how to display Window property inside Dialog Text box using DLL

Here is the problem: how come you don't have a pointer to the instance of MFC CWnd (or whatever you use)?

Certainly, as CWnd installs it's Window procedure for handling raw Windows messages, it should keep "this" pointer in raw Windows API data, which is usually done by UI libraries build on Windows API. This is the way to keep mapping between the class instance with OOP-library and raw Window data. But — if this is your application, you already have this pointer; if not — getting this pointer is not helpful, because you cannot safely cast it to the MFC class you might assume, as nothing can guarantee that the window was build using MFC; and MFC of the same version as you use.

Also, do understand that Windows API registered Window class of the class derived from any MFC class are two absolutely different things.

Now, if you need just the window text, you can use GetWindowText, see http://msdn.microsoft.com/en-us/library/ms633520%28v=vs.85%29.aspx[^].

[EDIT]
To extract other information from the raw Window handle, use the function GetWindowLong, see http://msdn.microsoft.com/en-us/library/ms633584(v=vs.85).aspx[^]. Potentially the pointer to the instance of the UI library class can be stored at the index GWL_USERDATA, but nothing can guarantee you any certain interpretation of this pointer or valid type cast you can use if this is not your application.

—SA