I know there is another question with exact the same problem, but I went trough all those answers, and none helped me. :( (This was the question.)
I just created a new A
Another thing that you should check is the Target Framework of any referenced projects to make sure that the calling project is using the same or later version of the framework.
I had this issue, I tried all of the previously suggested answers and then on a hunch checked the frameworks. One of projects being referenced was targeting 4.6.1 when the calling project was only 4.5.2.