In the .NET world, developers can place commonly used/referenced assemblies into the Global Assembly Cache. In the Silverlight world, it isn't always clear where where Visual Studio is picking up the assemblies in the Add References dialog. Fortunately, Silverlight Reference Viewer lists the exact directories associated with Visual Studio's Add Reference dialog, allowing you to see exactly where your Silverlight assembly references are coming from.
Silverlight Reference Viewer requires .NET Framework 4. After that, run the MSI installer, and you're off!
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