Using Web.config Transformations When Debugging Web Applications in Visual Studio
SlowCheetah is a very useful Visual Studio extension which builds upon Visual Studio's built-in support for Web.config transformations. It adds similar XDT transformation file support to non-web projects which can be used even during debugging, while built-in solution for web projects only works for web deployment. There is already a feature request for SlowCheetah for it, but until it's implemented, you'll need to find a different solution.