Build Your Own ASP.NET 2.0 Web Site Using C# & VB, Second Edition

Be Careful when Changing Global.asax

Be cautious when you add and modify code within the Global.asax file.

Any additions or modifications you make within this file will cause the ap-

plication to restart, so you’ll lose any data stored in application state.

Using Application State

You can store the variables and objects you want to use throughout an entire

application in a special object called Application. The data stored in this object

is called application state. The Application object also provides you with

methods that allow you to share application state data between all the pages in

a given ASP.NET application very easily.

Application state is closely related to another concept: session state. The key

difference between the two is that session state stores variables and objects for

one particular user for the duration of that user’s current visit, whereas application

state stores objects and variables that are shared between all users of an application

at the same time. Thus, application state is ideal for storing data that’s used by

all users of the same application.

In ASP.NET, session and application state are both implemented as collections,

or sets of name-value pairs. You can set the value of an application variable named

SiteName like this:

Visual Basic

Application("SiteName") = "Dorknozzle Intranet Application"

C#

Application["SiteName"] = "Dorknozzle Intranet Application";

With SiteName set, any pages in the application can read this string:

Visual Basic

Dim appName As String = Application("SiteName")

C#

String appName = Application["SiteName"];

We can remove an object from application state using the Remove method, like

so:

Visual Basic

Application.Remove("SiteName")

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Using Application State

C#

Application.Remove("SiteName");

If you find you have multiple objects and application variables lingering in applic-

ation state, you can remove them all at once using the RemoveAll method:

Visual Basic

Application.RemoveAll()

C#

Application.RemoveAll();

It’s important to be cautious when using application variables. Objects remain

in application state until you remove them using the Remove or RemoveAll

methods, or shut down the application in IIS. If you continue to save objects

into the application state without removing them, you can place a heavy demand

on server resources and dramatically decrease the performance of your applica-

tions.

Let’s take a look at application state in action. Application state is very commonly

used to maintain hit counters, so our first task in this example will be to build

one! Let’s modify the Default.aspx page that Visual Web Developer created

for us. Double-click Default.aspx in Solution Explorer, and add a Label control

inside the form element. You could drag the control from the Toolbox (in either

Design View or Source View) and modify the generated code, or you could simply

enter the new code by hand. We’ll also add a bit of text to the page, and change

the Label’s ID to myLabel, as shown below:

File: Default.aspx (excerpt)

<form id="form1" runat="server">

<div>

The page has been requested

<asp:Label ID="myLabel" runat="server" />

times!

</div>

</form>

In Design View, you should see your label appear inside the text, as shown in

Figure 5.27.

Now, let’s modify the code-behind file to use an application variable that will

keep track of the number of hits our page receives. Double-click in any empty

space on your form; Visual Web Developer will create a Page_Load subroutine

automatically, and display it in the code editor.

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Chapter 5: Building Web Applications

Figure 5.27. The new label appearing in Design View

Visual Basic File: Default.aspx.vb (excerpt)

Partial Class _Default

Inherits System.Web.UI.Page

Protected Sub Page_Load(ByVal sender As Object, _

ByVal e As System.EventArgs) Handles Me.Load

End Sub

End Class

C# File: Default.aspx.cs (excerpt)

public partial class _Default : System.Web.UI.Page

{

protected void Page_Load(object sender, EventArgs e)

{

}

}

Now, let’s modify the automatically generated method by adding the code that

we want to run every time the page is loaded. Modify Page_Load as shown below:

Visual Basic File: Default.aspx.vb (excerpt)

Protected Sub Page_Load(ByVal sender As Object, _

ByVal e As System.EventArgs) Handles Me.Load

' Reset counter when it reaches 10

If Application("PageCounter") >= 10 Then

Application.Remove("PageCounter")

End If

' Initialize or increment page counter each time the page loads

If Application("PageCounter") Is Nothing Then

Application("PageCounter") = 1

Else

Application("PageCounter") += 1

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Using Application State