
  public static string Capitalize (this string s) {...}
}

x and y are equivalent and both evaluate to "Sausages", but x uses extension methods, whereas y uses static methods:

string x = "sausage".Pluralize().Capitalize();
string y = StringHelper.Capitalize (StringHelper.Pluralize ("sausage"));

Ambiguity and Resolution

Namespaces

An extension method cannot be accessed unless its class is in scope, typically by its namespace being imported. Consider the extension method IsCapitalized in the following example:

using System;

namespace Utils
{
  public static class StringHelper
  {
    public static bool IsCapitalized (this string s)
    {
      if (string.IsNullOrEmpty(s)) return false;
      return char.IsUpper (s[0]);
    }
  }
}

To use IsCapitalized, the following application must import Utils in order to avoid a compile-time error:

namespace MyApp
{
  using Utils;

  class Test
  {
    static void Main() => Console.WriteLine ("Perth".IsCapitalized());
  }
}

Extension methods versus instance methods

Any compatible instance method will always take precedence over an extension method. In the following example, Test’s Foo method will always take precedence—even when called with an argument x of type int:

class Test
{
  public void Foo (object x) { }    // This method always wins
}

static class Extensions
{
  public static void Foo (this Test t, int x) { }
}

The only way to call the extension method in this case is via normal static syntax; in other words, Extensions.Foo(...).

Extension methods versus extension methods

If two extension methods have the same signature, the extension method must be called as an ordinary static method to disambiguate the method to call. If one extension method has more specific arguments, however, the more specific method takes precedence.

To illustrate, consider the following two classes:

static class StringHelper
{
  public static bool IsCapitalized (this string s) {...}
}
static class ObjectHelper
{
  public static bool IsCapitalized (this object s) {...}
}

The following code calls StringHelper’s IsCapitalized method:

bool test1 = "Perth".IsCapitalized();

Classes and structs are considered more specific than interfaces.

Anonymous Types

An anonymous type is a simple class created by the compiler on the fly to store a set of values. To create an anonymous type, use the new keyword followed by an object initializer, specifying the properties and values the type will contain. For example:

var dude = new { Name = "Bob", Age = 23 };

The compiler translates this to (approximately) the following:

internal class AnonymousGeneratedTypeName
{
  private string name;  // Actual field name is irrelevant
  private int    age;   // Actual field name is irrelevant

  public AnonymousGeneratedTypeName (string name, int age)
  {
    this.name = name; this.age = age;
  }

  public string  Name { get { return name; } }
  public int     Age  { get { return age;  } }

  // The Equals and GetHashCode methods are overridden (see Chapter 6).
  // The ToString method is also overridden.
}
...

var dude = new AnonymousGeneratedTypeName ("Bob", 23);

You must use the var keyword to reference an anonymous type because it doesn’t have a name.

The property name of an anonymous type can be inferred from an expression that is itself an identifier (or ends with one). For example:

int Age = 23;
var dude = new { Name = "Bob", Age, Age.ToString().Length };

is equivalent to:

var dude = new { Name = "Bob", Age = Age, Length = Age.ToString().Length };

Two anonymous type instances declared within the same assembly will have the same underlying type if their elements are named and typed identically:

var a1 = new { X = 2, Y = 4 };
var a2 = new { X = 2, Y = 4 };
Console.WriteLine (a1.GetType() == a2.GetType());   // True

Additionally, the Equals method is overridden to perform equality comparisons:

Console.WriteLine (a1 == a2);         // False
Console.WriteLine (a1.Equals (a2));   // True

You can create arrays of anonymous types as follows:

var dudes = new[]
{
  new { Name = "Bob", Age = 30 },
  new { Name = "Tom", Age = 40 }
};

Anonymous types are used primarily when writing LINQ queries (see Chapter 8), and were added in C# 3.0.

Dynamic Binding

Dynamic binding defers binding—the process of resolving types, members, and operations—from compile time to runtime. Dynamic binding is useful when at compile time you know that a certain function, member, or operation exists, but the compiler does not. This commonly occurs when you are interoperating with dynamic languages (such as IronPython) and COM and in scenarios when you might otherwise use reflection.

A dynamic type is declared with the contextual keyword dynamic:

dynamic d = GetSomeObject();
d.Quack();

A dynamic type tells the compiler to relax. We expect the runtime type of d to have a Quack method. We just can’t prove it statically. Since d is dynamic, the compiler defers binding Quack to d until runtime. To understand what this means requires distinguishing between static binding and dynamic binding.

Static Binding Versus Dynamic Binding

The canonical binding example is mapping a name to a specific function when compiling an expression. To compile the following expression, the compiler needs to find the implementation of the method named Quack:

d.Quack();

Let’s suppose the static type of d is Duck:

Duck d = ...
d.Quack();

In the simplest case, the compiler does the binding by looking for a parameterless method named Quack on Duck. Failing that, the compiler extends its search to methods taking optional parameters, methods on base classes of Duck, and extension methods that take Duck as its first parameter. If no match is found, you’ll get a compilation error. Regardless of what method gets bound, the bottom line is that the binding is done by the compiler, and the binding utterly depends on statically knowing the types of the operands (in this case, d). This makes it static binding.

Now let’s change the static type of d to object:

object d = ...
d.Quack();

Calling Quack gives us a compilation error, because although the value stored in d can contain a method called Quack, the compiler cannot know it since the only information it has is the type of the variable, which in this case is object. But let’s now change the static type of d to dynamic:

dynamic d = ...
d.Quack();

A dynamic type is like object—it’s equally nondescriptive about a type. The difference is that it lets you use it in ways that aren’t known at compile time. A dynamic object binds at runtime based on its runtime type, not its compile-time type. When the compiler sees a dynamically bound expression (which in general is an expression that contains any value of type dynamic), it merely packages up the expression such that the binding can be done later at runtime.

At runtime, if a dynamic object implements IDynamicMetaObjectProvider, that interface is used to perform the binding. If not, binding occurs in almost the same way as it would have had the compiler known the dynamic object’s runtime type. These two alternatives are called custom binding and language binding.

Note

COM interop can be considered to use a third kind of dynamic binding (see Chapter 25).

Custom Binding

Custom binding occurs when a dynamic object implements IDynamicMetaObjectProvider (IDMOP). Although you can implement IDMOP on types that you write in C#, and that is useful to do, the more common case is that you have acquired an IDMOP object from a dynamic language that is implemented in .NET on the DLR, such as IronPython or IronRuby. Objects from those languages implicitly implement IDMOP as a means by which to directly control the meanings of operations performed on them.

We will discuss custom binders in greater detail in Chapter 20, but we will write a simple one now to demonstrate the feature:

using System;
using System.Dynamic;

public class Test
{
  static void Main()
  {
    dynamic d = new Duck();
    d.Quack();                  // Quack method was called
    d.Waddle();                 // Waddle method was called
  }
}

public class Duck : DynamicObject
{
  public override bool TryInvokeMember (
    InvokeMemberBinder binder, object[] args, out object result)
  {
    Console.WriteLine (binder.Name + " method was called");
    result = null;
    return true;
  }
}

The Duck class doesn’t actually have a Quack method. Instead, it uses custom binding to intercept and interpret all method calls.

Language Binding

Language binding occurs when a dynamic object does not implement IDynamicMetaObjectProvider. Language binding is useful when working around imperfectly designed types or inherent limitations in the .NET type system (we’ll explore more scenarios in Chapter 20). A typical problem when using numeric types is that they have no common interface. We have seen that methods can be bound dynamically; the same is true for operators:

static dynamic Mean (dynamic x, dynamic y) => (x + y) / 2;

static void Main()
{
  int x = 3, y = 4;
  Console.WriteLine (Mean (x, y));
}

The benefit is obvious—you don’t have to duplicate code for each numeric type. However, you lose static type safety, risking runtime exceptions rather than compile-time errors.

Note

Dynamic binding circumvents static type safety, but not runtime type safety. Unlike with reflection (Chapter 19), you can’t circumvent member accessibility rules with dynamic binding.

By design, language runtime binding behaves as similarly as possible to static binding, had the runtime types of the dynamic objects been known at compile time. In our previous example, the behavior of our program would be identical if we hardcoded Mean to work with the int type. The most notable exception in parity between static and dynamic binding is for extension methods, which we discuss in “Uncallable Functions”.

Note

Dynamic binding also incurs a performance hit. Because of the DLR’s caching mechanisms, however, repeated calls to the same dynamic expression are optimized—allowing you to efficiently call dynamic expressions in a loop. This optimization brings the typical overhead for a simple dynamic expression on today’s hardware down to less than 100 ns.

RuntimeBinderException

If a member fails to bind, a RuntimeBinderException is thrown. You can think of this like a compile-time error at runtime:

dynamic d = 5;
d.Hello();                  // throws RuntimeBinderException

The exception is thrown because the int type has no Hello method.

Runtime Representation of Dynamic

There is a deep equivalence between the dynamic and object types. The runtime treats the following expression as true:

typeof (dynamic) == typeof (object)

This principle extends to constructed types and array types:

typeof (List<dynamic>) == typeof (List<object>)
typeof (dynamic[]) == typeof (object[])

Like an object reference, a dynamic reference can point to an object of any type (except pointer types):

dynamic x = "hello";
Console.WriteLine (x.GetType().Name);  // String
x = 123;  // No error (despite same variable)
Console.WriteLine (x.GetType().Name);  // Int32

Structurally, there is no difference between an object reference and a dynamic reference. A dynamic reference simply enables dynamic operations on the object it points to. You can convert from object to dynamic to perform any dynamic operation you want on an object:

object o = new System.Text.StringBuilder();
dynamic d = o;
d.Append ("hello");
Console.WriteLine (o);   // hello

Note

Reflecting on a type exposing (public) dynamic members reveals that those members are represented as annotated objects. For example:

public class Test
{
  public dynamic Foo;
}

is equivalent to:

public class Test
{
  [System.Runtime.CompilerServices.DynamicAttribute]
  public object Foo;
}

This allows consumers of that type to know that Foo should be treated as dynamic, while allowing languages that don’t support dynamic binding to fall back to object.

Dynamic Conversions

The dynamic type has implicit conversions to and from all other types:

int i = 7;
dynamic d = i;
long j = d;        // No cast required (implicit conversion)

For the conversion to succeed, the runtime type of the dynamic object must be implicitly convertible to the target static type. The preceding example worked because an int is implicitly convertible to a long.

The following example throws a RuntimeBinderException because an int is not implicitly convertible to a short:

int i = 7;
dynamic d = i;
short j = d;      // throws RuntimeBinderException

var Versus dynamic

The var and dynamic types bear a superficial resemblance, but the difference is deep:

var says, “Let the compiler figure out the type.”
dynamic says, “Let the runtime figure out the type.”

To illustrate:

dynamic x = "hello";  // Static type is dynamic, runtime type is string
var y = "hello";      // Static type is string, runtime type is string
int i = x;            // Runtime error      (cannot convert string to int)
int j = y;            // Compile-time error (cannot convert string to int)

The static type of a variable declared with var can be dynamic:

dynamic x = "hello";
var y = x;            // Static type of y is dynamic
int z = y;            // Runtime error (cannot convert string to int)

Dynamic Expressions

Fields, properties, methods, events, constructors, indexers, operators, and conversions can all be called dynamically.

Trying to consume the result of a dynamic expression with a void return type is prohibited—just as with a statically typed expression. The difference is that the error occurs at runtime:

dynamic list = new List<int>();
var result = list.Add (5);         // RuntimeBinderException thrown

Expressions involving dynamic operands are typically themselves dynamic, since the effect of absent type information is cascading:

dynamic x = 2;
var y = x * 3;       // Static type of y is dynamic

There are a couple of obvious exceptions to this rule. First, casting a dynamic expression to a static type yields a static expression:

dynamic x = 2;
var y = (int)x;      // Static type of y is int

Second, constructor invocations always yield static expressions—even when called with dynamic arguments. In this example, x is statically typed to a StringBuilder:

dynamic capacity = 10;
var x = new System.Text.StringBuilder (capacity);

In addition, there are a few edge cases where an expression containing a dynamic argument is static, including passing an index to an array and delegate creation expressions.

Dynamic Calls Without Dynamic Receivers

The canonical use case for dynamic involves a dynamic receiver. This means that a dynamic object is the receiver of a dynamic function call:

dynamic x = ...;
x.Foo();          // x is the receiver

However, you can also call statically known functions with dynamic arguments. Such calls are subject to dynamic overload resolution and can include:

Static methods
Instance constructors
Instance methods on receivers with a statically known type

In the following example, the particular Foo that gets dynamically bound is dependent on the runtime type of the dynamic argument:

class Program
{
  static void Foo (int x)    { Console.WriteLine ("1"); }
  static void Foo (string x) { Console.WriteLine ("2"); }

  static void Main()
  {
    dynamic x = 5;
    dynamic y = "watermelon";

    Foo (x);                // 1
    Foo (y);                // 2
  }
}

Because a dynamic receiver is not involved, the compiler can statically perform a basic check to see whether the dynamic call will succeed. It checks that a function with the right name and number of parameters exists. If no candidate is found, you get a compile-time error. For example:

class Program
{
  static void Foo (int x)    { Console.WriteLine ("1"); }
  static void Foo (string x) { Console.WriteLine ("2"); }

  static void Main()
  {
    dynamic x = 5;
    Foo (x, x);          // Compiler error - wrong number of parameters
    Fook (x);            // Compiler error - no such method name
  }
}

Static Types in Dynamic Expressions

It’s obvious that dynamic types are used in dynamic binding. It’s not so obvious that static types are also used—wherever possible—in dynamic binding. Consider the following:

class Program
{
  static void Foo (object x, object y) { Console.WriteLine ("oo"); }
  static void Foo (object x, string y) { Console.WriteLine ("os"); }
  static void Foo (string x, object y) { Console.WriteLine ("so"); }
  static void Foo (string x, string y) { Console.WriteLine ("ss"); }

  static void Main()
  {
    object o = "hello";
    dynamic d = "goodbye";
    Foo (o, d);               // os
  }
}

The call to Foo(o,d) is dynamically bound because one of its arguments, d, is dynamic. But since o is statically known, the binding—even though it occurs dynamically—will make use of that. In this case, overload resolution will pick the second implementation of Foo due to the static type of o and the runtime type of d. In other words, the compiler is “as static as it can possibly be.”

Uncallable Functions

Some functions cannot be called dynamically. You cannot call:

Extension methods (via extension method syntax)
Members of an interface, if you need to cast to that interface to do so
Base members hidden by a subclass

Understanding why this is so is useful in understanding dynamic binding.

Dynamic binding requires two pieces of information: the name of the function to call, and the object upon which to call the function. However, in each of the three uncallable scenarios, an additional type is involved, which is known only at compile time. As of C# 6, there’s no way to specify these additional types dynamically.

When calling extension methods, that additional type is implicit. It’s the static class on which the extension method is defined. The compiler searches for it given the using directives in your source code. This makes extension methods compile-time-only concepts, since using directives melt away upon compilation (after they’ve done their job in the binding process in mapping simple names to namespace-qualified names).

When calling members via an interface, you specify that additional type via an implicit or explicit cast. There are two scenarios where you might want to do this: when calling explicitly implemented interface members and when calling interface members implemented in a type internal to another assembly. We can illustrate the former with the following two types:

interface IFoo   { void Test();        }
class Foo : IFoo { void IFoo.Test() {} }

To call the Test method, we must cast to the IFoo interface. This is easy with static typing:

IFoo f = new Foo();   // Implicit cast to interface
f.Test();

Now consider the situation with dynamic typing:

IFoo f = new Foo();
dynamic d = f;
d.Test();             // Exception thrown

The implicit cast shown in bold tells the compiler to bind subsequent member calls on f to IFoo rather than Foo—in other words, to view that object through the lens of the IFoo interface. However, that lens is lost at runtime, so the DLR cannot complete the binding. The loss is illustrated as follows:

Console.WriteLine (f.GetType().Name);    // Foo

A similar situation arises when calling a hidden base member: you must specify an additional type via either a cast or the base keyword—and that additional type is lost at runtime.

Attributes

You’re already familiar with the notion of attributing code elements of a program with modifiers, such as virtual or ref. These constructs are built into the language. Attributes are an extensible mechanism for adding custom information to code elements (assemblies, types, members, return values, parameters, and generic type parameters). This extensibility is useful for services that integrate deeply into the type system, without requiring special keywords or constructs in the C# language.

A good scenario for attributes is serialization—the process of converting arbitrary objects to and from a particular format. In this scenario, an attribute on a field can specify the translation between C#’s representation of the field and the format’s representation of the field.

Attribute Classes

An attribute is defined by a class that inherits (directly or indirectly) from the abstract class System.Attribute. To attach an attribute to a code element, specify the attribute’s type name in square brackets, before the code element. For example, the following attaches the ObsoleteAttribute to the Foo class:

[ObsoleteAttribute]
public class Foo {...}

This attribute is recognized by the compiler and will cause compiler warnings if a type or member marked obsolete is referenced. By convention, all attribute types end in the word Attribute. C# recognizes this and allows you to omit the suffix when attaching an attribute:

[Obsolete]
public class Foo {...}

ObsoleteAttribute is a type declared in the System namespace as follows (simplified for brevity):

public sealed class ObsoleteAttribute : Attribute {...}

The C# language and the .NET Framework include a number of predefined attributes. We describe how to write your own attributes in Chapter 19.

Named and Positional Attribute Parameters

Attributes may have parameters. In the following example, we apply XmlElementAttribute to a class. This attribute tells XML serializer (in System.Xml.Serialization) how an object is represented in XML and accepts several attribute parameters. The following attribute maps the CustomerEntity class to an XML element named Customer, belonging to the http://oreilly.com namespace:

[XmlElement ("Customer", Namespace="http://oreilly.com")]
public class CustomerEntity { ... }

Attribute parameters fall into one of two categories: positional or named. In the preceding example, the first argument is a positional parameter; the second is a named parameter. Positional parameters correspond to parameters of the attribute type’s public constructors. Named parameters correspond to public fields or public properties on the attribute type.

When specifying an attribute, you must include positional parameters that correspond to one of the attribute’s constructors. Named parameters are optional.

In Chapter 19, we describe the valid parameter types and rules for their evaluation.

Attribute Targets

Implicitly, the target of an attribute is the code element it immediately precedes, which is typically a type or type member. You can also attach attributes, however, to an assembly. This requires that you explicitly specify the attribute’s target.

Here is an example of using the CLSCompliant attribute to specify CLS compliance for an entire assembly:

[assembly:CLSCompliant(true)]

Specifying Multiple Attributes

Multiple attributes can be specified for a single code element. Each attribute can be listed either within the same pair of square brackets (separated by a comma) or in separate pairs of square brackets (or a combination of the two). The following three examples are semantically identical:

[Serializable, Obsolete, CLSCompliant(false)]
public class Bar {...}

[Serializable] [Obsolete] [CLSCompliant(false)]
public class Bar {...}

[Serializable, Obsolete]
[CLSCompliant(false)]
public class Bar {...}

Caller Info Attributes (C# 5)

From C# 5, you can tag optional parameters with one of three caller info attributes, which instruct the compiler to feed information obtained from the caller’s source code into the parameter’s default value:

[CallerMemberName] applies the caller’s member name
[CallerFilePath] applies the path to caller’s source code file
[CallerLineNumber] applies the line number in caller’s source code file

The Foo method in the following program demonstrates all three:

using System;
using System.Runtime.CompilerServices;

class Program
{
  static void Main() => Foo();

  static void Foo (
    [CallerMemberName] string memberName = null,
    [CallerFilePath] string filePath = null,
    [CallerLineNumber] int lineNumber = 0)
  {
    Console.WriteLine (memberName);
    Console.WriteLine (filePath);
    Console.WriteLine (lineNumber);
  }
}

Assuming our program resides in c:\source\test\Program.cs, the output would be:

Main
c:\source\test\Program.cs
6

As with standard optional parameters, the substitution is done at the calling site. Hence, our Main method is syntactic sugar for this:

static void Main() => Foo ("Main", @"c:\source\test\Program.cs", 6);

Caller info attributes are useful for logging—and for implementing patterns such as firing a single change notification event whenever any property on an object changes. In fact, there’s a standard interface in the .NET Framework for this called INotifyPropertyChanged (in System.ComponentModel):

public interface INotifyPropertyChanged
{
  event PropertyChangedEventHandler PropertyChanged;
}

public delegate void PropertyChangedEventHandler
  (object sender, PropertyChangedEventArgs e);

public class PropertyChangedEventArgs : EventArgs
{
  public PropertyChangedEventArgs (string propertyName);
  public virtual string PropertyName { get; }
}

Notice that PropertyChangedEventArgs requires the name of the property that changed. By applying the [CallerMemberName] attribute, however, we can implement this interface and invoke the event without ever specifying property names:

public class Foo : INotifyPropertyChanged
{
  public event PropertyChangedEventHandler PropertyChanged = delegate { };

  void RaisePropertyChanged ([CallerMemberName] string propertyName = null)
  {
    PropertyChanged (this, new PropertyChangedEventArgs (propertyName));
  }

  string customerName;
  public string CustomerName
  {
    get { return customerName; }
    set
    {
      if (value == customerName) return;
      customerName = value;
      RaisePropertyChanged();
      // The compiler converts the above line to:
      // RaisePropertyChanged ("CustomerName");
    }
  }
}

Unsafe Code and Pointers

C# supports direct memory manipulation via pointers within blocks of code marked unsafe and compiled with the /unsafe compiler option. Pointer types are primarily useful for interoperability with C APIs but may also be used for accessing memory outside the managed heap or for performance-critical hotspots.

Pointer Basics

For every value type or reference type V, there is a corresponding pointer type V*. A pointer instance holds the address of a variable. Pointer types can be (unsafely) cast to any other pointer type. The main pointer operators are:

Operator	Meaning
`&`	The address-of operator returns a pointer to the address of a variable
`*`	The dereference operator returns the variable at the address of a pointer
`->`	The pointer-to-member operator is a syntactic shortcut, in which `x->y` is equivalent to `(*x).y`

Unsafe Code

By marking a type, type member, or statement block with the unsafe keyword, you’re permitted to use pointer types and perform C++ style pointer operations on memory within that scope. Here is an example of using pointers to quickly process a bitmap:

unsafe void BlueFilter (int[,] bitmap)
{
  int length = bitmap.Length;
  fixed (int* b = bitmap)
  {
    int* p = b;
    for (int i = 0; i < length; i++)
      *p++ &= 0xFF;
  }
}

Unsafe code can run faster than a corresponding safe implementation. In this case, the code would have required a nested loop with array indexing and bounds checking. An unsafe C# method may also be faster than calling an external C function, since there is no overhead associated with leaving the managed execution environment.

The fixed Statement

The fixed statement is required to pin a managed object, such as the bitmap in the previous example. During the execution of a program, many objects are allocated and deallocated from the heap. In order to avoid unnecessary waste or fragmentation of memory, the garbage collector moves objects around. Pointing to an object is futile if its address could change while referencing it, so the fixed statement tells the garbage collector to “pin” the object and not move it around. This may have an impact on the efficiency of the runtime, so fixed blocks should be used only briefly, and heap allocation should be avoided within the fixed block.

Within a fixed statement, you can get a pointer to any value type, an array of value types, or a string. In the case of arrays and strings, the pointer will actually point to the first element, which is a value type.

Value types declared inline within reference types require the reference type to be pinned, as follows:

class Test
{
  int x;
  static void Main()
  {
    Test test = new Test();
    unsafe
    {
       fixed (int* p = &test.x)   // Pins test
       {
         *p = 9;
       }
       System.Console.WriteLine (test.x);
    }
  }
}

We describe the fixed statement further in “Mapping a Struct to Unmanaged Memory” in Chapter 25.

The Pointer-to-Member Operator

In addition to the & and * operators, C# also provides the C++ style -> operator, which can be used on structs:

struct Test
{
  int x;
  unsafe static void Main()
  {
    Test test = new Test();
    Test* p = &test;
    p->x = 9;
    System.Console.WriteLine (test.x);
  }
}

Arrays

The stackalloc keyword

Memory can be allocated in a block on the stack explicitly using the stackalloc keyword. Since it is allocated on the stack, its lifetime is limited to the execution of the method, just as with any other local variable (whose life hasn’t been extended by virtue of being captured by a lambda expression, iterator block, or asynchronous function). The block may use the [] operator to index into memory:

int* a = stackalloc int [10];
for (int i = 0; i < 10; ++i)
   Console.WriteLine (a[i]);   // Print raw memory

Fixed-size buffers

The fixed keyword has another use, which is to create fixed-size buffers within structs:

unsafe struct UnsafeUnicodeString
{
  public short Length;
  public fixed byte Buffer[30];   // Allocate block of 30 bytes
}

unsafe class UnsafeClass
{
  UnsafeUnicodeString uus;

  public UnsafeClass (string s)
  {
    uus.Length = (short)s.Length;
    fixed (byte* p = uus.Buffer)
      for (int i = 0; i < s.Length; i++)
        p[i] = (byte) s[i];
  }
}
class Test
{
  static void Main() { new UnsafeClass ("Christian Troy"); }
}

The fixed keyword is also used in this example to pin the object on the heap that contains the buffer (which will be the instance of UnsafeClass). Hence, fixed means two different things: fixed in size and fixed in place. The two are often used together, in that a fixed-size buffer must be fixed in place to be used.

void*

A void pointer (void*) makes no assumptions about the type of the underlying data and is useful for functions that deal with raw memory. An implicit conversion exists from any pointer type to void*. A void* cannot be dereferenced, and arithmetic operations cannot be performed on void pointers. For example:

class Test
{
  unsafe static void Main()
  {
    short[ ] a = {1,1,2,3,5,8,13,21,34,55};
    fixed (short* p = a)
    {
      //sizeof returns size of value-type in bytes
      Zap (p, a.Length * sizeof (short));
    }
    foreach (short x in a)
      System.Console.WriteLine (x);   // Prints all zeros
  }

  unsafe static void Zap (void* memory, int byteCount)
  {
    byte* b = (byte*) memory;
      for (int i = 0; i < byteCount; i++)
        *b++ = 0;
  }
}

Pointers to Unmanaged Code

Pointers are also useful for accessing data outside the managed heap (such as when interacting with C DLLs or COM) or when dealing with data not in the main memory (such as graphics memory or a storage medium on an embedded device).

Preprocessor Directives

Preprocessor directives supply the compiler with additional information about regions of code. The most common preprocessor directives are the conditional directives, which provide a way to include or exclude regions of code from compilation. For example:

#define DEBUG
class MyClass
{
  int x;
  void Foo()
  {
    #if DEBUG
    Console.WriteLine ("Testing: x = {0}", x);
    #endif
  }
  ...
}

In this class, the statement in Foo is compiled as conditionally dependent upon the presence of the DEBUG symbol. If we remove the DEBUG symbol, the statement is not compiled. Preprocessor symbols can be defined within a source file (as we have done), and they can be passed to the compiler with the /define:symbol command-line option.

With the #if and #elif directives, you can use the ||, &&, and ! operators to perform or, and, and not operations on multiple symbols. The following directive instructs the compiler to include the code that follows if the TESTMODE symbol is defined and the DEBUG symbol is not defined:

#if TESTMODE && !DEBUG
  ...

Bear in mind, however, that you’re not building an ordinary C# expression, and the symbols upon which you operate have absolutely no connection to variables—static or otherwise.

The #error and #warning symbols prevent accidental misuse of conditional directives by making the compiler generate a warning or error given an undesirable set of compilation symbols. Table 4-1 lists the preprocessor directives.

Table 4-1. Preprocessor directives
Preprocessor directive	Action
`#define symbol`	Defines `symbol`
`#undef symbol`	Undefines `symbol`
`#if symbol [operator symbol2]...`	`symbol` to test
	`operator`s are `==`, `!=`, `&&`, and `\|\|` followed by `#else`, `#elif`, and `#endif`
`#else`	Executes code to subsequent `#endif`
`#elif symbol [operator symbol2]`	Combines `#else` branch and `#if` test
`#endif`	Ends conditional directives
`#warning text`	`text` of the warning to appear in compiler output
`#error text`	`text` of the error to appear in compiler output
`#pragma warning [disable \| restore]`	Disables/restores compiler warning(s)
`#line [ number ["file"] \| hidden]`	`number` specifies the line in source code; `file` is the filename to appear in computer output; `hidden` instructs debuggers to skip over code from this point until the next `#line` directive
`#region name`	Marks the beginning of an outline
`#endregion`	Ends an outline region

Conditional Attributes

An attribute decorated with the Conditional attribute will be compiled only if a given preprocessor symbol is present. For example:

// file1.cs
#define DEBUG
using System;
using System.Diagnostics;
[Conditional("DEBUG")]
public class TestAttribute : Attribute {}

// file2.cs
#define DEBUG
[Test]
class Foo
{
  [Test]
  string s;
}

The compiler will only incorporate the [Test] attributes if the DEBUG symbol is in scope for file2.cs.

Pragma Warning

The compiler generates a warning when it spots something in your code that seems unintentional. Unlike errors, warnings don’t ordinarily prevent your application from compiling.

Compiler warnings can be extremely valuable in spotting bugs. Their usefulness, however, is undermined when you get false warnings. In a large application, maintaining a good signal-to-noise ratio is essential if the “real” warnings are to get noticed.

To this effect, the compiler allows you to selectively suppress warnings with the #pragma warning directive. In this example, we instruct the compiler not to warn us about the field Message not being used:

public class Foo
{
  static void Main() { }

  #pragma warning disable 414
  static string Message = "Hello";
  #pragma warning restore 414
}

Omitting the number in the #pragma warning directive disables or restores all warning codes.

If you are thorough in applying this directive, you can compile with the /warnaserror switch—this tells the compiler to treat any residual warnings as errors.

XML Documentation

A documentation comment is a piece of embedded XML that documents a type or member. A documentation comment comes immediately before a type or member declaration and starts with three slashes:

/// <summary>Cancels a running query.</summary>
public void Cancel() { ... }

Multiline comments can be done either like this:

/// <summary>
/// Cancels a running query
/// </summary>
public void Cancel() { ... }

or like this (notice the extra star at the start):

/**
    <summary> Cancels a running query. </summary>
*/
public void Cancel() { ... }

If you compile with the /doc directive (in Visual Studio, go to the Build tab of Project Properties), the compiler extracts and collates documentation comments into a single XML file. This has two main uses:

If placed in the same folder as the compiled assembly, Visual Studio (and LINQPad) automatically read the XML file and use the information to provide IntelliSense member listings to consumers of the assembly of the same name.
Third-party tools (such as Sandcastle and NDoc) can transform the XML file into an HTML help file.

Standard XML Documentation Tags

Here are the standard XML tags that Visual Studio and documentation generators recognize:

<summary>

<summary>...</summary>

Indicates the tool tip that IntelliSense should display for the type or member; typically a single phrase or sentence.

<remarks>

<remarks>...</remarks>

Additional text that describes the type or member. Documentation generators pick this up and merge it into the bulk of a type or member’s description.

<param>

<param name="name">...</param>

Explains a parameter on a method.

<returns>

<returns>...</returns>

Explains the return value for a method.

<exception>

<exception [cref="type"]>...</exception>

Lists an exception that a method may throw (cref refers to the exception type).

<permission>

<permission [cref="type"]>...</permission>

Indicates an IPermission type required by the documented type or member.

<example>

<example>...</example>

Denotes an example (used by documentation generators). This usually contains both description text and source code (source code is typically within a <c> or <code> tag).

<c>

<c>...</c>

Indicates an inline code snippet. This tag is usually used inside an <example> block.

<code>

<code>...</code>

Indicates a multiline code sample. This tag is usually used inside an <example> block.

<see>

<see cref="member">...</see>

Inserts an inline cross-reference to another type or member. HTML documentation generators typically convert this to a hyperlink. The compiler emits a warning if the type or member name is invalid. To refer to generic types, use curly braces; for example, cref="Foo{T,U}".

<seealso>

<seealso cref="member">...</seealso>

Cross-references another type or member. Documentation generators typically write this into a separate “See Also” section at the bottom of the page.

<paramref>

<paramref name="name"/>

References a parameter from within a <summary> or <remarks> tag.

<list>

<list type=[ bullet | number | table ]>
  <listheader>
    <term>...</term>
    <description>...</description>
  </listheader>
  <item>
    <term>...</term>
    <description>...</description>
  </item>
</list>

Instructs documentation generators to emit a bulleted, numbered, or table-style list.

<para>

<para>...</para>

Instructs documentation generators to format the contents into a separate paragraph.

<include>

<include file='filename' path='tagpath[@name="id"]'>...</include>

Merges an external XML file that contains documentation. The path attribute denotes an XPath query to a specific element in that file.

User-Defined Tags

Little is special about the predefined XML tags recognized by the C# compiler, and you are free to define your own. The only special processing done by the compiler is on the <param> tag (in which it verifies the parameter name and that all the parameters on the method are documented) and the cref attribute (in which it verifies that the attribute refers to a real type or member and expands it to a fully qualified type or member ID). The cref attribute can also be used in your own tags and is verified and expanded just as it is in the predefined <exception>, <permission>, <see>, and <seealso> tags.

Type or Member Cross-References

Type names and type or member cross-references are translated into IDs that uniquely define the type or member. These names are composed of a prefix that defines what the ID represents and a signature of the type or member. The member prefixes are:

XML type prefix	ID prefixes applied to...
`N`	Namespace
`T`	Type (class, struct, enum, interface, delegate)
`F`	Field
`P`	Property (includes indexers)
`M`	Method (includes special methods)
`E`	Event
`!`	Error

The rules describing how the signatures are generated are well documented, although fairly complex.

Here is an example of a type and the IDs that are generated:

// Namespaces do not have independent signatures
namespace NS
{
  /// T:NS.MyClass
  class MyClass
  {
    /// F:NS.MyClass.aField
    string aField;

    /// P:NS.MyClass.aProperty
    short aProperty {get {...} set {...}}

    /// T:NS.MyClass.NestedType
    class NestedType {...};

    /// M:NS.MyClass.X()
    void X() {...}

    /// M:NS.MyClass.Y(System.Int32,System.Double@,System.Decimal@)
    void Y(int p1, ref double p2, out decimal p3) {...}

    /// M:NS.MyClass.Z(System.Char[ ],System.Single[0:,0:])
    void Z(char[ ] a1, float[,] p2) {...}

    /// M:NS.MyClass.op_Addition(NS.MyClass,NS.MyClass)
    public static MyClass operator+(MyClass c1, MyClass c2) {...}

    /// M:NS.MyClass.op_Implicit(NS.MyClass)~System.Int32
    public static implicit operator int(MyClass c) {...}

    /// M:NS.MyClass.#ctor
    MyClass() {...}

    /// M:NS.MyClass.Finalize
    ~MyClass() {...}

    /// M:NS.MyClass.#cctor
    static MyClass() {...}
  }
}

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Chapter 4. Advanced C#

Delegates

Note

Note

Writing Plug-in Methods with Delegates

Multicast Delegates

Note

Note

Multicast delegate example

Instance Versus Static Method Targets

Generic Delegate Types

The Func and Action Delegates

Note

Delegates Versus Interfaces

Delegate Compatibility

Type compatibility

Note

Parameter compatibility

Note

Return type compatibility

Generic delegate type parameter variance

Events

Note

Standard Event Pattern

Note

Note

Event Accessors

Note

Event Modifiers

Lambda Expressions

Note

Explicitly Specifying Lambda Parameter Types

Capturing Outer Variables

Note

Capturing iteration variables

Note

Warning

Anonymous Methods

Note

try Statements and Exceptions

Note

The catch Clause

Exception filters (C# 6)

The finally Block

The using statement

Throwing Exceptions

Rethrowing an exception

Note

Key Properties of System.Exception

Note

Common Exception Types

The TryXXX Method Pattern

Alternatives to Exceptions

Enumeration and Iterators

Enumeration

Note

Note

Collection Initializers

Iterators

Note

Iterator Semantics

yield break

Note

Iterators and try/catch/finally blocks

Composing Sequences

Figure 4-1. Composing sequences

Nullable Types

Nullable<T> struct

Implicit and explicit nullable conversions

Boxing and unboxing nullable values

Operator Lifting

Equality operators (== and !=)

Relational operators (<, <=, >=, >)

All other operators (+, −, *, /, %, &, |, ^, <<, >>, +, ++, --, !, ~)

Mixing nullable and non-nullable operators

bool? with & and | Operators

Nullable Types & Null Operators

Scenarios for Nullable Types

Alternatives to Nullable Types

Operator Overloading