Context:
We have a control library and want to release a Mango-compatible version. The library does not use any Mango-specific functionality.
After installing 7.1 SDK I found only one problem in the old code, which concerned the use of TemplateBinding in the ControlTemplate. Everything else just worked.
The library can be built for WindowsPhone or WindowsPhone71 targets. The (binary) code is nearly identical; the only important difference seems to be that the target is hardcoded in the dll file.
Now imagine the developer who installed the control library and wants to use it in his project. This project has to be targeted for either 7.0 or 7.1 environments. The control library is shown in the Add References dialog only when its target is compatible with the project target. (Can be bypassed by manually browsing for the dll file, but this is a subpar solution.)
Do you see any way how to produce a single control library dll file that could be used for both 7.0 and 7.1 targets?
If it is not dependent on Windows Phone Mango features, then you can simply compile the DLL for 7.0 and reuse it in 7.1 projects. That would be the best solution.
Even though it is not listed in Visual Studio, for most older libraries the “Browse” procedure is standard.