PRB: OLE Servers Don't Unload When Last Object Set to Nothing

ID: Q137031

The information in this article applies to:

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

SYMPTOMS

An OLE Automation server created with Visual Basic fails to unload from memory when the last instance of the object is destroyed.

CAUSE

A Visual Basic client can destroy the last instance of a Visual Basic OLE server it created by:

Setting it to Nothing (the preferred method).

Shutting down by unloading the last form (acceptable, but not as good).

Ending the program by using the End statement (this is not recommended; it can cause problems because the QueryUnload, Unload, and Terminate events don't fire)

If the OLE server has a non-visible form still loaded when the last instance is destroyed, the OLE server is not destroyed. This is usually handled in the Terminate event of the OLE server.

RESOLUTION

OLE servers are responsible for cleaning up any objects that they create or load. When an OLE server is unloaded, the Terminate event will be fired. This is where code should be located to unload any forms that the OLE server has in memory.

STATUS

This behavior is by design.

MORE INFORMATION

Steps to Reproduce Behavior in Visual Basic 4.0

Begin with a minimal OLE server and client application. For information on how to do this, please see the following article in the Microsoft Knowledge Base:
Q129801 How to Create and Use a Minimal OLE Automation Server

Add a form (Form1) to the OLE server.

Add a timer control (Timer1) to Form1.

Set the following properties:


   Object   Property    Value
   --------------------------
   Form1    Visible     False
   Timer1   Enabled     True
   Timer1   Internal    2000

Add the following code to the Timer1_Timer event procedure:


   Private Sub Timer1_Timer ()
      Beep
   End Sub

Add the following code to the General Declarations section of Class1:
```
   Dim frmX As New Form1 
```

Add the following line to the existing MyClass_Initialize event procedure:
```
   Load frmX 
```

Start the OLE server program by pressing the F5 key.

Minimize Visual Basic.

Start a second instance of Visual Basic.

Open (or create) the OLE client application described in Phase 2 of the article referenced in step 1.

Start the OLE client program running by pressing the F5 key.

Click Form1 of the OLE client program. The OLE server is created and begins beeping every two seconds.

Close the OLE client application. Notice that the OLE server continues to beep.

Steps to Resolve the Failure to Unload

With the above OLE server add the following code to the MyClass_Terminate event:


   Private Sub MyClass_Terminate ()
      Unload frmX
   End Sub

When the OLE server is terminated, the Terminate event fires, and the hidden form is unloaded.

Additional query words: 4.00 vb4win vb4all

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