Why can a .NET delegate not be declared static?

Question

When I try to compile the following:

public static delegate void MoveDelegate (Actor sender, MoveDirection args);

I receive, as an error: \

Answer 1

Delegate declaration is actually a type declaration. It cannot be static, just like you cannot define a static enum or structure.

However, I would rather use an interface instead of raw delegate.

Consider this:

public interface IGameStrategy {
    void Move(Actor actor, MoveDirection direction);
}

public class ConsoleGameStrategy : IGameStrategy {
    public void Move(Actor actor, MoveDirection direction)
    {
        // basic console implementation
        Console.WriteLine("{0} moved {1}", actor.Name, direction);
    }
}

public class Actor {
    private IGameStrategy strategy; // hold a reference to strategy

    public string Name { get; set; }    

    public Actor(IGameStrategy strategy)
    {
        this.strategy = strategy;
    }

    public void RunForrestRun()
    {
        // whenever I want to move this actor, I may call strategy.Move() method

        for (int i = 0; i < 10; i++)
            strategy.Move(this, MoveDirection.Forward);
    }
}

In your calling code:

var strategy = new ConsoleGameStrategy();

// when creating Actors, specify the strategy you want to use
var actor = new Actor(strategy) { Name = "Forrest Gump" };
actor.RunForrestRun(); // will write to console

This similar in spirit to Strategy design pattern and allows you to decouple Actor movement from the actual implementation strategy (console, graphic, whatever). Other strategy methods may later be required which makes it a better choice than a delegate.

Finally, you can use an Inversion of Control framework to automatically inject correct strategy instance in your Actor classes so there is no need for manual initialization.