Regular DLLs allows us to use MFC in the exported functions, but they do not require that the calling application should also use MFC. Thus regular DLL can be EASILY used in programs developed in other Windows programming languages like Visual Basic or Delphi. A big restriction of regular DLL is that it cannot export C++ classes. This restriction has been lifted in an Extension DLL. In an Extension DLL we can export classes. This forces the client application to be MFC application. The client can construct OBJECT of the exported class or derive classes from it. An extension DLL should meet two requirements:

1. An extension DLL dynamically links to the code in the MFC DLL. Hence an extension DLL requires that the client program be dynamically linked to the MFC LIBRARY.
2. Both the client program and the extension DLL be synchronized to the same version of the MFC DLLs.

Example:

Class AFX_EXT_CLASS CMyAboutBox: public CDialog
{
Public : void ShowDialog(void);
};

Regular DLLs allows us to use MFC in the exported functions, but they do not require that the calling application should also use MFC. Thus regular DLL can be easily used in programs developed in other Windows programming languages like Visual Basic or Delphi. A big restriction of regular DLL is that it cannot export C++ classes. This restriction has been lifted in an Extension DLL. In an Extension DLL we can export classes. This forces the client application to be MFC application. The client can construct object of the exported class or derive classes from it. An extension DLL should meet two requirements:

Example:

Class AFX_EXT_CLASS CMyAboutBox: public CDialog
{
Public : void ShowDialog(void);
};