MEF plugin implementation under C#.

In this example, MEF structure will be added and will be explained.

MEF is the framework which is provided by Microsoft natively under .NET. It provides API to bind methods dynamically. Hence, during compile time, the methods don't have to be statically bounded.

MEF is generally used for Plugin based structures where software is configurable based on user decisions. The purpose of using MEF is generally isolating the implementation from the caller method. Hence, if there is any change in implementation, there will not be any need of code change in caller methods. It will reduce the code amount to be changes, possible reduce the size of patch and provide elasticity to the software. Video processing softwares (i.e. directshow, gstreamer) are generally based on plugin based structures.

In this example, there are three projects.

• MefTrial : This is the main project. It searches for plugin directory and loads other dlls dinamically.
• MefInterface : This is the shared project which contains the interfaces (contracts) between implementation and caller.
• MefAuthentication : This is the implementation of contracts defined in MefInterface.

To define the bindings for MEF, two attributes are used: ImportAttribute and ExportAttribute.

In the following example, the line [Import(typeof(IAuthenticationManager))] informs that Authentication object has to be initialized with type IAuthenticationManager while composing the class. If a property is marked to be imported, the property has to be public.

  [Import(typeof(IAuthenticationManager))]
  public IAuthenticationManager Authentication;

In the following example, the line [Export(typeof(IAuthenticationManager))] informs that the class FirstAuthentication is an implementation of type IAuthentication. The attribute ExportMetadata is used to describe the implementation specific data. By using this data, without knowing the implementation class name, MEF can call correct binding.

 [Export(typeof(IAuthentication))]
 [ExportMetadata("AuthenticationType", "1")]
 public class FirstAuthentication : IAuthentication

As there are two important attributes Import and Export (with their metadata attributes), to add new bindings, we just need to define the property with Import attribute and the implementation with Export attribute. Whenever MEF composes the class with Import attribute, it will automatically inject the dependency