Creating a delegate type inside a method

Delegates are compiled to classes(a class that inherits from System.MulticastDelegate). In C#, you are not allowed to declare a class inside a method(see C# language specification). Thus you can't declare a delegate in a method, either.

Why do you want to create the delegate type within the method? What's wrong with declaring it outside the method? Basically, you can't do this - you can't declare a type (any kind of type) within a method.

One alternative would be to declare all the Func/Action generic delegates which are present in .NET 3.5 - then you could just do:

public void MyMethod(){
    Func<int, int, int> mySumImplementation = 
        delegate (int a, int b) { return a+b; };

    Console.WriteLine(mySumImplementation(1,1).ToString());
}

The declarations are on my C#/.NET Versions page.

The delegate type has to be defined outside the function. The actual delegate can be created inside the method as you do.

class MyClass {
  delegate int Sum(int a, int b);
  public void MyMethod(){

       Sum mySumImplementation=delegate (int a, int b) {return a+b;}

       Console.WriteLine(mySumImplementation(1,1).ToString());
  }

}

would be valid. The best solution may be to emulate .NET3.5, and create some generic delegate types globally, which can be used all over your solution, to avoid having to constantly redeclare delegate types for everything:

delegate R Func<R>();
delegate R Func<T, R>(T t);
delegate R Func<T0, T1, R>(T0 t0, T1 t1);
delegate R Func<T0, T1, T2, R>(T0 t0, T1 t1, T2 t2);

Then you can just use a Func<int, int, int> delegate in your code above.