Microsoft KB Archive/140599

= PRB: MFC DAO Functions Accepting COleVariant Strings May Fail =

Article ID: 140599

Article Last Modified on 11/21/2006

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APPLIES TO

 Microsoft Foundation Class Library 4.2, when used with:  Microsoft Visual C++ 4.0 Standard Edition

 Microsoft Visual C++ 4.1 Subscription

 Microsoft Visual C++ 4.2 Enterprise Edition

 Microsoft Visual C++ 4.2 Professional Edition</li></ul>

 Microsoft Visual C++ 4.2 Enterprise Edition</li></ul>

 Microsoft Visual C++ 4.2 Professional Edition</li></ul>

 Microsoft Visual C++ 5.0 Enterprise Edition</li></ul>

 Microsoft Visual C++ 5.0 Professional Edition</li></ul>

 Microsoft Visual C++ 6.0 Enterprise Edition</li></ul>

 Microsoft Visual C++ 6.0 Professional Edition</li></ul>

 Microsoft Visual C++ 6.0 Standard Edition</li></ul> </li></ul>

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<div class="notice_section">

This article was previously published under Q140599

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SYMPTOMS
The MFC DAO Classes contain methods that accept COleVariant arguments. For example, CDaoRecordset::Seek, CDaoRecordset::SetFieldValue, and CDaoRecordset::SetBookMark accept COleVariant arguments. You may find that these functions do not perform correctly if you are building a non-UNICODE build of your application and you are assigning a string to a COleVariant and passing the variant to one of these methods.

For example, look at the following code, which is shown in MFC Technote #53: COleVariant varFieldValue; rs.AddNew; varFieldValue = _T("MSFT"); rs.SetFieldValue(_T("Customer_ID"), varFieldValue); varFieldValue = _T("Microsoft"); rs.SetFieldValue(_T("Customer_Name"), varFieldValue); rs.Update; This code causes only the first letters of each string to be copied to the database with a non-UNICODE build. In this case, the letter M will be copied to both fields.

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CAUSE
MFC creates ANSI DAO objects when building non-UNICODE builds and creates UNICODE DAO objects when building UNICODE builds. For example, look at the following code in AfxDaoInit: GUID guidEngine; guidEngine = IID_IDAODBEngineW; guidEngine = IID_IDAODBEngine; The ANSI DAO objects require ANSI BSTRs for any strings that they receive. When you initialize a COleVariant object using: COleVariant varFieldValue(_T("some string")); -or- COleVariant varFieldValue; varFieldValue = _T("MSFT"); The COleVariant will contain a UNICODE BSTR regardless of whether the build is UNICODE or non-UNICODE. This is not what DAO expects.
 * 1) ifdef _UNICODE
 * 1) else
 * 1) endif

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RESOLUTION
There are two techniques you can use to initialize the COleVariant properly. If you are only concerned about setting the BSTR value of the COleVariant at construction time, use this code: COleVariant varFieldValue(_T("SomeString"), VT_BSTRT); The VT_BSTRT constant specifies that the BSTR will be a UNICODE BSTR with a UNICODE build and it will be a ANSI BSTR in a non-UNICODE build.

If you need to change the value of the variant often, you may want to create a helper function: void FillVariant(COleVariant & var, LPCTSTR lpszSrc) {       var.vt=VT_BSTR; if (lpszSrc==NULL) var.bstrVal=NULL; else {   #ifndef _UNICODE int nLen = lstrlen(lpszSrc); var.bstrVal = SysAllocStringByteLen(lpszSrc, nLen); #else var.bstrVal = SysAllocString(lpszSrc); #endif }   } With Visual C++ 4.2 or higher, COleVariant contains a SetString function that can be used instead of the FillVariant function above.

<div class="status_section">

STATUS
This behavior is by design.

Additional query words: Seek SetFieldValue SetCacheStart SetBookMark SetParamValue

Keywords: kbdatabase kbprogramming kbprb KB140599

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