Is there a Delegate which isn't a MulticastDel

问题:

I think the answer is NO? If there isn't, why do we have separated Delegate and MulticastDelegate classes? Maybe it's again because of "some other .NET languages"?

回答1:

EDIT: I thought this was part of ECMA 335, but I can't see it in there anywhere.

You can't create such a delegate type in C#, but you can in IL:

.class public auto ansi sealed Foo
       extends [mscorlib]System.Delegate
{
    // Body as normal
}

The C# compiler has no problems using such a delegate:

using System;

class Test
{
    static void Main()
    {
        Foo f = x => Console.WriteLine(x);
        f("hello");
    }
}

But the CLR does when it tries to load it:

Unhandled Exception: System.TypeLoadException: Could not load type 'Foo' from assembly 'Foo, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' because it cannot inherit directly from the delegate class. at Test.Main()

Basically the Delegate/MulticastDelegate separation is an historical accident. I believe that early alpha/beta versions did make the distinction, but it proved too confusing and generally not useful - so now every delegate derives from MulticastDelegate.

(Interestingly, the C# specification only mentions MulticastDelegate once, in the list of types which can't be used as generic constraints.)

回答2:

No, there isn't, because all delegates must naturally be able to be Delegate.Combineed. Delegate is there simply to wrap the non-multicasting functionality into a base class.

回答3:

System.MuticastDelegate is derived from System.Delegate. Each level within the delegate hierarchy provides a different set of services. System.Delegate is a container of the data for what method to call on a particular object. With System.MulticastDelegate comes the additional capability of not only invoking a method on a single object, but on a collections of objects. This enables multiple subscribers to an event.

Not sure, i have answered your question.

回答4:

No, the CLR does not allow that.

I recall something that they wanted to expose Delegate directly, but that was never needed.