Definition

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

UML class diagram

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Participants

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

• Command - declares an interface for executing an operation
• ConcreteCommand

defines a binding between a Receiver object and an action

implements Execute by invoking the corresponding operation(s) on Receiver

• Client - creates a ConcreteCommand object and sets its receiver
• Invoker - asks the command to carry out the request
• Receiver - knows how to perform the operations associated with carrying out the request.

Sample code in C#

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  /// <summary>
  /// MainApp startup class for Structural
  /// Command Design Pattern.
  /// </summary>
  class MainApp
  {
    /// <summary>
    /// Entry point into console application.
    /// </summary>
    static void Main()
    {
      // Create receiver, command, and invoker
      Receiver receiver = new Receiver();
      Command command = new ConcreteCommand(receiver);
      Invoker invoker = new Invoker();
 
      // Set and execute command
      invoker.SetCommand(command);
      invoker.ExecuteCommand();
 
      // Wait for user
      Console.ReadKey();
    }
  }
 
  /// <summary>
  /// The 'Command' abstract class
  /// </summary>
  abstract class Command
  {
    protected Receiver receiver;
 
    // Constructor
    public Command(Receiver receiver)
    {
      this.receiver = receiver;
    }
 
    public abstract void Execute();
  }
 
  /// <summary>
  /// The 'ConcreteCommand' class
  /// </summary>
  class ConcreteCommand : Command
  {
    // Constructor
    public ConcreteCommand(Receiver receiver) :
      base(receiver)
    {
    }
 
    public override void Execute()
    {
      receiver.Action();
    }
  }
 
  /// <summary>
  /// The 'Receiver' class
  /// </summary>
  class Receiver
  {
    public void Action()
    {
      Console.WriteLine("Called Receiver.Action()");
    }
  }
 
  /// <summary>
  /// The 'Invoker' class
  /// </summary>
  class Invoker
  {
    private Command _command;
 
    public void SetCommand(Command command)
    {
      this._command = command;
    }
 
    public void ExecuteCommand()
    {
      _command.Execute();
    }
  }

 

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