How to get class members without having access to Class type

I have a class (Plug_Comment) with properties which is passed as a member of another class (m_Descriptor)

I can also get the type

Inside I can see the property I need (useless) which is a double

but I can't get to it for the type Plug_Comment is not known here so I can't cast it.

-----EDIT---- I will try to explain that again. Sorry for not having made it clear.