I don't have a good explanation for this, or a good name, but it is something that I have seen occasionally. I really need to pay a bit more attention to it, but figured I'd ask here to see if anybody recognized it, and had a solution:

I have two different development computers, and this may happen when a project is moved from one to another, but it doesn't ALWAYS happen, and I'm not entirely sure when it does happen. What happens though is that ALL the libraries appear to go missing. The references are still there, but I just opened a modestly sized project to look for a bug, and was greeted with nearly 5000 errors. These were things like System.GUID is not defined, plus a whole lot of a different type which I will mention shortly.

When I click on one of those errors, the page opens with errors all over the place, then they all go away and the error count drops by hundreds. Just the act of opening the code pages fixes all the errors on the pages, normally. It's not just code pages, either, but some of the project pages that you don't normally look at, like AssemblyInfo.vb. Every file in the project has issues, and it is issues with simple things like basic types. Opening the file is enough.

That has left me with a series of errors that don't simply go away. Clicking on one takes me to a .designer.vb file (there are still a few hundred, so it may be every control in the project) and to the declaration of a control. There's no error showing on the line, and the line is fine. The error message is annoyingly difficult to get my hands on, so I'm going to try retyping one of them:
requested operation is not available because the runtime library function 'System.Runtime.CompilerServices.AccessedThroughPropertyAttribute.ctor' is not defined.
And this is the line I am taken to, which is clearly pretty innocent:
Code:
Friend WithEvents lbTotalRelSites As System.Windows.Forms.ListBox
These are not going away, and since there are about 400 of them, so they obscure everything else. I expect that they are some kind of problem related to the project file, or something like that, but I'm not sure.

I also have a whole bunch of simple types being undefined, as I mentioned earlier, but these are all in the file vbc. If I click on that, nothing happens. I don't have such a file in the project, so this must be something that the project is keeping track of. There are some other errors with a file called VBC (as opposed to the lower case vbc mentioned earlier).

I've seen this before, and just visiting all the errors caused them to vanish. This is the first time that I've had errors that didn't vanish.

Does anybody have any suggestions as to what is happening here?