Looks like I need to use Visual Studio to create library class. I recall vaguely something about WL6 using .NET framework 3.5 and 2 but not 4. Does either version of 2010 or 2008 Visual Studio C# Express make any difference with regards to this framework business?
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I think I just answered my own question. One can save the code in any of the targeted framework as desired.
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On a related note, SharpDevelop makes this step of changing target framework even easier.
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