Question

I just realized in a C# .Net 4.0 WPF background thread that this doesn’t work (compiler error):

Dispatcher.Invoke(DispatcherPriority.Normal, delegate()
{
    // do stuff to UI
});

From some examples I found out that it had to be casted like this: (Action)delegate(). However, in other examples it is casted to other classes, e.g. System.Windows.Forms.MethodInvoker.

Can anybody tell me what exactly is wrong with the example above? I also tried to reproduce it with other methods, but it was always working without casting:

delegate void MyAction();
void Method1(MyAction a) {
    // do stuff
}

void Method2(Action a) {
    // do stuff
}

void Tests()
{
    Method1(delegate()
    {
        // works
    });

    Method2(delegate()
    {
        // works
    });

    Method1(() =>
    { 
        // works
    });

    Method2(() =>
    {
        // works
    });

    Method2(new Action(delegate()
    {
        // works
    }));

    new System.Threading.Thread(delegate()
    {
        // works
    }).Start();
}

So whats the best (most elegant, less redundant) way to invoke the Dispatcher, and whats so special with it that delegates must be casted?

Answer 1

Look at the signature for Dispatcher.Invoke(). It doesn’t take Action or some other specific delegate type. It takes Delegate, which is the common ancestor of all delegate types. But you can’t convert anonymous method to this base type directly, you can convert it only to some specific delegate type. (The same applies to lambdas and method groups.)

Why does it take Delegate? Because you can pass to it delegates that take parameters or have return values.

The cleanest way is probably:

Action action = delegate()
{
    // do stuff to UI
};

Dispatcher.Invoke(DispatcherPriority.Normal, action);