Adding references in a shared (.shproj) project
You must add the reference in both WP and Windows project.
The reason for this is that shared project is not compiled into any output DLL - it is compiled into the project that references it, so it cannot reference any other project types except other Shared Projects.
I know this is an old question but I had a similar problem with a shared project. I needed a class in the shared project to use ConfigurationManager and the normal way would be to add a reference to the project, but you can't do that with a shared project.
In my case the solution was so simple it should have been obvious (but I was overthinking it!) - just give the full reference when using the configuration manager - EG:
var someVal = System.Configuration.ConfigurationManager.AppSettings["someKey"];Not very elegant but simple enough.