Resolution Scope

What is a scope

"In computer programming, a scope is the context within a computer program in which a variable name or other identifier is valid and can be used, or within which a declaration has effect. Outside of the scope of a variable name, the variable's value may still be stored, and may even be accessible in some way, but the name does not refer to it; that is, the name is not bound to the variable's storage" (Wikipedia).
In one sentence

Scope is a fixed duration of time or method calls in which an object exists.

What resolution scope means?

In term of Dependency Injection, the scope defines the life-cycle and visibility of instances.

A scope determines when a new instance of registered type with that scope is created and when an existing instance with that scope is destroyed. An instance of a scoped type is bound to the scope and is shared during the execution in the context of the scope.

Common resolution scopes

• No Scope (Transient, Prototype) : A new instance is created each time you request
• Singleton (Container): The same instance is used each time you request it from this container
• Thread: An instance is created for each thread

Common resolution scopes for web applications

• RequestScoped: Scopes a type definition to the life-cycle of a single HTTP request; that is each and every HTTP request will have its own instance of a single type definition. 
• SessionScoped: Scopes a single type definition to the life-cycle of a HTTP Session.
• ApplicationScoped: Scopes a single type definition to the life-cycle of a global HTTP Session (and usually binds to ApplicationContext).

Why resolution scope?

Using resolution scope has the following benefits:

• It implicitly separates state by context (for example, two HTTP requests imply different contexts).
• It reduces the necessity for state-aware application logic (which makes code much easier to test and reason about).

How to define a custom resolution scope

Sometimes, you can not find what you are looking in your selected DI framework. However, some of DI frameworks let you to extend built-in scopes or base classes to define your own custom scope; If not then you need to implement the custom scope yourself.
At the following, you can find out how to define a custom resolution scope in Unity, and use it wherever that you like.
To do that a base class named CustomScope has been defined. Any custom scope that you might use in the application inherits  from CustomScope. Following that, a sample code utilizing CustomScope has been demonstrated.

ICustomScope and CustomScope definition

CustomScope usage sample code

Further Reading
http://kohari.org/2009/03/06/cache-and-collect-lifecycle-management-in-ninject-20/
http://code.google.com/p/google-guice/wiki/Scopes
http://docs.jboss.org/webbeans/reference/1.0.0.PREVIEW1/en-US/html/scopescontexts.html