Class library usable by both .NET 3.5 (2.0?) and 4.0 applications?

I am starting a new class project, and I'd like for it to be usable by both .NET 3.5 and 4.0 projects.

If I build it for .NET 3.5, do I need to do anything beyond that? Will it be safely usable by .NET 4.0 projects without having to be compiled for it? Are there any incompatibilities? I know that, as an example, .NET 1.1 was supposed to be compatible with 1.0, but there were some subtle changes (that I don't remember.)

Or do I need to build it twice, once for 3.5 and once for 4.0? And... run unit tests for both?

I don't know if there will be any code that could take advantage of anything in 4.0 that isn't in 3.5, so I don't know if there is any real need to actually make a separate 4开发者_如何学Python.0-version.

So my question is: Do I have to, or should I?

Yes, It should work no problem. on the odd occasion something weird went on and it doesn't work - .net 4 now offers side by side frame work hosting

which will definitely work however it probably will not be needed as the 4.0 framework is backwards compatible with 3.5.