qobject_cast failed if an object is created by a library and cast in another project.

I have built a solution with visual studio 2015. it's has server projects, one main project(we call A project) which generate a exe file, another one （we call B Library）is a dynamic library which is depend by A project.

i have defined an base class C1 (inherit form QObject) in a header file.then in B library i inherit the base class(name it C2).

the problem is if the object is created in B library, and A project get the object use the interface B explored, then use qobject_cast cast the object to C1 will return NULL. but succeed if use dynamic_cast.