Cast delegate to Func in C#

Question

I have code:

public delegate int SomeDelegate(int p);

public static int Inc(int p) {
    return p + 1;
}

I can cast Inc to SomeDelegate or Func<int, int>:

SomeDelegate a = Inc;
Func<int, int> b = Inc;

but I can't cast Inc to SomeDelegate and after that cast to Func<int, int> with usual way like this:

Func<int, int> c = (Func<int, int>)a; // Сompilation error

How I can do it?

Answer 1

Try this:

Func<int, int> c = (Func<int, int>)Delegate.CreateDelegate(typeof(Func<int, int>), 
                                                           b.Target,
                                                           b.Method);

Answer 2

The problem is that:

SomeDelegate a = Inc;

Isn't actually a cast. It's the short-form of:

SomeDelegate a = new SomeDelegate(Inc);

Therefore there's no cast. A simple solution to your problem can be this (in C# 3.0)

Func<int,int> f = i=>a(i);

Answer 3

SomeDelegate a = Inc;
Func<int, int> b = Inc;

is short for

SomeDelegate a = new SomeDelegate(Inc); // no cast here
Func<int, int> b = new Func<int, int>(Inc);

You can't cast an instance of SomeDelegate to a Func<int, int> for the same reason you can't cast a string to a Dictionary<int, int> -- they're different types.

This works:

Func<int, int> c = x => a(x);

which is syntactic sugar for

class MyLambda
{
   SomeDelegate a;
   public Foo(SomeDelegate a) { this.a = a; }
   public int Invoke(int x) { return this.a(x); }
}

Func<int, int> c = new Func<int, int>(new MyLambda(a).Invoke);

Answer 4

It is the same kind of problem as this:

public delegate int SomeDelegate1(int p);
public delegate int SomeDelegate2(int p);
...
  SomeDelegate1 a = new SomeDelegate1(Inc);
  SomeDelegate2 b = (SomeDelegate2)a;  // CS0030

which is the same kind of problem as:

public class A { int prop { get; set; } }
public class B { int prop { get; set; } }
...
  A obja = new A();
  B objb = (A)obja;  // CS0029

Objects cannot be casted from one type to an unrelated other type, even though the types are otherwise completely compatible. For lack of a better term: an object has type identity that it carries along at runtime. That identity cannot be changed after the object is created. The visible manifestation of this identity is Object.GetType().

Answer 5

There's a much simpler way to do it, which all the other answers have missed:

Func<int, int> c = a.Invoke;

See this blog post for more info.