This article provides an introduction to Custom Events, a new language feature in VB .NET 2005. Custom Events allow the user to specify what to do when AddHandler, RemoveHandler, and RaiseEvent are called for an event.
Visual Basic 2005 has a number of new language features not present in previous versions of VB.NET. One of these is the ability to create custom events such that the programmer can specify what is done when AddHandler, RemoveHandler and RaiseEvent are called.
Below is an example of VB Code which uses a custom event to log changes to event handling. Of course, this is just one example of how a custom event could be used. The methods could be used for basically any purpose.
Delegate Sub MyDelegate(ByVal message As String) Class MyClass1 Custom Event MyEvent As MyDelegate ' This code will be run when AddHandler MyEvent, D1 ' is called AddHandler(ByVal value As MyDelegate) Console.WriteLine("Adding Handler for MyEvent") MyEventHandler = value End AddHandler ' This code will be run when RemoveHandler MyEvent, D1 ' is called RemoveHandler(ByVal value As MyDelegate) Console.WriteLine("Removing Handler for MyEvent") MyEventHandler = Nothing End RemoveHandler ' This code will be run when RaiseEvent MyEvent(string) ' is called RaiseEvent(ByVal message As String) If Not MyEventHandler Is Nothing Then MyEventHandler.Invoke(message) Else Console.WriteLine("No Handler for Raised MyEvent") End If End RaiseEvent End Event Public MyEventHandler As MyDelegate Public Sub Raise_Event() RaiseEvent MyEvent("MyEvent Was Raised") End Sub End Class Module DelegateModule Dim Var1 As MyClass1 Dim D1 As MyDelegate Sub Main() Var1 = New MyClass1 D1 = New MyDelegate(AddressOf MyHandler) AddHandler Var1.MyEvent, D1 Var1.Raise_Event() RemoveHandler Var1.MyEvent, D1 End Sub Sub MyHandler(ByVal message As String) Console.WriteLine("Event Handled: " & message) End Sub End Module