Multicast Delegates must have a return type of void. Why ?

Question

Multicast Delegates must have a return type of void Otherwise it will throw an exception.

I want to know whats the reason behind it, what if multiple methods could have a same return type as of a delegate ?

Vulpes · Answer

As I've just done a delegate example in another thread I thought I'd adapt it so that the delegate returns a value to illustrate the point I made in my previous post. First we define a delegate type MyDelegate which takes an int parameter and returns an int result. This means that the delegate is compatible with both the Treble and Square methods. We then create a variable 'del' of the delegate type to which we can assign compatible methods. Initially we just call the Treble method with an argument of 10 by invoking the delegate. However we then add the Square method to the delegate (making it 'multicast') and invoke both methods with an argument of 20: using System; delegate int MyDelegate(int number); class Test { static void Main() { MyDelegate del = Treble; int result = del(10); Console.WriteLine(result); // add another method to 'del's invocation list del += Square; result = del(20); Console.WriteLine(result); Console.ReadKey(); } static int Treble(int number) { return number * 3; } static int Square(int number) { return number * number; } } The output should be: 30 400 So on the second invocation of the delegate (with two methods attached) the result of calling the Treble method (3 x 20 = 60) is discarded and only the result of calling the Square method (20 x 20 = 400) is returned to the caller. Notice that there is no compiler error/warning or runtime exception to alert you that this will happen.

Sameer Jha · Answer

thanks for your reply.... it has helped....

Vulpes · Answer

Thanks Shivanand. As long as they get over the point you're trying to make I've always felt that simple examples are best :)

Shivanand Arur · Answer

Good post Vulpes.... The example you have given makes it very easy to understand... Keep up the good work.

Vulpes · Answer

It's not actually necessary for multicast delegates to have a void return type. However the problem is that if they return a value there's no way to access the values returned by all of the methods in the delegate's invocation list. What happens is that only the value returned by the last method invoked is returned to the caller of the delegate. All the other values are discarded. Consequently it's usually best to avoid multicast delegates which don't have a void return type unless of course you're not interested in the values returned.