PRB: SetParent Does Not Change Standard Forms into Child Windows (253814)


The information in this article applies to:

  • Microsoft Visual Basic Enterprise Edition for Windows 5.0
  • Microsoft Visual Basic Enterprise Edition for Windows 6.0
  • Microsoft Visual Basic Professional Edition for Windows 5.0
  • Microsoft Visual Basic Professional Edition for Windows 6.0
This article was previously published under Q253814

SYMPTOMS

When you call the SetParent function to allow a standard form to become a child form of a multiple-document interface (MDI) application, many child windowing features do not work.

CAUSE

This problem occurs because, by design, a standard window cannot be completely altered at run time to mimic a child window.

RESOLUTION

To work around this problem, use a dynamic MDI application in place of the SetParent call. ActiveX controls can be dynamically loaded onto a form. An MDI child form can dynamically load an ActiveX control that takes over the entire client area of the window. For more information on this approach, see the "References" section.

MORE INFORMATION

Using the SetParent function to change the ownership of a window in Visual Basic seldom provides the desired results. Therefore, Microsoft does not recommend or support this approach.

Use the sample code in the following steps to examine the problems that occur when you try to use the SetParent call to change a standard form into a child form.

Steps to Reproduce the Behavior

  1. Create a new Standard EXE project in Visual Basic. Form1 is created by default.
  2. On the Project menu, click Remove Form1.
  3. On the Project menu, click Add MDI Form. MDIForm1 is added to Project1.
  4. On the Project menu, click Project1 Properties, and set the Startup object to MDIForm1.
  5. Add the following code behind MDIForm1:
    Private Sub MDIForm_Click()
    Dim x As Class1
    Set x = New Class1
    x.ShowChildForm MDIForm1.hWnd
End Sub
  6. On the File menu, click Add Project. To create a project group, add a new ActiveX DLL project. Class1 is created by default.
  7. Add the following code to Class1:
    Private Declare Function GetWindowLong Lib "user32" _
        Alias "GetWindowLongA" _
        (ByVal hwnd As Long, ByVal nIndex As Long) As Long

Private Declare Function SetParent Lib "user32" _
        (ByVal hWndChild As Long, ByVal hWndNewParent As Long) As Long

Private Declare Function SetWindowLong Lib "user32" _
        Alias "SetWindowLongA" _
        (ByVal hwnd As Long, ByVal nIndex As Long, _
        ByVal dwNewLong As Long) As Long

Const GWL_STYLE = (-16)
Const WS_POPUP = &H80000000

Public Function ShowChildForm(ByVal hWndMDIForm As Long)
    Dim Ret As Long
    Dim CurrentStyle As Long
    
    Ret = SetParent(Form1.hwnd, hWndMDIForm)
    CurrentStyle = GetWindowLong(Form1.hwnd, GWL_STYLE)
    Ret = SetWindowLong(Form1.hwnd, GWL_STYLE, CurrentStyle Or WS_POPUP)
    Form1.Show
End Function
  8. On the Project menu, click Add Form. Form1 is added to Project2.
  9. In the Project Explorer, click Project1 to activate the Project1.
  10. On the Project menu, click References, and add a reference to Project2.
  11. Press the F5 key to run the program.
  12. Click the client area of MDIForm1. Notice that Form1 (from Project2) is now visible in the client area of the MDI form.
The following list includes some of the problems that you may encounter when the SetParent approach is implemented:
  • The title bar of the MDI parent does not show that it is the active window when a form from the dynamic-link library (DLL) has focus.
  • A menu from a form in the DLL does not merge with a menu of the MDI parent form.
  • When a form from the DLL is maximized, the title bar of the DLL form is still visible.
  • When MDI child forms that are part of the MDI project receive focus, they do not appear on top of a form from the DLL.
  • A form from the DLL can be moved over a toolbar that exists in the MDI project.
  • When a DLL form is moved out of the visible area of the MDI form, scroll bars do not appear on the MDI parent.

REFERENCES

For additional information about how to dynamically add intrinsic and ActiveX controls, click the article number below to view the article in the Microsoft Knowledge Base:

190670 HOWTO: Dynamically Add Controls to a Form with Visual Basic 6.0

For additional information about similar problems using the SetParent function to manipulate ActiveX components, click the article number below to view the article in the Microsoft Knowledge Base:

289498 PRB: Form Cannot Get Focus After You Use SetParent to Move Controls to Another Form

Modification Type:MajorLast Reviewed:7/15/2002
Keywords:kbAPI kbDLL kbDSupport kbprb KB253814
©2002 Microsoft Corporation. All rights reserved.