Why would I get an err msg about something not being defined or imported, when I have no reference to it in code?

Question

Trying to build a .DLL, I get, "The predefined type 'System.Runtime.CompilerServices.IsVolatile' is not defined or imported"

Yet, a solution-wide search for "IsVolatile" only turns up this:

//Keep around, might prove useful.
//namespace System.Runtime.CompilerServices
//{
//    public sealed class IsVolatile
//    {
//        private IsVolatile()
//        {
//        }
//    }
//}
// use: public volatile bool test;
//               ^
//               |-- volatile allow OS to interpret value. 

As you can see, every reference to "IsVolatile" is commented out. Even when I cut it out altogether, I still get that same err msg. Visual Studio (2003) itself doesn't know where it is, for 2-clicking the err msgs (there are three identical ones) goes nowhere.

What could cause that, and how can I rectify it?