Object Creational Patterns and Instantiation
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by Brian Mains
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Suppose you had a series of objects that needed constructed, but your application needs to specialize how they are built. This is where the builder pattern comes into play.  It handles performing the building of the necessary objects (also referred as the "product"); the builder class has that sole responsibility. The builder class also supports inheritance, which the product that the builder creates is usually involved in the inheritance scheme, meaning that the derived builder objects are actually building their related product object.

The builder pattern has a director that oversees the build process. The director has a reference to the builder(s) available and is responsible for invoking the correct method(s) in the builder. The builder has a common interface that all builders derive from and that the director knows about, and because of this, it can easily do its job.  Inheritance plays a key, as the builder's interface is abstract, and the derived builder classes actually do the work of building an object. The resulting class that it builds can make use of inheritance to provide a more specific implementation.  The builder class would be responsible for invoking the correct object.

It is possible that the builder/director relationship be dynamically created through the use of a configuration file, though that adds a lot of work to the software development process.  More simplistically and realistically, this relationship is statically created through a constructor or an initialization method.

Listing 14

public class SecurityZone 
 private IAuthorizationManager _manager = nullprivate IIdentity _user = nullpublic SecurityZone(string userID, IAuthorizationManager manager) 
 _manager = manager; 
 private void GetUserInstance() 
 if (_user != nullreturnif (_manager == nullthrow new ArgumentNullException("_manager"); 
 _user = _manager.GetuserInstance(); 
 public bool IsInRole(string roleName) 
 return Roles.IsUserInRole(_user.Name, roleName); 

In this case, the end product is the User, which is build from a class that implements the IAuthorizationManager interface.  The director is the SecurityZone class that oversees the authorization process.

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