In ASP.NET 1.1, a code-behind file for an ASP.NET page defines a class that derives from the Page class. The .aspx page in turn represents another class that derives from the code-behind class. The code-behind class is a complete class definition; it contains instance variables for all controls on the page, explicit event binding using delegates, and so on. ASP.NET also supported a code-behind model that was used primarily in designer-based tools such as Visual Studio 2005.

In the code-behind model for ASP.NET 1.1, the markup for the page is maintained in an .aspx page, and the code is maintained in a separate class file. The link between the .aspx page and its corresponding class file is made the @ Page directive. A typical directive looks like the following:

<%@ Page Language="vb" AutoEventWireup="false" Codebehind="SamplePage.aspx.vb" Inherits="SampleProject.SamplePage"%>

<%@ Page Language="c#" AutoEventWireup="false" Codebehind="SamplePage.aspx.cs" Inherits="SampleProject.SamplePage"%>

ASP.NET version 1.1 also supports a variant form of the code-behind model in which the Codebehind attribute of the @ Page directive was replaced with a Src attribute.

Code-Behind Model for ASP.NET version 2.0

The code-behind model for ASP.NET version 2.0 takes advantage of a new language feature known as partial classes. The code-behind file for a page is not a complete class definition. Instead, it includes only the application code you need, such as event handlers. The code-behind partial class does not need to include instance variables or explicit event binding. ASP.NET can infer the control instances and derive event bindings from the markup during compilation.

Note
ASP.NET pages that use Visual Basic as their programming language can use the Handles keyword on methods to explicitly bind the method to an event; doing so can result in slightly faster performance.

A code-behind file might look like the following:

Visual Basic

Partial Class SamplePage
        Inherits System.Web.UI.Page
    Protected Sub Button1_Click(ByVal sender As Object, _
            ByVal e As EventArgs) Handles Button1.Click
        Label1.Text = "Clicked at " & DateTime.Now.ToString()
    End Sub
End Class

C#

using System;
public partial class SamplePage : System.Web.UI.Page
{
    protected void Button1_Click(Object sender, EventArgs e)
    {
        Label1.Text = "Clicked at " + DateTime.Now.ToString();
    }
}

The linkage between the .aspx page and the code-behind page is similar to the linkage used with the earlier code-behind model. However, the @ Page directive uses a new CodeFile attribute instead of the Codebehind or Src attribute. In addition, the directive also includes an Inherits attribute to specify a class name for the page, as in the following example:

Visual Basic

<%@ Page language="VB" CodeFile="SamplePage.aspx.vb" 
  Inherits="SamplePage" AutoEventWireup="false" %>

C#

<%@ Page language="C#" CodeFile="SamplePage.aspx.cs" 
  Inherits="SamplePage" AutoEventWireup="true" %>

The new code-behind model uses a different inheritance model than the code-behind model from previous versions. For details, see ASP.NET Web Page Code Model.

The code-behind model introduced in ASP.NET version 2.0 offers the following improvements over the previous version:

• The code-behind file is simpler. It includes only the code you write yourself.

• You can include controls on a page without having to explicitly create instance variables for them in the code-behind class. The code-behind page cannot be out of synch with the controls declared in the markup.

• Because event binding can be inferred from the declarative controls, you do not need to explicitly bind delegates in the reserved InitializeComponent method.

  Note
  If you are creating controls and adding them to the page programmatically, you must still explicitly create event bindings. The code-behind model creates control instances and event bindings only for controls that are declared in markup.

Better Separation of Code and Content

Concepts