Why are all Delegate types incompatible with each other?

Question

In C# all delegate types are incompatible with one another, even if they have the same signature. As an example:

delegate void D1();
delegate void D2();

D1 d1 = MethodGroup;
D2 d2 = d1;                           // compile time error
D2 d2 = new D2 (d1);                  // you need to do this instead

What is the reasoning behind this behaviour and language design decision.

Answer 1

In C# all delegate types are incompatible with one another, even if they have the same signature. What is the reasoning behind this behaviour and language design decision?

First off, I think that it is fair to say that many of the runtime and language designers regret this decision. Structural typing on delegates -- that is, matching by signature -- is a frequently requested feature and it just seems strange that Func<int, bool> and Predicate<int> can't be freely assigned to each other.

The reasoning behind the decision as I understand it -- and I hasten to add that this decision was made about six years before I started on the C# team -- is that the expectation was that there would be delegate types with semantics. You want this to be a type error:

AnyFunction<int, int> af = x=> { Console.WriteLine(x); return x + y; };
PureFunction<int, int> pf = af;

A "pure" function is a function which produces and consumes no side effects, consumes no information outside of its arguments, and returns a consistent value when given the same arguments. Clearly af fails at least two of those, and so should not be assignable to pf as an implicit conversion.

But semantics-laden delegate types never happened, so it's a bit of a misfeature now.

Answer 2

Basically because the compiler makes two classes for you. The same reason you can't do:

class A {}
class B {}

void Main()
{
    A a = new A();
    B b = a;
}

---

For example, the following code

void Main() {}

delegate void D();
class C {}

The IL code is:

D.Invoke:

D.BeginInvoke:

D.EndInvoke:

D..ctor:

C..ctor:
IL_0000:  ldarg.0     
IL_0001:  call        System.Object..ctor
IL_0006:  ret