We have a large project targeting SharePoint 2010/.NET 3.5 built against the SharePoint.dll and we plan to expand it to support SharePoint 2013 as well, but the new SharePoint uses SharePoint.dll compiled in .NET 4.5.
Is it possible to somehow share code between .NET 3.5 and .NET 4.5 projects, something like Portable Class Libraries, but between .NET 3.5 and 4.5? Or the only solution is starting new project for .NET 4.5 and using referenced files from the first .NET 3.5 project?
The .NET build target is per project, so a 4.5 solution can include a project library targetting 3.5. Your can have a 4.5 library that extends the 3.5 solution where required and your main solution can build to 4.5 and include the 4.5 library.
Thus both the 3.5 solution and the 4.5 solution use the same 3.5 SharePoint code library, but the 4.5 solution can extend your SharePoint library to take advantage of new features not available in 2010 and .Net 4.5