About to create Silverlight Assemblies

I to create Assemblies in c# and want to use in Visual Studio 2008 with Silverlight.

in the Silverlight Project I add this assemblie, and I receive this message:

"You can't add a reference to usuario.dll as it was not built against the Silverlight runtime. Silverlight Project will only work with Silvelight assemblies"

Then, how is to Create Silverlight Assemblies?

Regards

Rub�©n
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