Why can't I create a assembly for silverlight and usual .NET?

Question

Why can't I create a assembly for silverlight and usual .NET?

Answer 1

The reason is that Silverlight uses a parallel implementation of the CLR (known as CoreCLR). The core libraries used to implement the runtime are different between Silverlight and .Net, even though they expose roughly the same services and conform to the same specifications.

Here is a nice article which explores CoreCLR in depth: http://msdn.microsoft.com/en-us/magazine/cc721609.aspx

Answer 2

I believe you can do this as of Silverlight 4, but not on earlier versions. MSDN has an article going into more details.

Prior to this, assembly portability hadn't been baked into .NET, so there was no set of common assemblies which could be targeted from both Silverlight and the desktop framework. At least, that's the way I read the above article...