Strange Error - CS0012: The type x is defined in an assembly that is not referenced

Question

The type \'x\' is defined in an assembly that is not referenced. You must add a reference to assembly \'abc123\'.

I have a .NET 2.0 web application that references m

Answer 1

I had the exact same error, but I had a public constructor in my class that used as a parameter, an object from another project.

I resolved the problem by making that constructor internal.