How to add an Action to an Event

Question

I have an event in a generic class

Public class SomeClass
    Public Event ChangeEvent(oldValue As T, newValue As T)

    ' some code..
End Class

And I have a complicated method to which I pass an Action(Of T) which should get added to the event and later removed again. It would look something like the following:

Public Sub SomeSub(listener As Action(Of T, T)
    AddHandler ChangeEvent, listener
    ' some code
    RemoveHandler ChangeEvent, listener
End Sub

But on both lines the compiler gives me the following error:

Value of type 'Action(Of T, T)' cannot be converted to 'ChangeEventHandler'.

The following works, but I can't remove the handler as it was a lambda expression.

Public Sub SomeSub(listener As Action(Of T, T)
    AddHandler ChangeEvent, Sub(x, y) listener(x, y)
End Sub

Is there a solution which doesn't involve me storing the lambda as a member? Please note that I cannot change the Event. I am only in control of the Method adding the listener.

Answer 1

It is better if you explicitly declare your event handler delegate, rather than letting Visual Basic do it implicitly:

Public Class SomeClass(Of T)
    Public Delegate Sub ChangeEventHandler(oldValue As T, newValue As T)
    Public Event ChangeEvent As ChangeEventHandler

    ' some code..
End Class

That way you can specify the delegate as a parameter of your method, thus giving the compiler the correct signature and you the ability to both add and remove it:

Public Sub SomeSub(listener As ChangeEventHandler)
    AddHandler ChangeEvent, listener
    ' some code...
    RemoveHandler ChangeEvent, listener
End Sub

Now you and the compiler are both happy! :)