I a have a Visual Studio 2015 Solution "S1" and in it contains one project "DAL". If I open the solution in VS it opens correctly, and all of my references are known. I can add a second Project to solution "S1" called "Model". Once again both "DAL" and "Model" work, build, execute, and all references are known.
If however I open a solution "S2" and in it contains the project "Model", and then I add an existing project "DAL", all of my references are unknown. All of them. System, System.Core, System.Data, EntityFramework, and the list goes on. If I remove the reference, save, and re-add the reference it is immediately unknown.
I have also tried to open "DAL" from its project file, and again the references are unknown. I have made sure that both projects are using the same version of the .NET Framework (4.5.2), and that both have the same Platform Target.
There is only one difference that I have found is that in the unknown references the Path property is blank and in the known reference it has C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.5.2\System.dll, which of course makes sense because the reference was found.
I just don't understand how if I open a project by its project file I have no references, but if I open the Solution I have references.