BUG: UserControl Containing Array of Controls Leaks Memory

MORE INFORMATION

Sample code is provided in this article to illustrate both the problem and its workaround.

To view the behavior of both the problem and the workaround, use a system tool that allows you to view available memory. On Windows NT 4.0 or Windows 2000, launch the Task Manager and watch the memory meter on the performance tab while running the application. On Windows 95, Windows 98, or Windows Me, use the Resource Meter utility.

The problem happens in both the development environment and as a compiled application.

Step-by-Step Example

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default.
2. Place a TextBox on Form1. Text1 is created by default. Now, enter the number '3' in the Text property. This value represents the number of times the called Form loads and unloads.
3. Place a CommandButton (Command1) on Form1. Change Commad1's Caption property to "Demonstrate Problem." NOTE: When this sample is completed, clicking this button will demonstrate the problem.
4. Place a second CommandButton (Command2) on Form1. Change Command2's Caption property to "Workaround." NOTE: When this sample is completed, clicking this button will demonstrate the workaround.
5. Add the following code to the Form1 code window:

         Option Explicit
   
         ' This code demonstrates the problem.
   
         Private Sub Command1_Click()
            Dim i As Long, n As Long
            n = Text1.Text
            For i = 1 To n
               Form2.Show
               Unload Form2
               Set Form2 = Nothing
             Next i
         End Sub
   
         ' This code demonstrates the workaround.
         Private Sub Command2_Click()
            Dim i As Long, n As Long
            n = Text1.Text
            For i = 1 To n
               Form3.Show
               Unload Form3
               Set Form3 = Nothing
            Next i
         End Sub
   
   					

6. From the Project menu, click Add User Control (UserControl1).
7. Add three ComboBox controls to UserControl1 as a Control Array. To do this: place a ComboBox from the Toolbox onto UserControl1, creating Combo1. Copy Combo1 (CTRL+C) and then Paste it (CTRL+V). A dialog box will ask if you want to create a Control Array; choose Yes. Then, paste a third ComboBox onto UserControl1. Note that the Properties window now shows the three ComboBox controls as Combo1(0), Combo1(1), and Combo1(2).
8. Add the following code to the UserControl1 code window:

         Option Explicit
         Dim MyCombo As ComboBox
   
         Private Sub UserControl_Initialize()
            Set MyCombo = Combo1(0)
         End Sub
   
         Private Sub UserControl_Terminate()
            MsgBox "Terminate"
         End Sub
   
   					

9. From the Project menu, click Add User Control (UserControl2).
10. Add three ComboBox controls to UserControl2 as a Control Array, as demonstrated in step 7.
11. Add the following code to the UserControl2 code window:

          Option Explicit
          Private MyCombo As ComboBox
    
          Private Sub UserControl_Initialize()
             Set MyCombo = Combo1(0)
          End Sub
    
          Public Sub CleanUp()
             Set MyCombo = Nothing
          End Sub
    
    					

12. From the Project menu, click Add Form (Form2).
13. Place an instance of UserControl1 onto Form2. Note that you must close the UserControl design window before you can place it onto Form2.
14. Add a third form to the project (Form3). From the Project menu, select Add Form.
15. From the Toolbox, place UserControl2 onto Form3.
16. Add the following code to the Form3 code window:

          Option Explicit
    
          Private Sub Form_Unload(Cancel As Integer)
             UserControl21.Cleanup
          End Sub
    
    					

17. Compile and Run your Project.

To illustrate the problem, click on the "Demonstrate Problem" CommandButton. If you are using a system tool that allows you to view available memory, you will see that memory usage increases every time you click the Demonstrate Problem CommandButton. Memory is not recovered after unloading Form2 or after setting Form2 to Nothing. Notice that UserControl1's Terminate event does not fire in this situation (the message box does not display).

To illustrate the workaround, click on the Workaround CommandButton. If you are using a system tool that allows you to view available memory, you will see that memory usage remains the same every time you click the Workaround CommandButton. Memory is recovered after unloading Form3. The workaround here is to release the memory from within UserControl2's CleanUp procedure, which is called during Form3's Form_Unload procedure.