EventHandler(Of TEventArgs) Delegate
Represents the method that will handle an event.
Assembly: mscorlib (in mscorlib.dll)
'Declaration Public Delegate Sub EventHandler(Of TEventArgs As EventArgs) ( _ sender As Object, _ e As TEventArgs _ )
The type of the event data generated by the event.
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.
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 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 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
The following code example declares event data and a generic delegate that uses the event data, and shows how the event is raised.
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! '
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.