Silverlight 4 101: Using Silverlight Assemblies in your non-Silverlight Projects
If you're like me, you have been coding in .NET for a long time (for me, pre-version 1.0) and you have many libraries containing classes you would like to reuse in your Silverlight applications.
However, if you try to reference a non-Silverlight assembly from a Silverlight project, you will get the following error message from Visual Studio:
You can't add a reference to <assembly name> as it was not built against the Silverlight runtime. Silverlight projects will only work with Silverlight assemblies.
Although you can't reference a non-Silverlight assembly from a Silverlight project, starting with Silverlight 4 and .NET 4, you CAN go the other way--reference a Silverlight assembly from a non-Silverlight project. This feature is known as "assembly portability". This portability currently only works with the following Silverlight assemblies:
So, as long as you convert your existing .NET class libraries to Silverlight projects and only referene the above assemblies, you can use these libraries from both Silverlight and non-Silverlight projects.
For more information, check out this CLR Team Blog post: http://blogs.msdn.com/b/clrteam/archive/2009/12/01/sharing-silverlight-assemblies-with-net-apps.aspx
Chief Software Architect, MM .NET Applicaiton Framework