Many developers and organizations have existing libraries (assemblies) used in their desktop solutions. Object Oriented (OOP) Developers love to reuse existing assets. However, desktop assemblies may not be reused in Silverlight projects. Only Silverlight assemblies may be referenced by Silverlight projects.
To include a class library, for example, it must be compiled specifically for Silverlight. Because Silverlight is an abbreviation of the .Net Framework, a simple recompile is not always possible.
The .Net framework has two key ingredients. First, the Framework Class Library (FCL). This contains base classes, interfaces, value types, and most of the Framework’s namespaces. However, there’s also the Base Class Library (BLC). This contains the Framework’s fundamental data types and shared functions.