This is because of smaller compilation modules in C/C++. In C/C++, each .c/.cpp file is compiled separately, creating an .obj module. Thus the compiler needs the information about types and variables, declared in other compilation modules. This information is supplied in form of forward declarations, usually in header files.
C#, on the other side, compiles several .cs files into one big compilation module at once.
In fact, when referencing different compiled modules from a C# program, the compiler needs to know the declarations (type names etc.) the same way as C++ compiler does. This information is obtained from the compiled module directly. In C++ the same information is explicitly separated (that's why you cannot find out the variable names from C++-compiled DLL, but can determine it from .NET assembly).
