Question

I’ve been using delegates for many years, and haven’t really given them much thought.
But I recently got egg on my face by assuming that delegates stored a this reference when referencing a class method.
The below example illustrates the gap in my understanding.

public class SomeClass
{
    public SomeClass(int someProperty)
    {
        SomeProperty = someProperty;
    }

    public int SomeProperty
    { 
        get; 
        set; 
    }

    // Throw in a Member field into the mix
    public int ClassAdd(int x, int y)
    {
        return x + y + SomeProperty;
    }
}

public static class SomeStaticClass
{
    public static int StaticAdd(int x, int y)
    {
        return x + y;
    }
}

Why is it that I can add both static and instance subscribers?

class TestClass
{
    delegate int myAddDelegate(int x, int y);

    private void UseDelegates()
    {
        myAddDelegate algorithm;
        algorithm = new SomeClass(3).ClassAdd;
        // Surprised that I could add static methods to my delegate?
        algorithm += SomeStaticClass.StaticAdd;

        // I'm fine with just one of the results being returned.
        int answer = algorithm(5, 10);
    }
}

What is actually going on? 😉

Answer 1

If you create a delegate referring to an instance method, it will capture this (or the relevant reference) in the field backing the Target property of the delegate. If you create a delegate referring to a static method, the Target will be null. Logically there’s no need to have an instance if you’re using a static method.

As one added complications, you can capture extension methods as if they were instance methods on the extended type:

static class Extensions
{
    public static void Foo(this string x)
    {
        Console.WriteLine("Calling Foo on " + x);
    }
}

class Test
{
    static void Main()
    {
        Action action = "text".Foo;
        Console.WriteLine(action.Target); // Prints "text"
    }
}

As for why you can do all of this: because it’s useful, and there’s no reason not to allow you to do it 🙂