I have a solution in VS 2008 that creates a DLL. I then use that DLL in another application. If I go in to the DLL projects property pages and change the following configuration for a DEBUG build then the built dll no long provides the desired functionality. If I change it back and rebuild the DLL, then the DLL does provide the correct functionality:

Property Pages => Configuration Properties => C/C++ => Code Generation => Runtime Library

If set to "Multi-threaded Debug DLL (/MDd)" then everything works as it should. I get the correct functionality from the DLL

If set to "Multi-threaded DLL (/MD)" then the DLL does not function properly...no runtime errors or anything, it just doesn't work (The DLL is supposed to plot some lines on a map but does not in this mode).

So the question is, why does using the /MDd flag result in correction functionality of the underlying code, while /MD results in incorrect functionality?

A little background...somebody else developed the DLL in C++ and I am using this DLL in a VB.net application.