proxy-theory.cs

  using System;

  // Proxy Pattern              Judith Bishop Dec 2006
  // Shows virtual and protection proxies
 
  class SubjectAccessor {  
    public interface ISubject {
    string Request ();
  }
 
  private class Subject {
    public string Request() {
      return "Subject Request " + "Choose left door\n";
    }
  }

  public class Proxy : ISubject {
    Subject subject;
 
    public string Request() {
      // A Virtual Proxy creates the object only on its first method call
      if (subject == null) {
        Console.WriteLine("Subject inactive");
        subject = new Subject();
      }        
      Console.WriteLine("Subject active");
        return "Proxy: Call to " + subject.Request();
      }  
    }
 
    public class ProtectionProxy : ISubject {
      // An Authentication Proxy first asks for a password
      Subject subject;
      string password = "Abracadabra";

      public string Authenticate (string supplied) {
        if (supplied!=password)
          return "Protection Proxy: No access";
        else
          subject = new Subject();
        return "Protection Proxy: Authenticated";
      }
      
      public string Request() {
         if (subject==null)
          return "Protection Proxy: Authenticate first";
        else
          return "Protection Proxy: Call to "+
        subject.Request();
      }  
    }
  }

  class Client  : SubjectAccessor {
    static void Main() {
      Console.WriteLine("Proxy Pattern\n");
    
      ISubject subject = new Proxy();
      Console.WriteLine(subject.Request());
      Console.WriteLine(subject.Request());
    
      subject = new ProtectionProxy();
      Console.WriteLine(subject.Request());
      Console.WriteLine((subject as ProtectionProxy).Authenticate("Secret"));
      Console.WriteLine((subject as ProtectionProxy).Authenticate("Abracadabra"));
      Console.WriteLine(subject.Request());
    }
  }

/* Output:

Proxy Pattern

Subject inactive
Subject active
Proxy: Call to Subject Request Choose left door

Subject active
Proxy: Call to Subject Request Choose left door

Protection Proxy: Authenticate first
Protection Proxy: No access
Protection Proxy: Authenticated
Protection Proxy: Call to Subject Request Choose left door
*/