Development Environment : - VS2010 - .Net Framework 4.0, 3.5, 2.0
Staging and Production Environments: - .Net Framework 3.5, 2.0
The project I'm working on is targeting .Net Framework 3.5. And today I used optional parameters feature, which is new to C#4, in this project and it worked fine. I think VS2010 is using C#4 compiler and is compiling the method with optional parameters to corresponding overloaded methods in IL.
I want to know if I can use all new C#4 features as well.
You cannot use is the dynamic feature. This relies on the C# runtime and DLR DLL's which are only available on the 4.0 version of the .Net framework. Versions of the DLR are available for 3.5 but I do not believe they are compatible with the one required by the C# compiler.
Additionally you cannot use NoPIA / Embedded Interop Types in a down targetted scenario. This feature requires CLR support that was added in 4.0.
What's great about down targeting in Visual Studio 2010 though is you don't have to be aware of every limitation. If your projects are set to down target 3.5 and you use an incompatible feature, Visual Studio will produce an error.
I bumped into this a couple of weeks ago actually. I used optional parameters even though the project targeted .net 3.5. You need to be very careful of this because if you install the application on a computer that only has .net 3.5 runtime installed then your program may not run. In my case, I used the optional params and the nightly build server only had 3.5 installed so the build failed.