Keep a Window Visible
Prevent users from moving windows offscreen.
by Karl E. Peterson

September 2002 Issue  Download the original Classic VB code from this column.

Technology Toolbox: VB6, VB5

Q: Keep Your Window Visible
How can I prevent a user from moving a window off the screen? I'd like to allow users to move a window around the desktop at will, but not allow them to move any part of the window off the desktop. The entire window must be viewable at all times.

A:
Hopefully, this is a client specification and not a method to keep some sort of popup advertisement perpetually in someone's face. (Online, a <g> would probably follow, but irritating users is rarely funny.) Assuming a legitimate need, the simple answer is that you'd want to hook your form's message stream and respond to incoming WM_MOVING messages (see Additional Resources). If you don't have a favorite drop-in subclassing module, I'd urge you to grab HookMe.zip from my Web site or this column's sample code.

Windows sends WM_MOVING messages to a window immediately prior to the user getting any feedback. These messages are accompanied by a pointer to a RECT structure in lParam that contains the drag rectangle coordinates Windows displays to the user. You're only given a pointer, so you need to copy the data at this address to a RECT structure declared within your hook procedure (see Listing 1).

At this point, you're free to examine the RECT coordinates and even modify them to suit your needs. In this case, you'd want to ensure that none of the edges go past the edge of the screen, and if they do, correct them to remain onscreen. After any necessary modifications, copy the updated structure back to the same address passed in lParam and tell Windows you've handled the message by returning True for the function result. K.E.P.


Updated Code Samples
  HookMe

About the Author
Karl E. Peterson is a GIS analyst with a regional transportation-planning agency and serves as a member of the Visual Studio Magazine Technical Review and Editorial Advisory Boards. Online, he's a Microsoft MVP and a section leader on several VSM forums. Find more of Karl's VB samples at www.mvps.org/vb.