Command Design Pattern - Execute method with return value

Question

I've implemented Command Design Pattern according to below UML Command

Where Command is is an interface called ICommand:

 public interface ICommand
    {
        CommandType CommandType { get; set; }
        Task  Execute();
    }

It was perfect design until i have been requested to return value from Task Execute(); for specific concrete command, So i thought "OK, lets add a new method Task<T> Execute<T>();" but it means that i need to add empty implementation to all classes that implement ICommand interface (around 10 classes).

I would like that ConcreteCommand will have only one of Execute method, is it possible? Is it must have empty implementation to other method for every ConcreteCommand ? Is there a way to combine those method into one using Task?

Just for clarification, I don't want to have two methods like this:

 public interface ICommand
    {
        CommandType CommandType { get; set; }
        Task  Execute();
        Task<T> Execute<T>();
    }

Thanks!

Rafael · Accepted Answer · 2020-07-06 15:39:51Z

2

Since the Invoker can change Commands at runtime, there must be loose type safety. For cases where you know the command's return value, you can pass its return type to execute():

using System;

class Receiver {
    public bool someAction() {
        return false;
    }
    
    public string someAction2() {
        return "some action value";
    }

    public void voidAction() {
        Console.WriteLine("void action");
    }
}

interface ICommand {
    object doAction();
};

class ConcreteCommand : ICommand {
    private Receiver receiver;
    
    public ConcreteCommand(Receiver recv) {
        receiver = recv;
    }
    
    public object doAction() {
        return receiver.someAction();
    }
}

class ConcreteCommand2 : ICommand {
    private Receiver receiver;
    
    public ConcreteCommand2(Receiver recv) {
        receiver = recv;
    }
    
    public object doAction() {
        return receiver.someAction2();
    }
}

class VoidCommand : ICommand {
    private Receiver receiver;
    
    public VoidCommand(Receiver recv) {
        receiver = recv;
    }
    
    public object doAction() {
        receiver.voidAction();
        return true;
    }
}

class Invoker {
    private ICommand command;
    
    public void setCommand(ICommand cmd) {
        command = cmd;
    }
    
    public T execute<T>() {
        return (T) command.doAction();
    }
}

public class Program
{
    public static void Main()
    {
        Invoker i = new Invoker();
        Receiver r = new Receiver();
        
        i.setCommand(new ConcreteCommand(r));
        Console.WriteLine(i.execute<bool>());
        
        i.setCommand(new ConcreteCommand2(r));
        Console.WriteLine(i.execute<string>());
        
        i.setCommand(new VoidCommand(r));
        i.execute<object>();

        ICommand[] commands = new ICommand[] {
            new ConcreteCommand(r),
            new ConcreteCommand(r),
            new ConcreteCommand2(r),
            new ConcreteCommand2(r),
            new VoidCommand(r)
        };
        
        foreach (ICommand c in commands) {
            i.setCommand(c);
            Console.WriteLine(i.execute<object>());
        }
    }
}

False
some action value
void action
False
False
some action value
some action value
void action
True

edited Jul 6, 2020 at 15:39

answered Jul 6, 2020 at 1:16

Rafael

7,79613 gold badges36 silver badges49 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Lior Dadon Over a year ago

Thanks for you answer! still wondering what about concrete commands that not have return value? how doAction will look like ?

Rafael Over a year ago

Since doAction() returns an object, it must return a value; use a dummy value such as true for void commands. (example added)

Collectives™ on Stack Overflow

Command Design Pattern - Execute method with return value

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related