c#-7.0

I have several projects in the solution, and the C# 7 features, such as tuples and throw expressions, work fine in all of the library projects, but there is a (non Core) web project that doesn’t compile due to errors on the C# 7 features. Right after compiling, the error window quickly clears itself, presumably because the IDE/editor compiles the same units without error. I have to use the output window to see the compiler errors. It is as though the IDE/editor are assuming C# 7, but the compiler used in the build is not.