SetParent API

Here's a fun one.  

We have a very large application that we need to partition out.  The application's interface is MDI.  The idea is to break each functional area out into smaller DLL's, where each DLL would (for the most part) correspond to a single screen (we have almost 200 screens).  The benefits of partitioning the application into DLLs provides us with better manageability, smaller EXE size and lower resource utilization.

Using VB5, each screen will be displayed modeless with the goal of the window being treated as an MDI child.  To accomplish this we are using the SetParent API call.  When the child screen is displayed, SetParent is invoked with the handle of the MDI parent form.

This works great - until we try to drag a window or click on its caption.  What happens next is the confusing part.  When we click on the title bar, the child form "jumps" to a lower-right offset.  We can drag the form around but the mouse pointer does not actually touch the child form’s title bar.  Once the mouse button is release, the child form "jumps" back to where it would be had everything worked correctly.

I have determined that the coordinate system of the MDI parent is not being assigned to the new child window.  Instead, the child window seems to retain the coordinate system of the desktop (the previous parent).

Does anyone know how to correct this behavior?

Incidentally, the Parent form does not need to be an MDI form.  You can duplicate this behavior with two normal forms, too.  Also, for testing, you do not need to create a DLL.  Simply create two forms in your project and use the following (pseudo)code.


Form1:
=====
Private Sub Form_Load()
     Form2.Load
End Sub

Form2:
=====
Private Sub Form_Load()
     Call SetParent(Form2.Hwnd, Form1.hWnd)
End Sub

Thanks!