支持旧框架版本中使用的库中较新的.NET框架功能

I have an asynchronous library which is compiled under .NET 3.5, since it's used by .NET 3.5 applications, but I want to be able to deal with .NET 4.0 CancellationTokens if someone wants to pass one in. In order to include .NET 4.0 stuff, I have to break compatibility with .NET 3.5. The only option I've come up with so far would be to just allow passing in "object" and then use reflection, but that makes for an ugly public interface... I want to make sure the client code is passing in the correct object type at compile time, not runtime.I have an asynchronous library which is compile