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AddHandler Statement

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Associates an event with an event handler at run time.

Syntax

AddHandler event, {AddressOf eventhandler | expression }

Parts

Part Description
event The name of the event to handle.
eventhandler The name of a procedure that handles the event.
expression A lambda expression that handles the event.

The parts AddressOf eventhandler and expression are mutually exclusive.

Remarks

The AddHandler and RemoveHandler statements allow you to start and stop event handling at any time during program execution.

The signature of the new event handler (the eventhandler procedure or the expression lambda) must match the signature of the event event.

The Handles keyword and the AddHandler statement both allow you to specify that particular procedures handle particular events, but there are differences. The AddHandler statement connects procedures to events at run time. Use the Handles keyword when defining a procedure to specify that it handles a particular event. For more information, see Handles.

A handler added with an explicit lambda CANNOT be removed later (using RemoveHandler). Indeed, if the lambda is not given a name, it is not possible to reference it later. But assigning the lambda to a variable and adding the handler through this variable allows to remove the handler using this variable.

Note

For custom events, the AddHandler statement invokes the event's AddHandler accessor. For more information on custom events, see Event Statement.

Example

Sub TestEvents()
    Dim Obj As New Class1
    ' Associate an event handler with an event.
    AddHandler Obj.Ev_Event, AddressOf EventHandler
    ' Call the method to raise the event.
    Obj.CauseSomeEvent()
    ' Stop handling events.
    RemoveHandler Obj.Ev_Event, AddressOf EventHandler
    ' This event will not be handled.
    Obj.CauseSomeEvent()
    ' Associate an event handler with an event, using a lambda.
    ' This handler cannot be removed.
    AddHandler Obj.Ev_Event, Sub ()
        MsgBox("Lambda caught event.")
    End Sub
    ' This event will be handled by the lambda above.
    Obj.CauseSomeEvent()
End Sub

Sub EventHandler()
    ' Handle the event.
    MsgBox("EventHandler caught event.")
End Sub

Public Class Class1
    ' Declare an event.
    Public Event Ev_Event()
    Sub CauseSomeEvent()
        ' Raise an event.
        RaiseEvent Ev_Event()
    End Sub
End Class

See also