Subclassing and Hooking with Visual Basic

Instance Subclassing: An Example

As we saw in Chapter 3, instance subclassing involves using the SetWindowLongPtr function to replace the window procedure of a specific window instance.

In this section, we’ll write our first simple subclassing application. The application will have two buttons, one to subclass the VB form and another to remove the subclass. The only other control will be a multiline text box that will display messages as they are sent to this form. I will add some more functionality to this application in the example following this one.

Let’s start with the layout of the form. It’s rather simple; the nondefault properties are listed in Table 4-1, and the form is displayed in Figure 4-2.

Table 4-1. Nondefault Properties of Form and Controls for Subclassing Example

Object	Property Name	Property Value
frmCh4	Caption	“Chapter 4 -- Subclassing Example”
frmCh4	ClientHeight	3612
frmCh4	ClientLeft	48
frmCh4	ClientTop	336
frmCh4	ClientWidth	6960
Text1	Height	3372
Text1	Left	1560
Text1	MultiLine	-1 `True’
Text1	ScrollBars	2 `Vertical’
Text1	Top	120
Text1	Width	5292
cmdUnSubclass	Caption	“Un-Subclass”
cmdUnSubclass	Height	372
cmdUnSubclass	Left	120
cmdUnSubclass	Top	600
cmdUnSubclass	Width	1332
cmdSubclass	Caption	“Subclass”
cmdSubclass	Height	372
cmdSubclass	Left	120
cmdSubclass	Top	120
cmdSubclass	Width	1332

Here is how it will work. The text box is initially blank. When you click the ...