Why do I get a warning icon when I add a reference to an MEF plugin project?

Solution 1:

As mentioned in the question's comments, differing .NET Framework versions between the projects can cause this. Check your new project's properties to ensure that a different default version isn't being used.

Solution 2:

Encountered the same issue with a ASP.Net Web App and two library class projects which needed to be referenced within the Web App. I had no information provided on why the build failed and the references were invalid.

Solution was to ensure all projects had the same Target Framework:

In Visual Studio 2015- Right Click project > Properties > Application > Target Framework

Save, Clean and Rebuild solution. The project references should no longer appear as yellow warnings and the solution will compile.

My Web App was targeting .Net 4.5 whereas the other two dependent library class projects targeted .Net v4.5.2