• Provide a surrogate or placeholder for another object to control access to it.
• In object-oriented languages, objects do the work they advertise through their public interface.
  • Clients of these objects expect this work to be done quickly and efficiently.
  • However, there are situations where an object is severely constrained and cannot live up to its responsibility.
  • Typically this occurs when there is a dependency on a remote resource (a call to another computer for example) or when an object takes a long time to load.
  • In situations like these, you apply the Proxy pattern and create a proxy object that "stands in" for the original object.
  • The Proxy forwards the request to a target object.
  • The interface of the Proxy object is the same as the original object and clients may not even be aware they are dealing with a proxy rather than the real object.
• There are 3 different types of proxies:
  • Remote proxies are responsible for encoding a request and for forwarding the encoded request to a real object in a different address space (app domain, process, or machine)
  • Virtual proxies may cache additional information about a real object so that they can postpone accessing it (this process is known by many names, such as, just-in-time loading, on-demand loading, or lazy loading)
  • Protection proxies check that the caller has the proper access permissions to perform the request.
  • A Smart Reference is a proxy for a pointer.
• In .NET the Proxy pattern manifests itself in the Remoting infrastructure.
  • In .NET Remoting, whenever an object requires access to an object in a different address space (app domain, process, or machine) a proxy is created that sends the request to the remote object and any data it needs.
  • As is common with proxies, the client is frequently not even aware that a proxy is at work.
  • Clients of WCF services also rely heavily on auto-generated proxy objects.

Participants

• Proxy (ServiceProxy)
  • 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 the real subject.
  • Controls access to the real subject and may be responsible for creating and deleting it.
  • Other responsibilities 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 information about the real subject so that they can postpone accessing it.
    • Protection proxies check that the caller has the access permissions required to perform a request.
• Subject (IService)
  • Defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected.
• RealSubject (Service)
  • Defines the real object that the Proxy represents.

IService service = new Service();
IService serviceProxy = new ServiceProxy(service);
var result = serviceProxy.Execute();

public interface IService
{
    string Execute();
}

class Service : IService
{
    public string Execute() => "Remote Result";
}

class ServiceProxy : IService
{
    private string _resultCached;
    private IService _service;

    public ServiceProxy(IService service)
    {
        _service = service;
    }

    public string Execute()
    {
        if (string.IsNullOrEmpty(_resultCached))
            _resultCached = _service.Execute();
        return _resultCached;
    }
}

Rules of Thumb

• The adapter provides a different interface to its subject.
  • Proxy provides the same interface.
  • The decorator provides an enhanced interface.
• Decorator and Proxy have different purposes but similar structures.
  • Both describe how to provide a level of indirection to another object, and the implementations keep a reference to the object to which they forward requests.