Hi

VS 2010 default target to .NET 4.0, you can integrate .NET 3.5 to 4.0 but you can't integrate 4.0 to 3.5.

I think in VS 2010 you can compile the project to .NET 3.5 dll.

in this way:

http://msdn.microsoft.com/en-us/library/bb772098(v=vs.90).aspx

http://msdn.microsoft.com/en-us/library/bb398202.aspx

Hope it helpful.