PRB: VB Does Not Support Dual Interfaces in OLE Controls

ID: Q151903

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1
- Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 4.0
- Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 4.0

SYMPTOMS

ActiveX Controls that implement a Dual Interface do not use early binding when placed on a Visual Basic form. Even if the Dual Interface is marked as the default interface in the ActiveX Control's .odl file, Visual Basic still uses the standard IDispatch interface for all automation calls.

STATUS

This behavior is by design.

MORE INFORMATION

Dual Interface provides an alternative to using the standard IDispatch interface when making Automation calls. This technique is also referred to as Early Binding because type checking is performed at compile time. Dual Interfaces are rapidly becoming popular because they provide increased performance over the standard IDispatch interface.

It is possible to add Dual Interface support to automation servers as well as ActiveX Controls. The ACDUAL sample provided with Visual C++ 4.1 demonstrates the addition of a Dual Interface to the AutoClick automation server. Tech Note 65, referenced in the References section below, outlines the changes you must make to an automation server to support a Dual Interface. Although ACDUAL and Tech Note 65 refer to automation servers, the information they provide is also applicable to ActiveX Controls.

Visual Basic does support early binding for automation servers that support a Dual Interface, but currently does not support Dual Interface ActiveX Controls. If you attempt to use the Dual Interface on an ActiveX Control in Visual Basic, the standard IDispatch interface is used instead. Future versions of Visual Basic may take advantage of Dual Interface ActiveX Controls, but Visual Basic 4.0 does not.

REFERENCES

ACDUAL Sample - MFC OLE Samples, Visual C++ 4.1

Tech Note 65: Dual-Interface Support for OLE Automation Servers

Additional query words:


Keywords          : kbole kbCtrl kbMFC kbVC 
Version           : winnt:4.0 4.1;win95:4.0,4.1
Platform          : Win95 winnt 
Issue type        : kbprb

Last Reviewed: July 28, 1999