porting apps?

Question

I hope this is the correct forum to post this (please tell me if it isn't). When I first heard about VC#, I had the understanding that MS claimed that you didn't need to re-compile your apps to port them to a different platform ('cause of the .net framework). Is this true? If so how in the world would I install the framework on, say, Linux or Mac (No I'm a hard-core microsoft fan, but some of my customers might not be). Thanks much.