Even though Shared Projects have been around since Visual Studio 2015 (maybe as early as VS 2013 update2), I've only recently learned about them. Today I spent time trying to learn how to use them following a tutorial I found Shared Project: An Impressive Feature of Visual Studio 2015 Preview. However, the one thing the author did in that tutorial, which won't work for us, is he created the Shared Project and 3 other projects, all within the same solution. Of course, you can do that, but in practice we're likely to want to create a Shared Project in some solution, and then as time goes by, include that Shared Project in other solutions.
So what I did is instead of putting the Windows Forms application into the same solution as the author of that C# Corner post did, I created a new solution with a Windows Forms project in it, then I tried to add the Shared Project from the first solution. First, I tried adding the .sln file. That failed miserably. Then I tried adding the .shproj file to the second solution. That failed miserably as well.
Next I shared here on SO for ways of addressing this. I found 2 posts: Adding references in a shared (.shproj) project and How do I add a reference to a Shared Code project (.shproj) from another project. The second one gave me an idea. I decided I would simply add the Shared Project, from the first solution, to the second solution by clicking on the second solution within Solution Explorer, then doing a "Add Existing Project". That worked.
But I wonder, is that the way you're supposed to use Shared Projects? If so, it seems to me as though I could just as well created a simple class library in the first solution and then added that class library project to the second solution. Is there something about Shared Projects that make them inherently better to use, if you add the Shared Project to a different solution, instead of just adding a regular class library project to a solution?
A class library compiles into its own DLL and your original project references that DLL, whereas a project using a Shared Project will compile into a single assembly. One scenario I could think of with shared projects is that you can have single code base but has platform specific code sections marked by directives.