I've been seeing a lot of C# 4.0 changes as of late. I really like some of them. Also though, I do not want to move on to .Net 4.0 for compatibility reasons just yet.
So, is there a comprehensive list of new C# 4.0 language features that will work on .Net 3.5 or lower?
As an example, do default parameters require the .NET 4.0 CLR, or are they a compiler feature? It's possible to use automatic properties (a C# 3.0 feature) and still target .NET 2.0, since that doesn't require Framework support, but not to use LINQ expressions, since that does require Framework 3.5.
Please don't say "of course C#4.0 won't work in .Net 3.5 cause it's older"
The C# compiler in 4.0 ships with a new multitargeting feature. The short version is that it will produce an assembly with whatever metadata version is present in the reference that defines System.Object (usually mscorlib.dll). This allows you to use it to compile assemblies for 2.0 and 3.5, as well as various versions of Silverlight. This feature was introduced in support of the multitargeting in Visual Studio 2010.
Therefore, you can use the C# 4.0 compiler to compile 3.5 assemblies, and make use of whatever C# 4 features you want, so long as there is no particular dependency on 4.0 libraries. For example, named arguments and optional parameters will work fine, because they don't use any 4.0 framework features. Dynamic, however, does, and so out of the box it won't work on 3.5.
There is a post about this here, that covers the basics.
http://blogs.msdn.com/ed_maurer/archive/2010/03/31/multi-targeting-and-the-c-and-vb-compilers.aspx
The short list (from Ed's post) is:
Things that won't work include dynamic and no-pia (our two biggest outlays in terms of time spent implementing them). The former requires framework support and the latter requires runtime support.