"Out" parameter (reference) with Action<>

Question

I have many methods similar to this:

 void GetColorSky(out float r, out float g, out float b)
 void GetColorGround(out float r, out float g, out float b)

"Similar" meaning they have the exact same method header excluding the method name. I also have several "colourPicker" controls, that accept R, G, B values.

I am trying to create a method that accepts a method as one of it's parameters, like so:

UpdateColourPicker(ColorPicker cp, CustomMethod cMethod)

and call it as follows:

UpdateColourPicker(cpSky, GetColorSky)
UpdateColourPicker(cpGround, GetColorGround)

What is the correct syntax for out and Action together? I've looked at this question but I still haven't managed.

Thanks!

Answer 1

Taking cue from that linked answer, this will work:

public delegate void GetColorDel(out float r, out float g, out float b);

void UpdateColourPicker(ColorPicker cp, GetColorDel cMethod) { }

UpdateColourPicker(cpSky, GetColorSky);
UpdateColourPicker(cpGround, GetColorGround);

Unfortunately, as stated in the answer to the question you link, Action and Func do not work with out and ref. This is simply because Action and Func are just delegate definitions stated (quite a few times to offer different overloads) with generics in the BCL. Adding ref and out variants would quickly cause re-definition compiler errors.

The full sample:

class Program
{        
    public delegate void GetColorDel(out float r, out float g, out float b);

    static void Main(string[] args)
    {
        UpdateColourPicker(null, GetColorSky);
        UpdateColourPicker(null, GetColorGround);

        Console.Read();
    }

    static void GetColorSky(out float r, out float g, out float b) 
    {
        r = g = b = 0f;
        Console.WriteLine("sky"); 
    }

    static void GetColorGround(out float r, out float g, out float b) 
    {
        r = g = b = 0f;
        Console.WriteLine("ground"); 
    }

    static void UpdateColourPicker(object cp, GetColorDel cMethod) 
    {
        float r, g, b;
        cMethod(out r, out g, out b);
    }
}

Answer 2

If you want to define a generic delegate with out parameters, it should be done like this:

delegate void ActionWithOutparameter<T>(out T x);
delegate void ActionWithOutparameter<T1, T2>(out T1 x, out T2 y);
// ...

Clearly the Action and Func delegates in the BCL do not match this signature.