Why do I get a warning icon when I add a reference to an MEF plugin project?
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.
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
For both of (or all of) the projects that you want to use together:
Right click on the project > Properties > Application > Target .NET framework
Make sure that both of (or all of) your projects are using the same .NET framework version.
Make sure all versions are same for each projects click each projects and see the version here Project > Properties > Application > Target .NET framework
a. Go to Tools > Nuget Package Manager > Package Manager Console Type Update-Package -Reinstall (if not working proceed to 2.b)
b. THIS IS CRITICAL BUT THE BIGGEST POSSIBILITY THAT WILL WORK. Remove < Target > Maybe with multiple lines < /Target > usually found at the bottom part of .csproj.
Save, load and build the solution.