PRB: HWNDs on OLE Controls Are Not Persistent

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

SYMPTOMS

Attempting to access the hWND property when it is invalid results in error 438:

   Object doesn't support this method or property.

CAUSE

OLE controls (such as the Data-Bound Grid, Data-Bound Listbox, and OLE Container control) provide an hWND property, but it is not guaranteed to be valid at every given instance.

RESOLUTION

Unlike previous versions of Visual Basic, where the hWND property was consistently valid for non-lightweight controls as long it was visible, OLE controls are more efficient regarding window usage; therefore, the hWND property may not be always valid.

Because each window, including most controls, in Microsoft Windows requires a certain amount of overhead and system resources, minimizing the number of windows increases the overall performance of the system.

OLE Controls typically have a valid hWND when the control is activated, however if the control is not currently being used, the OLE control frequently releases the hHWND, making the property invalid. Therefore, when using the hWND property of a OLE control, you need to take great care.

It is not advisable to store the hWnd property of OLE controls in a variable for later use because the value may have changed by the time you use it. You should always reference the hWnd of OLE controls by using the <controlname>.hWnd syntax.

STATUS

This behavior is by design.