The Enterprise Library includes an assembly named Common that contains useful elements that are shared among multiple application blocks. Any application that uses an Enterprise Library application block (with the exception of the Unity Application Block) must reference this assembly. By supplying a set of commonly used functions to all the application blocks, the Common assembly reduces the dependency of one application block on another.

The following are some criteria used to factor elements into the Common assembly:

• Elements could not depend on other application blocks. However, they could depend on other elements that are already in the Common assembly.
• Elements could not depend on any persistent state.
• Elements could not depend on any third-party technology.
• Elements could not imply a particular application model. They could be used equally well in any application model. For example, the elements should function as well in a smart client as in a Web service.

Most of the application blocks contain instrumentation. The types of instrumentation are the following:

• Performance counters
• Event log events
• WMI (Windows Management Instrumentation) events

The instrumentation classes are a part of the Enterprise Library Core and are located in the Common assembly. You can reuse this code in your own applications.

By default, instrumentation is disabled, but you can use the Enterprise Library configuration tools to enable each type of instrumentation. If you have administrative rights, you can use the Installutil.exe program to install the instrumentation.

Most other dependencies are encapsulated in individual providers. For example, the Exception Handling Application Block can use the Logging Application Block to publish exception information, but this is not a requirement. You can choose to publish exception information to other sources, such as the event log, or you can extend the Exception Handling Application Block by supplying your own logging provider.

Configuration uses the System.Configuration namespace, which makes it easier to use application blocks either with configuration files or without them. Factories (and the Unity Application Block) create application block objects from the data contained in configuration files. It is possible to use alternatives to XML configuration files as configuration sources. For example, you can use a SQL Server database. You can also create objects without using configuration files; you can do this by using constructors with primitive data types.

To support extensibility in Enterprise Library, configuration includes helper classes in addition to those classes found in the System.Configuration namespace. These helper classes support polymorphic collections, generic collections, and configuration sources. These enhancements require configuration to be based on the Dependency Injection pattern instead of the Provider pattern, as is the case with the System.Configuration namespace.

The configuration tools allow you to change an application block's configuration without directly editing the XML configuration files. You can use the design-time components that the configuration tools use with applications and application blocks that you create to give your users a similar experience to the Enterprise Library Configuration Console and Configuration Editor.