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

c# delegates multicastdelegate
samj28 picture samj28 · Sep 21, 2012 · Viewed 9.4k times · Source

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 ?

Answer

Marc Gravell picture Marc Gravell · Sep 21, 2012

The premise is wrong; it works fine:

Func<int> func = delegate { Console.WriteLine("first part"); return 5; };
func += delegate { Console.WriteLine("second part"); return 7; };
int result = func();

That is a multicast delegate with a non-void result, working fine. You can see from the console that both parts executed. The result of the last item is the one returned. We can demonstrate that this is a true multicast delegate:

if(func is MulticastDelegate) Console.WriteLine("I'm multicast");

and it will write "I'm multicast" even after just the first line (when there is only a single method listed).

If you need more control over individual results, then use GetInvocationList():

foreach (Func<int> part in func.GetInvocationList())
{
    int result = part();
}

which allows you to see each individual result.

In IL terminology:

.class public auto ansi sealed Func<+ TResult>
    extends System.MulticastDelegate`

which is to say: Func<T> inherits from MulticastDelegate. Basically, to all intents and purposes, all delegates in .NET are multicast delegates. You might be able to get a non-multicast delegate in managed C++, I don't know. But certainly not from C#.

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