Subclassing and Hooking with Visual Basic

A System-Wide Example

This example shows how to set up and install a system-wide WH_SYSMSGFILTER hook. You must place the filter function and the functions that install and remove the hook in a Win32 DLL. First I will show you the code contained in the VB EXE that loads and uses the DLL. Figure 16-2 shows a screenshot of the VB application.

Figure 16-2. Screenshot of the VB EXE

Table 16-2 presents the nondefault properties of the form and its controls.

Table 16-2. Nondefault Properties of the Form and Its Controls

Object	Property Name	Property Value
Form	BorderStyle	3-Fixed Dialog
Form	Caption	“No Hooks Installed”
Form	Top	330
Form	Left	45
Form	Height	1155
Form	Width	4905
Command Button1	Name	cmdHook
Command Button1	Caption	“SYSMSGFILTER”
Command Button1	Top	120
Command Button1	Left	1755
Command Button1	Height	315
Command Button1	Width	1395
Command Button2	Name	cmdUnhook
Command Button2	Caption	“UNHOOK”
Command Button2	Top	660
Command Button2	Left	1755
Command Button2	Height	315
Command Button2	Width	1395

The application contains a SYSMSGFILTER button that installs the system-wide hook and an UNHOOK button that removes the hook. The VB code that Example 16-1 illustrates is placed behind each button in the SysForm2.frm module.

Example 16-1. Event Procedures for the cmdHook and cmdUnhook Buttons

Private Sub cmdHook_Click( ) Call ...