How to compile a c# program under .net Framework 3.5 and above?

Question

.NET

Wrote a program. If I compile according to the 3.5 Framework, then it does not go to 4.5, if to 4.5, then it does not start on 3.5. How to compile universally so that it runs on 3.5 and higher netFramework?

Answer 1

1 solution
2 projects
one source code for both projects, one
reference files / libraries are different for two projects, the
final folders-compile files of two projects are different net35, net45 for example. to pick up from the nuget or handles more convenient.
what kind of unique libraries are used in the project that 3.5 is backwards incompatible with 4.5?