Encapsulating Action<T> and Func<T>?

Question

I'm trying to make a design for some sort of IExecutable interface. I will not get into details, but the point is that I have several Actions that need to be executed from a base class. They may take different parameters (no big deal), and they may/may not return a value.

So far, this is my design:

public abstract class ActionBase
{
    // ... snip ...
}

public abstract class ActionWithResultBase<T>: ActionBase
{
    public abstract T Execute();
}

public abstract class ActionWithoutResultBase: ActionBase
{
    public abstract void Execute();
}

So far, each of my concrete actions need to be a child from either ActionWithResultBase or ActionWithoutResult base, but I really don't like that. If I could move the definition of Execute to ActionBase, considering that the concrete class may or may not return a value, I will have achieved my goal.

Someone told me this could be done with using Func and Action, for which I totally agree, but I can't find a way to have that into one single class so that the caller would know if the action is going to return a value or not.

Brief: I want to do something like:

// Action1.Execute() returns something.
var a = new Action1();
var result = a.Execute();

// Action2.Execute() returns nothing.
var b = new Action2();
b.Execute();

Answer 1

If you want a lightweight solution, then the easiest option would be to write two concrete classes. One will contain a property of type Action and the other a property of type Func<T>:

public class ActionWithResult<T> : ActionBase { 
  public Func<T> Action { get; set; } 
}

public class ActionWithoutResult : ActionBase {
  public Action Action { get; set; }
}

Then you can construct the two types like this:

var a1 = new ActionWithResult<int> { 
  CanExecute = true,
  Action = () => { 
    Console.WriteLine("hello!");
    return 10; 
  }
}

If you don't want to make Action property read/write, then you could pass the action delegate as an argument to the constructor and make the property readonly.

The fact that C# needs two different delegates to represent functions and actions is quite annoying. One workaround that people use is to define a type Unit that represents "no return value" and use it instead of void. Then your type would be just Func<T> and you could use Func<Unit> instead of Action. The Unit type could look like this:

public class Unit {
  public static Unit Value { get { return null; } }
}

To create a Func<Unit> value, you'll write:

Func<Unit> f = () => { /* ... */ return Unit.Value; }