Microsoft KB Archive/143330

= FIX: Messagebox Shows in DDX_FieldCBString & DDX_FieldLBString =

Article ID: 143330

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 Professional Edition

 Microsoft Visual C++ 5.0 Learning Edition 

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This article was previously published under Q143330



SYMPTOMS
When you map a control to a recordset variable by using DDX_FieldCBString or DDX_FieldLBString, the following message may appear in a messagebox when UpdateData is called:

Please enter no more than 0 characters.



CAUSE
The DDX_FieldCBString and DDX_FieldLBString functions incorrectly check the length of the destination CString variable. The problem exists on lines 420 and 509 in Daoview.cpp. The code is performing the following check; if (nLen > value.GetAllocLength) ... It is possible that the buffer for the string has been emptied by a call to AddNew or SetFieldNull. This would cause the CString value to be empty, which causes the messagebox to be displayed.



RESOLUTION
Create a new function by copying the DDX_FieldCBString or DDX_FieldLBString from Daoview.cpp located in the \Msdev\Mfc\Src directory. In the new function, add an additional parameter to be used as the maximum allowable length (the size of the field). You'll then need to replace the DDX call in your DoDataExchange function with the new function you created.

For the DDX_FieldCBString function, change the function prototype to something like this: void DDX_MyFieldCBString(CDataExchange* pDX, int nIDC, CString& value,  CDaoRecordset* pRecordset, int nMaxFieldLength) Replace these lines: if (nLen > value.GetAllocLength) AfxFailMaxChars(pDX, value.GetAllocLength); // get known length ::GetWindowText(hWndCtrl, value.GetBuffer(0), nLen+1); with these lines: if (nLen > nFieldLength) AfxFailMaxChars(pDX, nFieldLength); // get known length ::GetWindowText(hWndCtrl, value.GetBuffer(nLen), nLen+1); Note that GetBuffer is no longer receiving 0 but is using the value in nLen.

For the DDX_FieldLBString function, change the prototype to: void DDX_MyFieldLBString(CDataExchange* pDX, int nIDC, CString& value,  CDaoRecordset* pRecordset, int MaxFieldLength) Replace these lines: if (nLen > value.GetAllocLength) AfxFailMaxChars(pDX, value.GetAllocLength); ::SendMessage(hWndCtrl, LB_GETTEXT, nIndex,                (LPARAM)(LPSTR)value.GetBuffer(0)); with these lines: if (nLen > nFieldLength) AfxFailMaxChars(pDX, nFieldLength); ::SendMessage(hWndCtrl, LB_GETTEXT, nIndex,               (LPARAM)(LPSTR)value.GetBuffer(nLen)); You will need to include a prototype for AfxFailMaxChars in your code. Here's the prototype: void _stdcall AfxFailMaxChars(CDataExchange* pDX, int nChars);

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STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 4.1.

Additional query words: kbVC400bug

Keywords: kbbug kbdatabase kbfix kbvc410fix KB143330

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