Subclassing and Hooking with Visual Basic

Peering into the Superclassing Application with Spy++

When using Spy++ to analyze this superclassing example application, we see several interesting things. Examining the window and window class information for each window of the application, we notice that all the information from the original class is copied over to the superclass. For example, the ThunderRT6FormDC base class information contains this data:

Class WndProc       6601FFCB
hInstance           66000000
Style               CS_DBLCLKS or CS_OWNDC
Class Extra Bytes   0
Wnd Extra Bytes     2
Menu                (none)
Icon Handle         (none)
Cursor Handle       IDC_ARROW
Bkgnd Brush         COLOR_WINDOW

Using GetClassInfoEx and RegisterClassEx, we made a copy of the ThunderRT6FormDC base class and registered it as the NewMainWndClass superclass. The NewMainWndClass superclass information contains this data:

Class WndProc       004032CE
hInstance           00400000
Style               CS_DBLCLKS or CS_OWNDC
Class Extra Bytes   0
Wnd Extra Bytes     2
Menu                (none)
Icon Handle         (none)
Cursor Handle       IDC_UPARROW
Bkgnd Brush         COLOR_WINDOW

The class name (lpszClassName), the class window procedure (lpfnWndProc), the instance handle (hInstance), and the Cursor Handle (hCursor) were the only class structure members modified. The above information reflects these changes. All other class information is copied into the NewMainWndClass superclass verbatim.

We superclass the BUTTON base class in a similar way, except hCursor is not modified. The global BUTTON base class contains this data:

Class WndProc 77E8BB63 hInstance NULL Style CS_PARENTDC ...