Can I have an Action<> or Func<> with an out param?

Question

I have a method with an out parameter, and I'd like to point an Action or Func (or other kind of delegate) at it.

This works fine:

static void Func(int a, int b) { }
Action<int,int> action = Func;

However this doesn't

static void OutFunc(out int a, out int b) { a = b = 0; }
Action<out int, out int> action = OutFunc; // loads of compile errors

This is probably a duplicate, but searching for 'out parameter' isn't particularly fruitful.

Answer 1

Action and Func specifically do not take out or ref parameters. However, they are just delegates.

You can make a custom delegate type that does take an out parameter, and use it, though.

For example, the following works:

class Program
{
    static void OutFunc(out int a, out int b) { a = b = 0; }

    public delegate void OutAction<T1,T2>(out T1 a, out T2 b);

    static void Main(string[] args)
    {
        OutAction<int, int> action = OutFunc;
        int a = 3, b = 5;
        Console.WriteLine("{0}/{1}",a,b);
        OutFunc(out a, out b);
        Console.WriteLine("{0}/{1}", a, b);
        Console.ReadKey();
    }
}

This prints out:

3/5
0/0

Answer 2

No, not with the builtin delegates. out and ref are special qualifiers and the delegate has to be setup with them explicitly since they are completely different calling styles.

However, if you defined your own delegate, you can do this:

delegate void OutAction<T1, T2>(out T1 a, out T2 b);