How are events implemented

Question

I\'m asking specifically about VB.NET, but I imagine the general principles are the same in other languages. I thought an event was a first-class concept in .NET, but it seems

Answer 1

An event is just a delegate. Here's some code to play with that works just like a regular event, using a delegate object instead:

Module Module1
    Sub Main()
        Dim obj As New Example
        obj.AnEvent = New EventHandler(AddressOf Handler)
        obj.Test()
        Console.ReadLine()
    End Sub

    Sub Handler(ByVal sender As Object, ByVal e As EventArgs)
        Console.WriteLine("got event")
    End Sub
End Module

Class Example
    Public AnEvent As EventHandler
    Public Sub Test()
        If AnEvent IsNot Nothing Then AnEvent(Me, EventArgs.Empty)
    End Sub
End Class

But do note the problem with this code. Some other code could mess with AnEvent as well. Like replacing it or setting it back to Nothing. That's disastrous in most any case, the code that subscribed the event first will stop working properly.

The Event keyword in VB.NET prevents this from happening. It wraps the delegate object and makes it inaccessible to other code, beyond the provided keywords. Somewhat similar to how a Property protects access to a field. AddHandler and RemoveHandler ensure that existing registrations cannot disappear. RaiseEvent fires the event without the need for the Nothing check.