Programming - Proxy Structural Design Pattern

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

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

Proxyand
and
- 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.
Subjectand
and
- defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected.
RealSubjectand
and and
- defines the real object that the proxy represents.

// Proxy pattern -- Structural example

                using
              
              System;

               namespace

DoFactory.GangOfFour.Proxy.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
// Create proxy and request a service
Proxy proxy = new Proxy();
proxy.Request();

// Wait for user
Console.Read();
}
}

// "Subject"

abstract class Subject
{
public abstract void Request();
}

// "RealSubject"

class RealSubject : Subject
{
public override void Request()
{
Console.WriteLine("Called RealSubject.Request()");
}
}

// "Proxy"

class Proxy : Subject
{
RealSubject realSubject;

public override void Request()
{
// Use 'lazy initialization'
if (realSubject == null)
{
realSubject = new RealSubject();
}

realSubject.Request();
}
}
}

Output expected:

Called RealSubject.Request()