Difficulty with preserving methods raising events with delegates after conversion from C#

Question

So for the last day or two I've been studying this article in order to get some exposure to dependency properties and routed commands, and hopefully leverage some of the sample code to solve a content scaling problem in another project. That project happens to be written in vb.net, and this sample code is in C#.

Okay, no problem. Most of the tutorials and demo projects that I have seen use C#, and I have found that reading the code and writing its equivalent in vb.net is a really good way to understand what's actually happening and to get more comfortable using either. It's time consuming, but worthwhile at my experience level (#00FF00)

It wasn't long before I encountered problems with events with callback methods. Consider this method:

public static class AnimationHelper
{

...

public static void StartAnimation(UIElement animatableElement,
                                  DependencyProperty dependencyProperty,
                                  double toValue,
                                  double animationDurationSeconds,
                                  EventHandler completedEvent)
{
    double fromValue = (double)animatableElement.GetValue(dependencyProperty);

    DoubleAnimation animation = new DoubleAnimation();
    animation.From = fromValue;
    animation.To = toValue;
    animation.Duration = TimeSpan.FromSeconds(animationDurationSeconds);

    animation.Completed += delegate(object sender, EventArgs e)
    {
        //
        // When the animation has completed bake final value of the animation
        // into the property.
        //
        animatableElement.SetValue(dependencyProperty, animatableElement.GetValue(dependencyProperty));
        CancelAnimation(animatableElement, dependencyProperty);

        if (completedEvent != null)
        {
            completedEvent(sender, e);
        }
    };

    animation.Freeze();

    animatableElement.BeginAnimation(dependencyProperty, animation);
}

Transcribing this method to vb.net is straightforward except for the way to properly handle DoubleAnimation's Completed event and the callback method. My best attempt looks like:

Public NotInheritable Class AnimationHelper

...

Public Shared Sub StartAnimation(...)

...

animation.Completed += Function(sender As Object, e As EventArgs)

            animatableElement.SetValue(dependencyProperty, animatableElement.GetValue(dependencyProperty))
            CancelAnimation(animatableElement, dependencyProperty)
            RaiseEvent completedEvent(sender, e)
        End Function
...

End Sub

This results in two complaints:

'completedEvent' is not an event of [namespace].AnimationHelper
'Public Event Completed(...)' is an event and cannot be called directly. Use RaiseEvent...

(1) is a bit of a mystery to me because completedEvent (As EventHandler) is one of the parameters in the method declaration. Removing RaiseEvent from the head of the line and calling it like an ordinary method appears to satisfy visual studio, but I don't whether that will work at runtime or if it's valid to do so at all. In (2) the syntax looks dubious to me, adding RaiseEvent to the head of the line causes similar complaints similar to (1).

I'm going to continue scouring stack and the greater internet for good primers on delegates and events in vb.net since it's obviously time I stopped avoiding learning how they work. In the meantime, advice/suggestions are most welcome.

Daniel Hilgarth Daniel Hilgarth · Accepted Answer · 2012-08-23T20:05:25

Why do you try raising that event handler? An EventHandler is just a delegate. Simply execute it as in C#:
```
if completedEvent IsNot Nothing then
    completedEvent(sender, e)
end if
```

Difficulty with preserving methods raising events with delegates after conversion from C#

2 Answers