What is Backward Compatibility in .Net

Backward compatibility means that an application that is developed for a particular version of a platform will also run on the later versions of that platform. The .NET Framework tries to maximize the backward compatibility: Source code written for one version of the .NET Framework should compile on the later versions of the .NET Framework, and the binaries that run on one version of the .NET Framework should behave identically on the later versions of the .NET Framework.

.NET Framework 4.5 and its point releases are backward-compatible with apps that were built with earlier versions of the .NET Framework. In other words, apps and components built with the previous versions will work without modification on the .NET Framework 4.5 as well. However, by default, apps do run on the version of the common language runtime for which they were developed, so you may have to provide a configuration file to enable your app to run on the .NET Framework 4.5.

?  Note that the .NET Framework 4.5 is not supported on Windows XP.

? .net 4.5 comes automatically with windows 8

? .net 3.5 sp1 comes automatically with windows 7 sp1