Metadata file '.dll' could not be found

I just had the same problem. Visual Studio isn't building the project that's being referenced.

Written Instructions:

1. Right click on the solution and click Properties.
2. Click Configuration on the left.
3. Make sure the check box under "Build" for the project it can't find is checked. If it is already checked, uncheck, hit apply and check the boxes again.
4. (Optional) You had to do it for both Release and Debug modes on the solution properties.

Screen capture Instructions:

• They say a picture is worth a thousand words. Click on the GIF to zoom in, and hopefully it will be easy to follow:

This can still happen in newer versions of Visual Studio (I just had it happen on Visual Studio 2013):

Another thing to try is to close Visual Studio and delete the .suo file that is next to the .sln file. (It will be re-generated the next time you Save all (or exit Visual Studio)).

I've had this problem when adding new projects to the solution on another machine and then pulling the revisions in, but the .suo file can be corrupted in other cases as well and lead to very strange Visual Studio behaviour, so deleting it is one of the things I always try.

Note that deleting the .suo file will reset the startup project(s) of the solution.

More on the .suo file is here.