Can I certainly do this ? If this is possible how do we do this ? What language features of C# 4.0 are tightly bound to the CLR of .net 4.0 framework if any? In case if no features are bound to CLR4.0 can we really use C#4.0 in a .Net3.5 application ?
You cannot use any version later than 3.5 in VS2008.
You can benefit from some of the syntactical enhancements (and other compiler goodness), and still compile as against 3.5 in VS2010 though, which may save having to deploy a newer version of the framework.