DOCUMENT:Q151903  06-MAY-2001  [visualc]
TITLE   :PRB: VB Does Not Support Dual Interfaces in OLE Controls
PRODUCT :Microsoft C Compiler
PROD/VER:winnt:4.0,4.1
OPER/SYS:
KEYWORDS:kbActiveX kbAutomation kbCOMt kbContainer kbCtrl kbMFC kbVBp400 kbVC400 kbVC410 kbGrpDS

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The information in this article applies to:

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

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Keywords          : kbActiveX kbAutomation kbCOMt kbContainer kbCtrl kbMFC kbVBp400 kbVC400 kbVC410 kbGrpDSVB 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.0,4.1
Issue type        : kbprb

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