events - Equivalent of VB's custom RaiseEvent blocks in C#?

Question

(I know the title sounds easy, but hold on—this probably isn't the question you think it is.)

In VB.NET I was able to write custom events. For an example, I had a separate thread that would periodically raise an event and on that event the GUI would need to be updated. I didn't want the busy thread to bother with UI calculations and I didn't want to put Me.Invoke(Sub() ...) in the event handler since it was also called from the GUI thread.

I came up with this very useful bit of code. The GUI thread would set EventSyncInvoke = Me (the main form). The thread could then simply raise the event TestEvent as usual, no special code, and it would be seamlessly executed on the GUI thread:

Private TestEventDelegate As EventHandler
Public EventSyncInvoke As System.ComponentModel.ISynchronizeInvoke

Public Custom Event TestEvent As EventHandler
    AddHandler(value As EventHandler)
        TestEventDelegate = [Delegate].Combine(TestEventDelegate, value)
    End AddHandler

    RemoveHandler(value As EventHandler)
        TestEventDelegate = [Delegate].Remove(TestEventDelegate, value)
    End RemoveHandler

    RaiseEvent(sender As Object, e As System.EventArgs)
        If EventSyncInvoke IsNot Nothing Then
            EventSyncInvoke.Invoke(TestEventDelegate, {sender, e})
        Else
            TestEventDelegate.Invoke({sender, e})
        End If
    End RaiseEvent
End Event

Now in C# I can do this much:

public event EventHandler TestEvent
    add
    {
        testEventDelegate = (EventHandler)Delegate.Combine(testEventDelegate, value);
    }
    remove

    {
        testEventDelegate = (EventHandler)Delegate.Remove(testEventDelegate, value);
    }


}

But where is the ability to do custom raising?

Answer 1

The other answers told me the fact that I couldn't do this directly in C#, but not the rationale behind why I can't and why I wouldn't want to. It took me a while to understand how C# events worked in comparison to VB.NET. So this explanation is for others who don't have a good grasp on this to start thinking along the right lines.

Honestly, I was so used to the boilerplate OnTestEvent format that I didn't quite like the idea of making it different from the rest of the helper methods. :-) But now that I understand the rationale, I see that it is actually the best place to put this stuff.

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VB.NET allows you to hide the background details of calling the delegates with the RaiseEvent keyword. RaiseEvent calls either the event delegate or your custom RaiseEvent section for a custom event.

In C#, there is no RaiseEvent. Raising an event is basically no more than calling a delegate. No custom RaiseEvent sections can be seamlessly called when all you're doing to raise it is calling a delegate. So for C#, custom events are like skeletons, implementing add and remove for events but not implementing the ability to raise them. It's like having to replace all your RaiseEvent TestEvent(sender, e) with the code from the custom RaiseEvent section.

For a normal event, raising looks roughly like NormalEvent(sender, e). But as soon as you put in a custom add and remove, you must use whatever variable you used in the add and remove because the compiler isn't doing it anymore. It's like automatic properties in VB.NET: once you put in a getter and setter manually, you have to declare and handle your own local variable. So instead of TestEvent(sender, e), use testEventDelegate(sender, e). That's where you rerouted the event delegates.

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I compared moving from VB.NET to C# with having to replace each of your RaiseEvents with your custom RaiseEvent code. A RaiseEvent code section is basically an event and a helper function rolled together. It's actually standard to only have one instance of a RaiseEvent in either VB.NET or C# inside a protected OnTestEvent method and call that method to raise the event. This allows any code with access to the protected (or private or public) OnTestEvent to raise the event. For what you want to do, just putting it in the method is easier, simpler and performs slightly better. This is best practice.

Now if you really want to want (or need) somehow to mimic VB.NET's RaiseEvent nitty-gritty-hiding call SomeDelegate(sender, e) and have the magic happen, you can simply hide the nitty-gritty inside a second delegate:

NiceTestEvent = (sender, e) => eventSyncInvoke.Invoke(testEventDelegate, new object[] { sender, e });

Now you can call NiceTestEvent(sender, e). You won't be able to call TestEvent(sender, e) though. TestEvent is only for outside code to add and remove through, as Visual Studio will tell you.