This error message in VS can arise because of different things.
1.One of them is simply that you are in Release mode instead of Debug Mode. (Does not generate the .pdb file with your .dll).
2.It can also be that you .pdb is corupt, just recompile your project.
3.The output property of your assembly is not where it should be. This usually appens with utilities dll you include in your project. If you do not set the output of the other projects the the same output folder as the main one, you may encounter that error message, or other weird behaviors.