In some cases the reference was added clearly, why vs also reported wrong "XXX" does not exist type or namespace (if assembly reference is missing)

Program Main structure: Two assembly DLLs, one Opticalalarm (main program), one Opticalalarm.common

Problem Description: Build the program framework, using the log4net for log processing, after the Opticalalarm.common is written to generate, successful error-free, in the main program reference OpticalAlarm.Common.DLL, run the main program, error.

Resolution: Guessing is the program used by the target framework, which is used by the log4net version. NET is 4.0, check that my program version is . NET 4.0 Client Profile (Lite version of 4.0) modified to 4.0 after the error disappears!

In some cases the reference was added clearly, why vs also reported wrong "XXX" does not exist type or namespace (if assembly reference is missing)