Anonymous method in Invoke call

Question

Having a bit of trouble with the syntax where we want to call a delegate anonymously within a Control.Invoke.

We have tried a number of different approaches, all to no avail.

For example:

myControl.Invoke(delegate() { MyMethod(this, new MyEventArgs(someParameter)); });

where someParameter is local to this method

The above will result in a compiler-error of

"Cannot convert anonymous method to type 'System.Delegate' because it is not a delegate type"

Any help is appreciated!

Answer 1

myControl.Invoke(new MethodInvoker(delegate() {...}))

Answer 2

You need to create a delegate type. The keyword 'delegate' in the anonymous method creation is a bit misleading. You are not creating an anonymous delegate but an anonymous method. The method you created can be used in a delegate. Like this:
myControl.Invoke(new MethodInvoker(delegate() { (MyMethod(this, new MyEventArgs(someParameter)); }));

Answer 3

Because Invoke/BeginInvoke accepts Delegate (rather than a typed delegate), you need to tell the compiler what type of delegate to create ; MethodInvoker (2.0) or Action (3.5) are common choices (note they have the same signature); like so:

control.Invoke((MethodInvoker) delegate {this.Text = "Hi";});

If you need to pass in parameters, then "captured variables" are the way:

string message = "Hi";
control.Invoke((MethodInvoker) delegate {this.Text = message;});

(caveat: you need to be a bit cautious if using captures async, but sync is fine - i.e. the above is fine)

Another option is to write an extension method:

public static void Invoke(this Control control, Action action)
{
    control.Invoke((Delegate)action);
}

then:

this.Invoke(delegate { this.Text = "hi"; });
// or simce we are using C# 3.0
this.Invoke(() => { this.Text = "hi"; });

You can of course do the same with BeginInvoke:

public static void BeginInvoke(this Control control, Action action)
{
    control.BeginInvoke((Delegate)action);
}

If you can't use C# 3.0, you could do the same with a regular instance method, presumably in a Form base-class.

Answer 4

Actually you do not need to use delegate keyword. Just pass lambda as parameter: control.Invoke((MethodInvoker)(()=> {this.Text = "Hi";}));

Answer 5

I had problems with the other suggestions because I want to sometimes return values from my methods. If you try to use MethodInvoker with return values it doesn't seem to like it. So the solution I use is like this (very happy to hear a way to make this more succinct - I'm using c#.net 2.0):

    // Create delegates for the different return types needed.
    private delegate void VoidDelegate();
    private delegate Boolean ReturnBooleanDelegate();
    private delegate Hashtable ReturnHashtableDelegate();

    // Now use the delegates and the delegate() keyword to create 
    // an anonymous method as required

    // Here a case where there's no value returned:
    public void SetTitle(string title)
    {
        myWindow.Invoke(new VoidDelegate(delegate()
        {
            myWindow.Text = title;
        }));
    }

    // Here's an example of a value being returned
    public Hashtable CurrentlyLoadedDocs()
    {
        return (Hashtable)myWindow.Invoke(new ReturnHashtableDelegate(delegate()
        {
            return myWindow.CurrentlyLoadedDocs;
        }));
    }