Which Net framework to use to cover all Win versions from XP to 8.1?

Question

Programming

Hello, there is an application that is written under the framework 4.0, and there are problems running on XP.
Under which framework is it better to write in order to cover everything from XP. What will happen when Win 10 comes out? Will the program run on the same framework? Is it possible to change the framework on an already finished project and what are the consequences of this?

Answer 1

1. Net framework 3.5
2. Win 10 will come out and the Net framework installed immediately in it will be 4.5.2+ (not exact info)
3. If you install 3.5 - it will be
4. If the code allows, then yes. And if you use innovations from the new Net framework, then it will not work.

Answer 2

Isn't it easier to specify the presence of at least 4.0 in the requirements for the application? There will still be ancient XP users who have the .net framework 2.0 installed. Let them update.