I have many libraries that are built using .Net 2.0 and 3.5
I now have a new winforms application that will build against .Net 4.0 but needs to reference the libraries.
Is this possible or is there a way to branch the *.csproj files of the libraries so that they have .Net 4 against them so I can use them in my new application
You can use them anyway, they don’t need to be .Net 4.0.
There is the chance you will deploy to a machine that only has .Net 4.0, but you can set your installer to check for 3.5 and 4.0 as dependencies for the application.