How to: Create Event Handlers in ASP.NET Web Pages

Article
10/22/2014

When you are editing an ASP.NET Web page in Visual Studio, you can create server event handlers for controls and for the page in a variety of ways. Some of the ways depend on which programming language you are working in. You can create handlers by using the facilities in Visual Web Developer, or you can create them declaratively or in code.

Note

You can also add events for client script, which execute when the control is running in the browser as an element in the page. For details, see How to: Add Client Script Events to ASP.NET Web Server Controls.

If a control supports more than one event, it is usually configured so that one event is the default. For example, for the Button control, the default event is the Click event.

Creating Event Handlers in Visual Web Developer

To create an event handler for the default event

In Design view, double-click the page or double-click the control for which you want to create a default event handler.

Visual Web Developer creates a handler for the default event and opens the code editor with the insertion point in the event handler.

To create an event handler in the Properties window

In Design view, select the control for which you want to create an event handler.

Note

You cannot select the page in Design view, so you cannot create a page-level event using this procedure. Use the previous procedure instead.
In Properties, click the events symbol .

The Properties window displays a list of events for the selected control.
In the box next to an event name, do one of the following:
- Double-click to create a new event handler for that event. The designer will name the handler using the convention controlID_event.
- Type the name of the handler to create.
- In the drop-down list, select the name of an existing handler.
  
  The drop-down list displays a list of methods that have the correct signature for the event.
When you are finished, Visual Web Developer switches to the code editor and puts the insertion point in the handler.

To create an event handler in the code editor

Switch to the code editor by using one of the following techniques:
- For a single-file Web page, switch to Source view.
- For a code-behind page, in Solution Explorer, right-click the name of the page, and then click View Code.
  
  Note
  
  In code-behind pages that use C#, this technique will not work because the control name will not appear in the drop-down list in the last step. Use one of the other procedures instead.
At the top of the code editor window, select the control from the drop-down list located on the left side, and then select the event from the drop-down list located on the right side.

Note

Events for which methods already exist are displayed in bold text.

Creating Event Handlers Declaratively

When you use Visual Web Developer to create a handler, Visual Web Developer creates the skeleton handler and configures the control to call the handler. You can do the same yourself without using Visual Web Developer.

To create a handler for page events

In the code editor, create a method with the name Page_event.

For example, to create a handler for the page's Load event, create a method named Page_Load.

Note

Page event handlers are not required to take parameters the way that other controls event handlers are.

ASP.NET pages automatically bind page events to methods that have the name Page_event. This automatic binding is configured by the AutoEventWireup attribute in the @ Page directive, which is set to true by default. If you set AutoEventWireup to false, the page does not automatically search for methods that use the Page_event naming convention.

The following code example shows a handler for the page's Load event.
```
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If Page.IsPostBack Then
        Response.Write("<br>Page has been posted back.")
    End If
End Sub
```
```
protected void Page_Load(object sender, EventArgs e)
{
    if (IsPostBack)
    {
        Response.Write("<br>Page has been posted back.");
    }
}
```

To add a handler declaratively for a control

In the code editor, create an event-handling method with the appropriate signature.

The method can be named anything you like.

In the markup of the control, use the attributes that are named On<event name> to hook up the event handler. Use the event handler method name as the value.

The following code example shows the markup for a TextBox control for which the TextChanged event is bound to the event handler method named NameChange:

Security Note:
This example contains a text box that accepts user input, which is a potential security threat. By default, ASP.NET Web pages validate that user input does not include script or HTML elements. For more information, see Script Exploits Overview.

Protected Sub NameChange(ByVal sender As Object, ByVal e As System.EventArgs) Handles textbox1.TextChanged
    'Sub for the OnTextChanged event
End Sub

<asp:TextBox ID="textbox1" Runat="server" 
Text="" 
OnTextChanged="NameChange" />

Add the event method to your code. For example, the following code example shows the code for the event handler method named NameChange:
```
protected void NameChange(object sender, EventArgs e)
{
    //Method for the OnTextChanged event.
}
```

To add an even handler implicitly in Visual Basic

In the code editor, create an event-handling method with the appropriate signature.

The method can be named anything you like.
Use the Handles keyword, specifying the control and event to which the method should bind.

The following code example shows an event handler for the Click event of a button named SampleButton. The method in the example is named Clicked.
```
Protected Sub SampleButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) _
        Handles SampleButton.Click
    ' Code goes here.
End Sub
```
Note

If you use the Handles keyword, do not include an event attribute in the control markup. If you do, the handler will be called twice.
If the method handles multiple events, add the names of the additional events to the Handles clause, separated by a comma.

The following code example shows a method that handles the Click event for several buttons. In the handler, the code tests the sender argument to determine which button was clicked.
```
Protected Sub AnyClicked(ByVal sender As Object, ByVal e As System.EventArgs) _
        Handles Button1.Click, Button2.Click, Button3.Click
    Dim b As Button = CType(sender, Button)
    Response.Write("You clicked the button labeled " & b.ID)
End Sub
```

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How to: Create Event Handlers in ASP.NET Web Pages

Creating Event Handlers in Visual Web Developer

To create an event handler for the default event

To create an event handler in the Properties window

To create an event handler in the code editor

Creating Event Handlers Declaratively

To create a handler for page events

To add a handler declaratively for a control

To add an even handler implicitly in Visual Basic

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