A .NET Forms application uses a completely different run time then what Silverlight uses. The two run times are not interoperable. You do have a few options however but they may not be the solution you are looking for.
First option, convert your Sliverlight user controls into Windows Form controls.
Second option, create parallel Silverlight and Forms controls and use link files to keep core logic in a single file. This is messy, I've personally done this in the past and it caused a lot of headaches amongst developers that didn't understand how to use link files.
Share code with Add as Link[
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Your third option would to take a look at out-of-browser support for Silverlight. You can run a Silverlight application without using browser so you have the appearance of a desktop application.
Out-of-browser Support[
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A possible fourth option would be to switch to WPF. The XAML and libraries used in WPF and Silverlight are very similar and migrating code between the two technologies, for the most part, is painless.
Hope this helps you out.
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