Editing private members

Question

Here is my problem guys. 
I have two CS files:

CS1:

namespace BothSame:

.......


CS2:

namespace BothSame:

......

CS1 calls a method in CS2 passing it an object reference.  This object has a private member which I would like to edit in this method (in CS2).

If both CS1 and CS2 are compiled in the same project i can do this easily, no problem.

However when I compile CS2 separately as a DLL and include it in CS1, the method is called fine, however when i go to edit the private member, it won't change.   No exceptions are thrown either.

Any ideas?