Base type is not CLS-compliant, what reasons of this warning?

I guess you have a derived type marked as CLSCompliant but the base isn't.

Writing CLS Compliant Code The canonical example is using a UInt32 which is not part of the Common Language Specification (CLS) - hence you need to use Int64 to be CLS compliant or remove the attribute (declare yourself non-cls compliant).

Need more code to identify the offending line of code. More info on the error you're getting.

You probably have [assembly:CLSCompliant(true)] somewhere in that specific project. This triggers the compiler to check all types to be CLS compliant. You can override this for a type or method or something with [CLSCompliant(false)].