Visual Basic 2012 Programmer's Reference

OVERRIDING WNDPROC

The Windows operating system sends all sorts of messages to applications that tell them about changes in the Windows environment. Messages tell forms to draw, move, resize, hide, minimize, close, respond to changes in the Windows environment, and do just about everything else related to Windows.

All Windows applications have a subroutine tucked away somewhere that responds to those messages. That routine is traditionally called a WindowProc. A Visual Basic .NET form processes these messages in a routine named WndProc. You can override that routine to take special actions when the form receives certain messages.

Example program FixedAspectRatio, which is available on the book’s website, looks for WM_SIZING messages. When it finds those messages, it adjusts the form’s new width and height so they always have the same aspect ratio (ratio of height to width).

WNDPROC WARNING

When you override the WndProc method, it is very important that the new method calls the base class’s version of WndProc as shown in the following statement:

MyBase.WndProc(m)

If the program doesn’t do this, it won’t respond properly to events. For example, the form won’t be able to draw itself correctly, resize or move itself, or even create itself properly.

When you override the WndProc method, you must also figure out what messages to intercept, what parameters those messages take, and what you can do to affect them safely. One way to learn about messages is to insert the following ...