PRB: Setting Form to Nothing Does Not Unload Form

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic, 16-bit and 32-bit, for Windows, version 4.0

SYMPTOMS

If you use the following code to set a Form variable to nothing, the form does not unload:

   Set Form1 = Nothing

CAUSE

Setting a form to nothing releases the handle or reference that Visual Basic has to the form, but it does not cause the underlying form to be unloaded. In fact, if you set a form to nothing, this invalidates the form variable as a reference to the form so that later calls such as this fail:

   Unload Form1

RESOLUTION

Use the Unload statement to explicitly unload your forms.

STATUS

This behavior is by design.

MORE INFORMATION

The following syntax is perfectly valid for OLE Automation servers and objects that are created in Visual Basic:

   Dim Obj As Object
   Set Obj = Nothing

Rules for Instantiating, Loading, Unloading, and Uninstantiating Forms

The rules for instantiating, loading, unloading, and uninstantiating are as follows:

A form is instantiated when one of the following occurs:

• It is used in a Set New statement such as Set x = New Form1. However, note that Dim As New does not cause instantiation.
• Any member of the form is referenced.
• It is passed as an argument.

• The Load statement is used on it.
• One of the built-in (not user-defined) properties or methods is referenced.
• A property or method of a control or a control array on the form is referenced.
• A control that is a member of a control array is referenced.
• It is the MDI form, and a child form is loaded (this causes a show of the MDI form too).

• The Unload statement is used on it.

• All references to it or any controls or control arrays on it are released, AND it is unloaded.

The Initialize event occurs when a form is instantiated. The Load event occurs when it is loaded. The Unload event occurs when it is unloaded, and the Terminate event occurs when it is uninstantiated.

Steps to Reproduce Behavior

1. Start a new project in Visual Basic. Form1 is created by default.

2. Add a Module (Module1.BAS) to your Project.

3. Add the following code to the General Declarations section of Module1.Bas:

   Global FormVar As Form

      Sub Main()
         Set FormVar = New Form1
         Form1.Show
         Load FormVar
      End Sub
   
   

4. Choose Options from the Tools menu. Choose the Project tab on the dialog and change the Startup Form to Sub Main.

5. Add three Command buttons (Command1, Command2, and Command3) to Form1.

6. Add the following code to the appropriate procedures:

      Private Sub Command1_Click()
         Set Form1 = Nothing
         Set FormVar = Nothing
         Unload Form1
         ' Unload FormVar
         ' The above line generates an Error if executed.
            ' Error 91 is now:
            ' "Object variable or With block variable not set."
         Msgbox Forms.Count & " forms are still loaded!"
      End Sub
   
      Sub Command2_Click ()
         For I = Forms.Count-1 to 0 Step -1
            Unload Forms(I)
         Next
      End Sub
   
      Sub Command3_Click ()
         Unload Form1
         Set Form1 = Nothing
         Unload FormVar
         Set FormVar = Nothing
         Msgbox Forms.Count & " forms are still loaded!"
      End Sub
   
   

7. Start the program by choosing Start from the Run menu or by pressing the F5 key.

8. Click the Command1 button. Note that although Form1 has been set to Nothing and explicitly unloaded on the next line, the form does not unload.

9. Click the Command2 button to unload Form1 via the Forms collection.