EventHandler<TEventArgs> Delegate
Microsoft Silverlight will reach end of support after October 2021. Learn more.
Represents the method that will handle an event.
Namespace: System
Assembly: mscorlib (in mscorlib.dll)
Syntax
'Declaration
Public Delegate Sub EventHandler(Of TEventArgs As EventArgs) ( _
sender As Object, _
e As TEventArgs _
)
public delegate void EventHandler<TEventArgs>(
Object sender,
TEventArgs e
)
where TEventArgs : EventArgs
Type Parameters
- TEventArgs
The type of the event data generated by the event.
Parameters
- sender
Type: System.Object
The source of the event.
- e
Type: TEventArgs
An EventArgs that contains the event data.
Remarks
The event model in the .NET Framework is based on having an event delegate that connects an event with its handler. To raise an event, two elements are needed:
A delegate that refers to a method that provides the response to the event.
A class that holds the event data.
The delegate is a type that defines a signature, that is, the return value type and parameter list types for a method. You can use the delegate type to declare a variable that can refer to any method with the same signature as the delegate.
The standard signature of an event handler delegate defines a method that does not return a value, whose first parameter is of type Object and refers to the instance that raises the event, and whose second parameter is derived from type EventArgs and holds the event data. If the event does not generate event data, the second parameter is simply an instance of EventArgs. Otherwise, the second parameter is a custom type derived from EventArgs and supplies any fields or properties needed to hold the event data.
EventHandler<TEventArgs> is a predefined delegate that represents an event handler method for an event, regardless of whether the event generates event data. If your event does not generate event data, substitute EventArgs for the generic type parameter; otherwise, supply your own custom event data type and substitute it for the generic type parameter.
The advantage of using EventHandler<TEventArgs> is that you do not need to code your own custom delegate if your event generates event data. Additionally, the .NET Framework needs only one implementation to support EventHandler<TEventArgs> regardless of the event data type you substitute for the generic type parameter.
To associate the event with the method that will handle the event, add an instance of the delegate to the event. The event handler is called whenever the event occurs, unless you remove the delegate.
For more information about event handler delegates, see Events Overview for Silverlight
Examples
The following code example declares event data and a generic EventHandler<TEventArgs> delegate that uses the event data, and shows how the event is raised.
.gif "Note") Note: |
|---|
To run this example, see Building Examples That Use a Demo Method and a TextBlock Control. |
' This example demonstrates the EventHandler(Of T) delegate.
Public Class MyEventArgs
Inherits EventArgs
Private msg As String
Public Sub New(ByVal messageData As String)
msg = messageData
End Sub
Public Property Message() As String
Get
Return msg
End Get
Set(ByVal value As String)
msg = value
End Set
End Property
End Class
Public Class HasEvent
' Declare an event of delegate type EventHandler of
' MyEventArgs.
Public Event SampleEvent As EventHandler(Of MyEventArgs)
Public Sub DemoEvent(ByVal val As String)
RaiseEvent SampleEvent(Me, New MyEventArgs(val))
End Sub
End Class
Public Class Example
Private Shared outputBlock As System.Windows.Controls.TextBlock
Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
Example.outputBlock = outputBlock
Dim he As New HasEvent()
AddHandler he.SampleEvent, AddressOf ExampleEventHandler
he.DemoEvent("Hey there, Bruce!")
he.DemoEvent("How are you today?")
he.DemoEvent("I'm pretty good.")
he.DemoEvent("Thanks for asking!")
End Sub
Private Shared Sub ExampleEventHandler(ByVal src As Object, _
ByVal mea As MyEventArgs)
outputBlock.Text &= mea.Message & vbCrLf
End Sub
End Class
'---------------------------------------------------------
'
'This example produces the following results:
'
'Hey there, Bruce!
'How are you today?
'I'm pretty good.
'Thanks for asking!
'
// This example demonstrates the EventHandler<T> delegate.
using System;
using System.Collections.Generic;
//---------------------------------------------------------
public class MyEventArgs : EventArgs
{
private string msg;
public MyEventArgs(string messageData)
{
msg = messageData;
}
public string Message
{
get { return msg; }
set { msg = value; }
}
}
//---------------------------------------------------------
public class HasEvent
{
// Declare an event of delegate type EventHandler of
// MyEventArgs.
public event EventHandler<MyEventArgs> SampleEvent;
public void DemoEvent(string val)
{
// Copy to a temporary variable to be thread-safe.
EventHandler<MyEventArgs> temp = SampleEvent;
if (temp != null)
temp(this, new MyEventArgs(val));
}
}
//---------------------------------------------------------
public class Example
{
private static System.Windows.Controls.TextBlock outputBlock;
public static void Demo(System.Windows.Controls.TextBlock outputBlock)
{
Example.outputBlock = outputBlock;
HasEvent he = new HasEvent();
he.SampleEvent +=
new EventHandler<MyEventArgs>(ExampleEventHandler);
he.DemoEvent("Hey there, Bruce!");
he.DemoEvent("How are you today?");
he.DemoEvent("I'm pretty good.");
he.DemoEvent("Thanks for asking!");
}
private static void ExampleEventHandler(object src, MyEventArgs mea)
{
outputBlock.Text += mea.Message + "\n";
}
}
//---------------------------------------------------------
/*
This example produces the following results:
Hey there, Bruce!
How are you today?
I'm pretty good.
Thanks for asking!
*/
Version Information
Silverlight
Supported in: 5, 4, 3
Silverlight for Windows Phone
Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0
XNA Framework
Supported in: Xbox 360, Windows Phone OS 7.0
Platforms
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.
See Also