重建/清理构建解决方案后，缺少.dll

I have 3 projects in my solution and a bunch of 3rd party or company dlls. Each time I rebuild my project or clean solution, a lot of this dll's are deleted, therefore missing. It is pretty annoying to reference this dll files again and again after rebuilding. Can someone explain how to avoid this? Thanks in advance.I have 3 projects in my solution and a bunch of