Introduction to the Policy Injection Application Block

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Applications include a mix of business logic and crosscutting concerns, and the two are typically intermingled—which can make the code harder to read and maintain. Each task or feature of an application is referred to as a "concern." Concerns that implement the features of an object within the application, such as the business logic, are core concerns. Crosscutting concerns are the necessary tasks, features, or processes that are common across different objects—for example, logging, authorization, validation, and instrumentation. The purpose of the Policy Injection Application Block is to separate the core concerns and crosscutting concerns.

Developers can use the Policy Injection Application Block to specify crosscutting behavior of objects in terms of a set of policies. A policy is the combination of a series of handlers that execute when client code calls methods of the class and—with the exception of attribute-based policies—a series of matching rules that select the classes and class members (methods and properties) to which the application block attaches the handlers. Attribute-based policies rely on attributes directly applied to the members of the target class to identify those to which the application block will apply policies. The result is a mechanism for declaratively applying a chain of handlers to members of the target class instance. This chain of handlers is a pipeline.

Figure 1 illustrates a high-level view of the way the Policy Injection Application Block implements a handler pipeline for a specified member of a specified class.

Figure 1
High-level view of the way the Policy Injection Application Block implements a handler pipeline

The Policy Injection Application Block provides a mechanism for automatically applying policies to object instances; this helps developers to better manage crosscutting concerns and maximize separation of concerns and encapsulation of behavior. Developers define the set of policies for the target classes and their members through configuration of the Policy Injection Application Block or by applying attributes to individual members of the target class.

The matching rules for each policy encompass a range of capabilities for selecting classes and members, including the name of the assembly, the namespace, the type, the member name, the member signature, attributes that are applied to the member, and more.

A factory method of the Policy Injection Application Block creates a proxy for each configured class selected by the matching rules or the directly applied attributes; the method attaches the handler pipelines to each proxy and connects them to the appropriate methods and properties of a new instance of the target object. If there are no policies for the object, the application block detects this and returns the object itself instead of a proxy to that object.

The handlers within each pipeline are reusable and independent of the object. Each handler can implement a specific requirement, such as validating a property value or checking a user's authorization status. In fact, many of the crosscutting tasks regularly occurring within applications that use Enterprise Library are calls to other application blocks within the library, such as the Validation Application Block, the Logging Application Block, the Exception Handling Application Block, the Security Application Block, and the Instrumentation and Performance Counters features of the Enterprise Library Core.

The Policy Injection Application Block ships with pre-built handlers for these application blocks that speed up development when using the Enterprise Library to help manage crosscutting concerns. Developers can also create custom handlers and policies that carry out almost any required interception processing for methods and properties of their objects.

The implementation of a system that automatically creates a proxy and handler pipelines for methods is similar to the aspect-oriented programming (AOP) approach. However, the Policy Injection Application Block is not an AOP framework implementation for the following reasons:

• It uses interception to enable only pre-processing handlers and post-processing handlers.
• It does not insert code into methods.
• It does not provide interception for class constructors.

Retired Content
This content is outdated and is no longer being maintained. It is provided as a courtesy for individuals who are still using these technologies. This page may contain URLs that were valid when originally published, but now link to sites or pages that no longer exist.
The latest Enterprise Library information can be found at the Enterprise Library site.