All framework versions (1.0, 1.1, 2.0, 3.0, 3.5) can run side by side. For safty's sake you should test your apps under any proposed framework versioin change. For WinForm apps usually you target one framework version and the app will use that one but if its not designed for it then it may break if it is changed from an upgrade install of a new FW version. Now if you have multiple versions of th framework then it wil use what it was targeted for.

Here is an old chart on the different loading conditions for the first 3 framework versions.




With web apps its a be a bit different.

You can configure your site to use a particular framework version using IIS. Drawback is that only one specific version can be targetted and/or used per site. Separate sites can have different versions of the framework as their configured target. So no issues there.

So bottom line is yes but if you want to take advantage of the new FW version for a site, it must be individually configured to it.



Edit: WOOT! 40,000 POSTS!