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

when some external indicator changes, but that kind of task is a little beyond the

scope of this discussion.

Later in the code, we could use the cached object as follows:

Visual Basic

employeesTable = Cache("Employees")

C#

employeesTable = Cache["Employees"];

Objects in the cache can expire, so it’s good practice to verify that the object

you’re expecting does actually exist, to avoid any surprises:

Visual Basic

employeesTable = Cache("Employees")

If employeesTable Is Nothing Then

' Read the employees table from another source

Cache("Employees") = employeesTable

End If

C#

employeesTable = Cache["Employees"];

if (employeesTable == null)

{

// Read the employees table from another source

Cache["Employees"] = employeesTable;

}

This sample code checks to see if the data you’re expecting exists in the cache.

If not, it means that this is the first time the code has been executed, or that the

item has been removed from the cache. Thus, we can populate employeesTable

from the database, remembering to store the retrieved data into the cache. The

trip to the database server is made only if the cache is empty or not present.

Using Cookies

If you want to store data related to a particular user, you could use the Session

object, but this approach has an important drawback: its contents are lost when

the user closes the browser window.

To store user data for longer periods of time, you need to use cookies. Cookies

are pieces of data that your ASP.NET application can save on the user’s browser,

to be read later by your application. Cookies aren’t lost when the browser is

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Using Cookies

closed (unless the user deletes them), so you can save data that helps identify

your user in a cookie.

In ASP.NET, a cookie is represented by the HttpCookie class. We read the user’s

cookies through the Cookies property of the Request object, and we set cookies

though the Cookies property of the Response object. Cookies expire by default

when the browser window is closed (much like session state), but their points of

expiration can be set to dates in the future; in such cases, they become persistent

cookies.

Let’s do a quick test. First, open Default.aspx and remove the text surrounding

myLabel:

File: Default.aspx (excerpt)

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

<div>

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

</div>

</form>

Then, modify Page_Load in the code-behind file as shown:

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

Protected Sub Page_Load(ByVal sender As Object, _

ByVal e As System.EventArgs) Handles Me.Load

' Declare a cookie variable

Dim userCookie As HttpCookie

' Try to retrieve user's ID by reading the UserID cookie

userCookie = Request.Cookies("UserID")

' Verify if the cookie exists

If userCookie Is Nothing Then

' Display message

myLabel.Text = "Cookie doesn't exist! Creating a cookie now."

' Create cookie

userCookie = New HttpCookie("UserID", "JoeBlack")

' Set cookie to expire in one month

userCookie.Expires = DateTime.Now.AddMonths(1)

' Save the cookie on the client

Response.Cookies.Add(userCookie)

Else

' Display message

myLabel.Text = "Welcome back, " & userCookie.Value

End If

End Sub

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Chapter 5: Building Web Applications

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

protected void Page_Load(object sender, EventArgs e)

{

// Declare a cookie variable

HttpCookie userCookie;

// Try to retrieve user's ID by reading the UserID cookie

userCookie = Request.Cookies["UserID"];

// Verify if the cookie exists

if (userCookie == null)

{

// Display message

myLabel.Text =

"Cookie doesn't exist! Creating a cookie now.";

// Create cookie

userCookie = new HttpCookie("UserID", "JoeBlack");

// Set cookie to expire in one month

userCookie.Expires = DateTime.Now.AddMonths(1);

// Save the cookie on the client

Response.Cookies.Add(userCookie);

}

else

{

// Display message

myLabel.Text = "Welcome back, " + userCookie.Value;

}

}

The first time you load the page, you’ll be notified that the cookie doesn’t exist,

and that a new cookie is being created, via a message like the one shown in Fig-

ure 5.31.

Figure 5.31. Creating a new cookie

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Using Cookies