EventHandler Delegate
Represents the method that will handle an event that has no event data.
Assembly: mscorlib (in mscorlib.dll)
<SerializableAttribute> <ComVisibleAttribute(True)> Public Delegate Sub EventHandler ( sender As Object, e As EventArgs )
Parameters
- sender
-
Type:
System.Object
The source of the event.
- e
-
Type:
System.EventArgs
An object that contains no event data.
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 identifies the method that provides the response to the event.
Optionally, a class that holds the event data, if the event provides 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. This method's first parameter is of type Object and refers to the instance that raises the event. Its 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 the value of the EventArgs.Empty field. Otherwise, the second parameter is a type derived from EventArgs and supplies any fields or properties needed to hold the event data.
The EventHandler delegate is a predefined delegate that specifically represents an event handler method for an event that does not generate data. If your event does generate data, you must use the generic EventHandler(Of TEventArgs) delegate class.
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 Handling and Raising Events.
The following example shows an event named ThresholdReached that is associated with an EventHandler delegate. The method assigned to the EventHandler delegate is called in the OnThresholdReached method.
Module Module1 Sub Main() Dim c As Counter = New Counter(New Random().Next(10)) AddHandler c.ThresholdReached, AddressOf c_ThresholdReached Console.WriteLine("press 'a' key to increase total") While Console.ReadKey(True).KeyChar = "a" Console.WriteLine("adding one") c.Add(1) End While End Sub Sub c_ThresholdReached(sender As Object, e As ThresholdReachedEventArgs) Console.WriteLine("The threshold of {0} was reached at {1}.", e.Threshold, e.TimeReached) Environment.Exit(0) End Sub End Module Class Counter Private threshold As Integer Private total As Integer Public Sub New(passedThreshold As Integer) threshold = passedThreshold End Sub Public Sub Add(x As Integer) total = total + x If (total >= threshold) Then Dim args As ThresholdReachedEventArgs = New ThresholdReachedEventArgs() args.Threshold = threshold args.TimeReached = DateTime.Now OnThresholdReached(args) End If End Sub Protected Overridable Sub OnThresholdReached(e As ThresholdReachedEventArgs) RaiseEvent ThresholdReached(Me, e) End Sub Public Event ThresholdReached As EventHandler(Of ThresholdReachedEventArgs) End Class Class ThresholdReachedEventArgs Inherits EventArgs Public Property Threshold As Integer Public Property TimeReached As DateTime End Class
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1