This problem is making me officially crazy, until now everything was working correctly, now the program is showing a lot of monstrous errors.
In practice, I clicked with the right mouse button on the project and I selected "Clean", when I clicked "Compile" I got so many errors like:
The type or namespace does not exist in the namespace
I looked for the error on the internet and I found these:
Type or namespace name does not exist
but it did not help me, also I'm working with 5 projects, and everyone has set up version as 4.6.1