Knowledge Base

BUG: DBGrid on a Modal Form Can Cause a Program to Hang

Article ID: 150206

Article Last Modified on 6/28/2004

APPLIES TO

This article was previously published under Q150206

SYMPTOMS

A program hangs if a DBGrid control is placed on a Modal Form, which itself was invoked from a Modal form from an MDIChild window, and the form containing the DBGrid is then closed. Usually the window receiving the focus after the form with the DBGrid control is closed begins to flash, and does not respond to user input.

STATUS

Microsoft has confirmed this to be an issue in the Microsoft products listed at the beginning of this article. Microsoft is researching this issue and will post new information here in the Microsoft Knowledge Base as it becomes available.

WORKAROUND

Show the form containing the DBGrid in a non-modal fashion, and disable the other forms in the project until the non-modal form is dismissed. The routine below shows a form that appears as a modal form: 
   Public Sub ShowModalForm(frmTarget As Form)

   Dim c As Collection
   Set c = New Collection

   'Disable all the forms
   For Each ofrm In Forms
       If ofrm.Enabled = True Then
       c.Add ofrm
       ofrm.Enabled = False
       End If

   Next ofrm

   'Now show the target form non-modal
   frmTarget.Show

   'If the frmTarget was disabled by the loop above
   '(because it was invisible) make sure it is now enabled

   frmTarget.Enabled = True

   'Sit in a loop until the target form is dismissed
				
   Do While frmTarget.Visible = True
   DoEvents
   Loop

   'FIX: Unload the Form
   UnLoad frmTarget

   'We have left the loop, so the dialog has been closed
   'Now Enable the forms that were disabled, and exit the procedure

   For Each ofrm In c
       ofrm.Enabled = True
   Next ofrm

   End Sub
Because a non-modal form cannot be shown from a modal form, the routine above must also be used to show the form prior to the modal form containing the DBGrid control.

MORE INFORMATION

Steps to Reproduce Problem

  1. Start a new project in the 32-bit edition of Microsoft Visual Basic. Form1 is created by default.
  2. From the Insert menu, select MDI Form to insert an MDI Parent form.
  3. From the Insert menu, select Form to insert two more regular forms into the project.
  4. Choose Form1, and select the Properties window by pressing F4. Change the MDIChild property to True.
  5. Place a Command button in Form1, and in the Click event place the following code: 
       Private Sub Command1_Click()
      Form2.Show vbModal
   End Sub
  6. Place a Command button on Form2, and in the Click event, place the following code: 
       Private Sub Command1_Click()
      Form3.Show vbModal
   End Sub
  7. On Form3, place a DBGrid control and a Data Control. Change the Databasename property of the Data control to "biblio.mdb" in the Visual Basic directory. Change the Recordsource property of the Data control to "Authors." Change the Datasource property of the DBGrid control to "Data1."
  8. Run the project by pressing F5. Press the Command button on Form1 to show Form2 modally. Press the Command button on Form2 to show Form3 modally. Close Form3 using the Control box of the window. Note that Form2 flashes, and the program hangs.
To correct the problem in the Steps to Reproduce Problem section above, paste the ShowModalForm routine contained above into a standard code module, and then change the Command1_Click event in Form1 to: 
   'FIX: Flag to avoid unloading the form when the user closes it
   Private mblnIgnoreUnload as boolean

   'FIX: Additional Code for Form_Load of Form2
   Private Sub Form_Load
       mblnIgnoreUnload = True
   End Sub

   'FIX: Additional Code for Form_QueryUnLoad of Form2
   Private Sub Form_QueryUnLoad
       If mblnIgnoreUnload Then
           Me.Hide ' Make Form2 Invisible
           Cancel = True ' Do not unload Form2
           mblnIgnoreUnload = False ' Next time we come here unload Form2
       End If
   End Sub

   Private Sub Command1_Click()
      ShowModalForm Form2
   End Sub
and change the Command1_Click event in Form2 to: 
   Private Sub Command1_Click()
      ShowModalForm Form3
   End Sub
Now the forms correctly display and act like Modal forms, but the program does not hang when Form3 is dismissed.

Additional query words: 4.00 vb4win vb4all buglist4.00

Keywords: kbbug KB150206


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