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How-To Q&A: How do I decide between an imported field or an importing constructor in MEF?

Kathleen Dollard explains the choices developers face when building applications in MEF.

A reader asks: I am building an application using Managed Extensibility Framework (MEF). How do I decide whether to use an imported field or an importing constructor?

Kathleen Answers: If the import does not appear in a base class, you have the choice of using an imported field, imported property or importing constructor. Importing constructors are generally easier to test because you can pass values directly, rather than constructing and using a MEF container. When using a constructor, you sometimes need to add an attribute on individual parameters, such as specifying cardinality. You can do this by placing the attribute ahead of each parameter:

  <ImportingConstructor()>
   Public Sub New(<Import(AllowDefault:=True)> ByVal typeRetriever As ITypeRetriever)
      _typeRetriever = typeRetriever
   End Sub

The downside of importing constructors is it can become messy when you have many or long parameter attributes.

If the import appears in a base class, you must use an imported field or property and cannot use an importing constructor. Only leaf constructors are accessed with MEF. While you could use an importing constructor on the leaf class and pass the value on to the base class, this breaks encapsulation of the base class. If you frequently use imports in base classes, you’ll lose the testing benefit and might find imported fields easier.

About the Author

Kathleen is a consultant, author, trainer and speaker. She’s been a Microsoft MVP for 10 years and is an active member of the INETA Speaker’s Bureau where she receives high marks for her talks. She wrote "Code Generation in Microsoft .NET" (Apress) and often speaks at industry conferences and local user groups around the U.S. Kathleen is the founder and principal of GenDotNet and continues to research code generation and metadata as well as leveraging new technologies springing forth in .NET 3.5. Her passion is helping programmers be smarter in how they develop and consume the range of new technologies, but at the end of the day, she’s a coder writing applications just like you. Reach her at kathleen@mvps.org.

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