Command Design Pattern

 UML diagram
 sample code in C#


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

Frequency of use:   medium high

UML class diagram


    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.

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// Command pattern -- Structural example

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.

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// Command pattern -- Real World example

This .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, object initializers, automatic properties, etc.

Show code

// Command pattern -- .NET optimized

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Design Pattern
Framework 4.5

C# and VB.NET


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