Why can't I create an assembly for silverlight and regular .NET?
The reason is that Silverlight uses a parallel CLR implementation (known as CoreCLR). The main libraries used to implement the runtime differ between Silverlight and .Net, although they provide approximately the same services and meet the same specifications.
Here is a good article that explores CoreCLR in detail: http://msdn.microsoft.com/en-us/magazine/cc721609.aspx
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