C# lets you overload operators to work with operands that are custom
classes or structs using operators. An operator is a static method with the
keyword operator
preceding the operator to overload
(instead of a method name), parameters representing the operands, and return
types representing the result of an expression. Table 4-1 lists the available
overloadable operators.
Table 4-1. Overloadable operators
+
|
-
|
!
|
~
|
++
|
--
|
+
|
-
|
* (binary only)
|
/
|
%
|
& (binary only)
|
|
|
^
|
<<
|
>>
|
==
|
!=
|
>
|
<
|
>=
|
<=
|
Literals that also act as overloadable operators are true
and false
.
A pair of references exhibit referential equality when both
references point to the same object. By default, the
==
and !=
operators will compare
two reference-type variables by reference. However, it is occasionally
more natural for the ==
and !=
operators to exhibit value equality, whereby the comparison is based
on the value of the objects that the references point to.
Whenever overloading the ==
and
!=
operators, you should always override the
virtual Equals
method to route its functionality to
the ==
operator. This allows a class to be used
polymorphically (which is essential if you want to take advantage of
functionality such as the collection classes). It also provides
compatibility with other .NET languages that don’t overload
operators.
Tip
A good guideline for knowing whether to implement the
==
and !=
operators is if it is
natural for the class to overload other operators too, such as
<
, >
, +
, or
-
; otherwise, don’t bother — just implement
the Equals
method. For structs, overloading the
==
and !=
operators provides a more
efficient implementation than the default one.
class Note { int value; public Note(int semitonesFromA) { value = semitonesFromA; } public static bool operator ==(Note x, Note y) { return x.value == y.value; } public static bool operator !=(Note x, Note y) { return x.value != y.value; } public override bool Equals(object o) { if(!(o is Note)) return false; return this ==(Note)o; } } Note a = new Note(4); Note b = new Note(4); Object c = a; Object d = b; // To compare a and b by reference Console.WriteLine((object)a ==(object)b; // false //To compare a and b by value: Console.WriteLine(a == b); // true //To compare c and d by reference: Console.WriteLine(c == d); // false //To compare c and d by value: Console.WriteLine(c.Equals(d)); // true
The C# compiler enforces operators that are logical pairs to
both be defined. These operators are == !=
, < >
, and <= >=
.
As explained in the discussion on types, the rationale behind implicit
conversions is they are guaranteed to succeed and do not lose information
during the conversion. Conversely, an explicit conversion is 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) { return 440*Math.Pow(2,(double)x.value/12); } // Convert from hertz(only accurate to nearest semitone) public static explicit operator Note(double x) { return 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
The true
and false
keywords are
used as operators when defining types with three-state logic to enable these
types to work seamlessly with constructs that take boolean expressions — namely,
the if
, do
, while
, for
,
and conditional (?:)
statements. The System.Data.SQLTypes.SQLBoolean
struct
provides this functionality:
public struct SQLBoolean ... { ... public static bool operator true(SQLBoolean x) { return x.value == 1; } public static bool operator false(SQLBoolean x) { return x.value == -1; } public static SQLBoolean operator !(SQLBoolean x) { return new SQLBoolean(- x.value); } public bool IsNull { get { return value == 0;} } ... } class Test { void Foo(SQLBoolean a) { if (a) Console.WriteLine("True"); else if (! a) Console.WriteLine("False"); else Console.WriteLine("Null"); } }
The &&
and ||
operators
are automatically evaluated from &
and |
,
so they do not need to be overloaded. The []
operators
can be customized with indexers (see Section 3.1.5 in Chapter 3).
The assignment operator =
cannot be overloaded, but all
other assignment operators are automatically evaluated from their corresponding
binary operators (e.g., +=
is evaluated from +
).
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