Recently I went through a tricky problem while working with VS 2010. I was trying to reference a Class library project in a Console project. So I opened Add Reference window from Console App, selected Projects, on top left corner of the window and located Class Library Project that I wanted to reference and I clicked on Add button. It was there in the Reference list, but with an yellow exclamation mark.
I looked around on Google and found some posts which were suggesting that the physical dll file might not be there while VS is trying to load it and hence, it’s showing that exclamation mark.