Quote Add after lifetime achievement error, hint reference does not exist

These two days to write a WinForm program, code or something is correct, but from the quote does not exist, and then remove the reference and then add the correct, a "regenerate" and then continue the error, good tangled, have never encountered such a. Baidu does not have a result, today suddenly think of what version may be wrong, open the program's properties a look, sure enough, the framework version is not correct,:

It's clear. Take this as a ring!

Quote Add after lifetime achievement error, hint reference does not exist