我从文件->新建->现有项目中打开一个文件夹.

I open a folder from file->new->existing project.

就我而言，您应该打开 .sln 文件而不是文件夹.

As far as I'm concerned you should open a .sln file instead of a folder.

您只能运行 .sln 文件(Visual Studio 解决方案)而不是 .cpp 文件.

You could only run the .sln file (Visual Studio Solution) not a .cpp file.