Definition

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Participants

The classes and/or objects participating in this pattern are:

• Command (Command)
  • Declares an interface for executing an operation
• ConcreteCommand (CalculatorCommand)
  • Defines a binding between a Receiver object and an action
  • Implements Execute by invoking the corresponding operation(s) on Receiver
• Client (CommandApp)
  • Creates a ConcreteCommand object and sets its receiver
• Invoker (User)
  • Asks the command to carry out the request
• Receiver (Calculator)
  • Knows how to perform the operations associated with carrying out the request.

Sample Code in C#

This structural code demonstrates the Command pattern which stores requests as objects allowing clients to execute or playback the requests.

// --------------------------------------------------------------------------------------------------------------------// <copyright company="Chimomo's Company" file="Program.cs">// Respect the work.// </copyright>// <summary>// Structural Command Design Pattern.// </summary>// --------------------------------------------------------------------------------------------------------------------namespace CSharpLearning{    using System;    /// <summary>    /// Startup class for Structural Command Design Pattern.    /// </summary>    internal static class Program    {        #region Methods        /// <summary>        /// Entry point into console application.        /// </summary>        private static void Main()        {            // Create receiver, command, and invoker            var receiver = new Receiver();            Command command = new ConcreteCommand(receiver);            var invoker = new Invoker();            // Set and execute command            invoker.SetCommand(command);            invoker.ExecuteCommand();        }        #endregion    }    /// <summary>    /// The 'Command' abstract class    /// </summary>    internal abstract class Command    {        #region Fields        /// <summary>        /// The receiver.        /// </summary>        protected readonly Receiver Receiver;        #endregion        // Constructor        #region Constructors and Destructors        /// <summary>        /// Initializes a new instance of the <see cref="Command"/> class.        /// </summary>        /// <param name="receiver">        /// The receiver.        /// </param>        protected Command(Receiver receiver)        {            this.Receiver = receiver;        }        #endregion        #region Public Methods and Operators        /// <summary>        /// The execute.        /// </summary>        public abstract void Execute();        #endregion    }    /// <summary>    /// The 'ConcreteCommand' class    /// </summary>    internal class ConcreteCommand : Command    {        // Constructor        #region Constructors and Destructors        /// <summary>        /// Initializes a new instance of the <see cref="ConcreteCommand"/> class.        /// </summary>        /// <param name="receiver">        /// The receiver.        /// </param>        public ConcreteCommand(Receiver receiver)            : base(receiver)        {        }        #endregion        #region Public Methods and Operators        /// <summary>        /// The execute.        /// </summary>        public override void Execute()        {            this.Receiver.Action();        }        #endregion    }    /// <summary>    /// The 'Receiver' class    /// </summary>    internal class Receiver    {        #region Public Methods and Operators        /// <summary>        /// The action.        /// </summary>        public void Action()        {            Console.WriteLine("Called Receiver.Action()");        }        #endregion    }    /// <summary>    /// The 'Invoker' class    /// </summary>    internal class Invoker    {        #region Fields        /// <summary>        /// The command.        /// </summary>        private Command command;        #endregion        #region Public Methods and Operators        /// <summary>        /// The execute command.        /// </summary>        public void ExecuteCommand()        {            this.command.Execute();        }        /// <summary>        /// The set command.        /// </summary>        /// <param name="command">        /// The command.        /// </param>        public void SetCommand(Command command)        {            this.command = command;        }        #endregion    }}// Output:/*Called Receiver.Action()*/

This real-world code demonstrates the Command pattern used in a simple calculator with unlimited number of undo's and redo's. Note that in C#  the word 'operator' is a keyword. Prefixing it with '@' allows using it as an identifier.

// --------------------------------------------------------------------------------------------------------------------// <copyright company="Chimomo's Company" file="Program.cs">// Respect the work.// </copyright>// <summary>// Real-World Command Design Pattern.// </summary>// --------------------------------------------------------------------------------------------------------------------namespace CSharpLearning{    using System;    using System.Collections.Generic;    /// <summary>    /// Startup class for Real-World Command Design Pattern.    /// </summary>    internal static class Program    {        #region Methods        /// <summary>        /// Entry point into console application.        /// </summary>        private static void Main()        {            // Create user and let her compute            var user = new User();            // User presses calculator buttons            user.Compute('+', 100);            user.Compute('-', 50);            user.Compute('*', 10);            user.Compute('/', 2);            // Undo 4 commands            user.Undo(4);            // Redo 3 commands            user.Redo(3);        }        #endregion    }    /// <summary>    /// The 'Command' abstract class    /// </summary>    internal abstract class Command    {        #region Public Methods and Operators        /// <summary>        /// The execute.        /// </summary>        public abstract void Execute();        /// <summary>        /// The un execute.        /// </summary>        public abstract void UnExecute();        #endregion    }    /// <summary>    /// The 'ConcreteCommand' class    /// </summary>    internal class CalculatorCommand : Command    {        #region Fields        /// <summary>        /// The calculator.        /// </summary>        private readonly Calculator _calculator;        /// <summary>        /// The operand.        /// </summary>        private readonly int _operand;        /// <summary>        /// The operator.        /// </summary>        private readonly char _operator;        #endregion        // Constructor        #region Constructors and Destructors        /// <summary>        /// Initializes a new instance of the <see cref="CalculatorCommand"/> class.        /// </summary>        /// <param name="calculator">        /// The calculator.        /// </param>        /// <param name="operator">        /// The @operator.        /// </param>        /// <param name="operand">        /// The operand.        /// </param>        public CalculatorCommand(Calculator calculator, char @operator, int operand)        {            this._calculator = calculator;            this._operator = @operator;            this._operand = operand;        }        #endregion        // Execute new command        #region Public Methods and Operators        /// <summary>        /// The execute.        /// </summary>        public override void Execute()        {            this._calculator.Operation(this._operator, this._operand);        }        /// <summary>        /// Unexecute the last command.        /// </summary>        public override void UnExecute()        {            this._calculator.Operation(this.Undo(this._operator), this._operand);        }        #endregion        // Returns opposite operator for given operator        #region Methods        /// <summary>        /// The undo.        /// </summary>        /// <param name="operator">        /// The operator.        /// </param>        /// <returns>        /// The <see cref="char"/>.        /// </returns>        private char Undo(char @operator)        {            switch (@operator)            {                case '+':                    return '-';                case '-':                    return '+';                case '*':                    return '/';                case '/':                    return '*';                default:                    throw new ArgumentException("@operator");            }        }        #endregion    }    /// <summary>    /// The 'Receiver' class    /// </summary>    internal class Calculator    {        #region Fields        /// <summary>        /// The current value.        /// </summary>        private int curr;        #endregion        #region Public Methods and Operators        /// <summary>        /// The operation.        /// </summary>        /// <param name="operator">        /// The operator.        /// </param>        /// <param name="operand">        /// The operand.        /// </param>        public void Operation(char @operator, int operand)        {            switch (@operator)            {                case '+':                    this.curr += operand;                    break;                case '-':                    this.curr -= operand;                    break;                case '*':                    this.curr *= operand;                    break;                case '/':                    this.curr /= operand;                    break;            }            Console.WriteLine("Current value = {0,3} (following {1} {2})", this.curr, @operator, operand);        }        #endregion    }    /// <summary>    /// The 'Invoker' class    /// </summary>    internal class User    {        // Initializers        #region Fields        /// <summary>        /// The calculator.        /// </summary>        private readonly Calculator calculator = new Calculator();        /// <summary>        /// The commands.        /// </summary>        private readonly List<Command> commands = new List<Command>();        /// <summary>        /// The current command.        /// </summary>        private int current;        #endregion        #region Public Methods and Operators        /// <summary>        /// The compute.        /// </summary>        /// <param name="operator">        /// The @operator.        /// </param>        /// <param name="operand">        /// The operand.        /// </param>        public void Compute(char @operator, int operand)        {            // Create command operation and execute it            Command command = new CalculatorCommand(this.calculator, @operator, operand);            command.Execute();            // Add command to undo list            this.commands.Add(command);            this.current++;        }        /// <summary>        /// The redo.        /// </summary>        /// <param name="levels">        /// The levels.        /// </param>        public void Redo(int levels)        {            Console.WriteLine("\n---- Redo {0} levels ", levels);            // Perform redo operations            for (int i = 0; i < levels; i++)            {                if (this.current < this.commands.Count - 1)                {                    Command command = this.commands[this.current++];                    command.Execute();                }            }        }        /// <summary>        /// The undo.        /// </summary>        /// <param name="levels">        /// The levels.        /// </param>        public void Undo(int levels)        {            Console.WriteLine("\n---- Undo {0} levels ", levels);            // Perform undo operations            for (int i = 0; i < levels; i++)            {                if (this.current > 0)                {                    Command command = this.commands[--this.current];                    command.UnExecute();                }            }        }        #endregion    }}// Output:/*Current value = 100 (following + 100)Current value =  50 (following - 50)Current value = 500 (following * 10)Current value = 250 (following / 2)---- Undo 4 levelsCurrent value = 500 (following * 2)Current value =  50 (following / 10)Current value = 100 (following + 50)Current value =   0 (following - 100)---- Redo 3 levelsCurrent value = 100 (following + 100)Current value =  50 (following - 50)Current value = 500 (following * 10)*/