Proxy Design Pattern

 UML diagram
 sample code in C#


Provide a surrogate or placeholder for another object to control access to it.

Frequency of use:   medium high

UML class diagram


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

  • Proxy   (MathProxy)
    • maintains a reference that lets the proxy access the real subject. Proxy may refer to a Subject if the RealSubject and Subject interfaces are the same.
    • provides an interface identical to Subject's so that a proxy can be substituted for for the real subject.
    • controls access to the real subject and may be responsible for creating and deleting it.
    • other responsibilites depend on the kind of proxy:
      • remote proxies are responsible for encoding a request and its arguments and for sending the encoded request to the real subject in a different address space.
      • virtual proxies may cache additional information about the real subject so that they can postpone accessing it. For example, the ImageProxy from the Motivation caches the real images's extent.
      • protection proxies check that the caller has the access permissions required to perform a request.
  • Subject   (IMath)
    • defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected.
  • RealSubject   (Math)
    • defines the real object that the proxy represents.

sample code in C#

This structural code demonstrates the Proxy pattern which provides a representative object (proxy) that controls access to another similar object.

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// Proxy pattern -- Structural example

This real-world code demonstrates the Proxy pattern for a Math object represented by a MathProxy object.

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// Proxy 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.

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// Proxy pattern -- .NET optimized

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Design Pattern
Framework 4.5

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