The question Can I use a .NET 4 feature while targeting .NET 3.5 SP1? has been closed as a dup by Jeff Atwood. Unfortunately, it's quite obviously not a dup of the "possible duplicate", Conditional Compilation and Framework Targets.
The question that was closed is asking about runtime behavior, while the possible duplicate is about compile-time behavior, which is completely unrelated.
I've flagged it and voted to reopen, but I'm not sure that either method I've tried will help since Jeff closed it. Can the question please be re-opened?
I ask because I have an actual answer to the question (as opposed to the current answers which are all wrong).