I have a .NET 4 website in VS2010 stored in a TFS 2010 team project. I need to add a reference to System.Data.Linq.dll to the website. I am referencing a LINQ DataContext that is defined in another project and I get build errors saying that I need the reference to System.Data.Linq. I go up to the "Add Reference" menu option and add it like I would any normal reference, and it even shows up in the Web.config and in the Properties pages for the website... BUT if I build I still get the same error.

So I found a place in my code where I was referencing the LINQ count function and it told me it was invalid because I was missing a reference and it offered to add the reference automatically. I told it to add the reference automatically and it is at this point that I get the error mentioned in the subject:

TF14040: The folder $/Folder/Subfolder may not be checked out. No items were checked out

I've done some research online but I haven't been able to find much. I saw on a blog that making the folder not readonly fixed the issue for him, but it didn't seem to work for me unless I misunderstood something.

I tried loading up the project from source control onto a fresh computer where that project had never been loaded before and I can reproduce the issue the same way. Help would be greatly appreciated.