我有同样的问题，并通过确保其他项目引用的每个项目都包含在解决方案中来解决。

I had the same problem, and solved it by making sure that every project referenced by other projects were included in the solution.

右键单击第一个项目，选择引用...，并确保包含引用下列出的每个项目。去下一个，做同样的。

Right-click on the first project, choose References... and make sure every project listed under References are included. Go to the next and do the same.

这解决了我的问题项目文件已被重命名或不再在解决方案。不知道A build is already in progress的问题。

That solved my problem with "The Project file '' has been renamed or is no longer in the solution". Not sure about the problem with "A build is already in progress".