Q149594: HOWTO: Calling DAO Directly from MFC to Change a User’s Password
Article: Q149594
Product(s): Microsoft C Compiler
Version(s): 4.0 4.1
Operating System(s):
Keyword(s): kbcode kbprogramming kbDAOsearch kbDatabase kbMFC kbVC
Last Modified: 30-JUL-2001
-------------------------------------------------------------------------------
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
-------------------------------------------------------------------------------
SUMMARY
=======
The MFC DAO database classes offer a subset of the Data Access Object Hierarchy.
It is possible to manipulate the entire hierarchy with the help of OLE DAO
interfaces available within the MFC Library. This article offers a demonstration
of how to manipulate these interfaces in order to change the password of a user
object found in the Users collection.
NOTE: Beginning with Visual C++ version 4.2, you will find this information and
more in MFC Technical Note #54 in the Visual C++ online documentation.
MORE INFORMATION
================
The OLE interfaces for each object in the DAO Access Object Hierarchy are
defined in the file found in the \Msdev\Include directory. These interfaces
provide methods that allow you to manipulate the entire DAO Hierarchy. For many
of the methods in the DAO Interfaces, you will need to manipulate a BSTR object
(a length-prefixed string used in OLE Automation). The BSTR object typically is
encapsulated within the VARIANT data type. The MFC class COleVariant itself
inherits from the VARIANT data type.
Depending on whether you build your project for ANSI or UNICODE, the DAO
interfaces will return ANSI or UNICODE BSTRs. There are two macros (V_BSTR and
V_BSTRT) that are useful for assuring that the DAO interface gets the BSTR of
the expected type.
V_BSTR will extract the bstrVal member of a COleVariant. This macro is typically
used when you need to pass the contents of a COleVariant to a method of a DAO
interface. The following code fragment shows both the declarations and actual
use for two methods of DAOUser that take advantage of the V_BSTR macro:
COleVariant varOldName;
COleVariant varNewName( _T("NewUser"), VT_BSTRT );
// Code to assign pUser to a valid value omitted
DAOUser *pUser = NULL;
// These method declarations were taken from Dbdaoint.h
// STDMETHOD(get_Name) (THIS_ BSTR FAR* pbstr) PURE;
// STDMETHOD(put_Name) (THIS_ BSTR bstr) PURE;
DAO_CHECK( pUser->get_Name ( &V_BSTR(&varOldName) ) );
DAO_CHECK( pUser->put_Name ( V_BSTR (&varNewName) ) );
NOTE: VT_BSTRT is required in the constructor of COleVariant as described in the
following article in the Microsoft Knowledge Base:
Q140599 PRB: MFC DAO Functions Accepting COleVariant Strings May Fail
V_BSTRT will extract either a ANSI or UNICODE bstrVal member of COleVariant
depending on the type of build (ANSI or UNICODE). The following code
demonstrates how to extract the BSTR value from a COleVariant into a CString:
COleVariant varName( _T( "MyName" ), VT_BSTRT );
CString str = V_BSTRT( &varName );
The V_BSTRT macro, along with other tricks to crack open other types stored
within COleVariant, is demonstrated in the DAOView sample (DAOView is included
with Visual C++ 4.x). Specifically, this translation is performed in the
CCrack::strVARIANT method. This method, where possible, translates the value of
a COleVariant into an instance of CString.
The "Sample Code" section of this article uses these techniques to demonstrate
calling the DAO OLE interfaces to change a user's password. This is done by
manipulating two interfaces in particular: DAOUser and DAOUsers. DAOUsers is a
collection of user objects, and like many of the collections inherits from the
_DAODynaCollection interface. This interface offers two methods:
- Append
- Delete
In addition, the _DAODynaCollection inherits from _DAOCollection. This interface
offers three methods:
- get_Count
- Refresh
- _newEnum
This provides a common set of methods for DAO collection interfaces.
Finally, in order to change the user's password in this example, you will need to
create a user named NewUser with a password of PassWord. The easiest way to do
this is to use Microsoft Access. Note that the default password for a newly
created user in Microsoft Access is blank. You will need to log in as that user
in order to set the password. For more information, please see the Microsoft Jet
Database Engine Programmer's Guide, Chapter 10, "Managing Security."
Sample Code
-----------
// The following code presents several functions you can use to manage
// MFC DAO classes in a way that allows you to call the DAO interfaces.
// It then shows how to use those functions within InitInstance().
// Set the system database that the DAO database engine will use:
void AfxDaoSetSystemDB(CString & strSystemMDB)
{
COleVariant varSystemDB(strSystemMDB,VT_BSTRT );
// Initialize DAO for MFC:
AfxDaoInit();
DAODBEngine* pDBEngine = AfxDaoGetEngine();
ASSERT(pDBEngine != NULL);
// Call put_SystemDB method to set the system database for DAO engine:
DAO_CHECK( pDBEngine->put_SystemDB( varSystemDB.bstrVal ) );
}
void AfxDaoSetDefaultUser(CString & strUserName, CString & strPassword)
{
COleVariant varUserName( strUserName, VT_BSTRT );
COleVariant varPassword( strPassword, VT_BSTRT );
DAODBEngine* pDBEngine = AfxDaoGetEngine();
ASSERT(pDBEngine != NULL);
// Set default user:
DAO_CHECK( pDBEngine->put_DefaultUser(varUserName.bstrVal));
// Set default password:
DAO_CHECK( pDBEngine->put_DefaultPassword(varPassword.bstrVal));
}
void AfxDaoChangePassword(CString & strUserName, CString &
strOldPassword, CString & strNewPassword)
{
// Create(Open) Workspace:
CDaoWorkspace wsp;
CString strWspName = _T("Temp Workspace");
wsp.Create( strWspName, strUserName, strOldPassword );
wsp.Append();
// Determine how many objects there are in the Users collection:
short nUserCount;
short i;
DAOUser *pUser = NULL;
DAOUsers *pUsers = NULL;
// Side-effect is implicit OLE AddRef() on DAOUser object:
DAO_CHECK( wsp.m_pDAOWorkspace->get_Users( &pUsers ) );
// Side-effect is implicit OLE AddRef() on DAOUsers object:
DAO_CHECK( pUsers->get_Count( &nUserCount ) );
TRACE( "# of users in collection = %d\n", nUserCount );
// Traverse through the list of users and change password for userid
// used to create/open the workspace:
for( i = 0; i < nUserCount; i++ )
{
COleVariant varIndex( i, VT_I2 );
COleVariant varName;
// Retrieve information for user i
DAO_CHECK( pUsers->get_Item( varIndex, &pUser ) );
// Retrieve name for user i
DAO_CHECK( pUser->get_Name( &V_BSTR(&varName)) );
TRACE( "User # %d Name = %s\n", i, V_BSTRT(&varName) );
CString strTemp = V_BSTRT(&varName);
// If there is a match, change the password:
if( strTemp == strUserName )
{
COleVariant varOldPwd( strOldPassword, VT_BSTRT );
COleVariant varNewPwd( strNewPassword, VT_BSTRT );
DAO_CHECK( pUser->NewPassword( V_BSTR( &varOldPwd ),
V_BSTR( &varNewPwd ) ) );
TRACE( "\t Password is changed\n" );
}
}
// Clean up: decrement the usage count on the OLE objects:
pUser->Release();
pUsers->Release();
wsp.Close();
}
BOOL CMyApp::InitInstance()
{
// Specify path to the Microsoft Access system database:
CString strSystemDB =
_T("c:\\Program Files\\MSOffice\\access\\System.mdw");
// Set system database before MFC initilizes DAO:
AfxDaoSetSystemDB(strSystemDB);
// User name and password manually added by using Microsoft Access:
CString strUserName = _T("NewUser");
CString strOldPassword = _T("Password");
CString strNewPassword = _T("NewPassword");
// Set default user so that MFC will be able to log in by default
// using the user name and password from the system database:
AfxDaoSetDefaultUser(strUserName,strOldPassword);
// Change the password. You should be able to call this function from
// anywhere in your MFC application:
ChangePassword(strUserName,strOldPassword,strNewPassword);
// Remainder of your InitInstance code:
...
}
REFERENCES
==========
Microsoft Jet Database Engine Programmer's Guide
ISBN# 1-55615-877-7
Chapter 10 "Managing Security"
- Tech Note #54, "Calling DAO Directly While Using MFC DAO Classes."
- DAOVIEW Sample that ships with Visual C++ 4.x.
- The following articles in the Microsoft Knowledge Base:
Q146876 PRB: DAO Error 3028 When Creating a DAO Workspace
Q140599 PRB: MFC DAO Functions Accepting COleVariant Strings May Fail
Q143408 BUG: Assertion Failure After Using CDaoWorkspace::Create()
Additional query words:
======================================================================
Keywords : kbcode kbprogramming kbDAOsearch kbDatabase kbMFC kbVC
Technology : kbAudDeveloper kbMFC
Version : 4.0 4.1
Issue type : kbhowto
=============================================================================
THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.
Copyright Microsoft Corporation 1986-2002.