HOWTO: Using the DAO SDK dbDao Classes with Visual C++ 4.x (149392)

MORE INFORMATION

Compile and Link Settings

1. After installing the DAO SDK (found on the Visual C++ 4.x CD-ROM), modify settings in the Visual C++ Developer Studio to use the directories where the DAO SDK is installed. On the Tools menu, click Options. Then add the Daosdk\Include directory to the list of directories for include files. For example, if you used the Visual C++ defaults, you need to add the following directory:

   C:\MSDEV\DAOSDK\INCLUDE

   Failure to do so will generate this error:
   fatal error C1083: Cannot open include file: 'dbdao.h':
   No such file or directory
   In addition, add the Daosdk\Lib and Daosdk\Lib\Debug entries to the list of directories for library files. The first directory contains ANSI and UNICODE builds of the dbDao library, and the second contains debug versions of each. Failure to add these libraries will generate an error similar to this one:
   LINK : fatal error LNK1104: cannot open file "dbdao3.lib"
2. Copy the debug ANSI and UNICODE versions of the .dll and .pdb files found in the \Msdev\Daosdk\Lib\Debug directory into your \Windows\System directory. This will allow you to step into the dbDao library in debug builds.
3. Add one of the following libraries to the "Object/Library Modules" entry in the Link tab found in the Project Settings dialog invoked by clicking Settings on the Build menu:

   dbdao3.lib - ANSI Release export library
   dbdao3d.lib - ANSI Debug export library
   dbdao3u.lib - UNICODE Release export library
   dbdao3ud.lib - UNICODE Debug export library

4. Link with the MFC Library. On the Build menu, click Settings, and then click the General tab to do this. Failure to do so will cause LNK2001 unresolved external error messages on the CdbDBEngine and CdbOleObject symbols.
   
   If you are creating a console application, you will still have to link to MFC. MFC wasn't designed, tested, or intended to be used in a true console application. However, dbDao uses only a few, high-level abstract base classes such as CObject, CException, and COleVariant.
   
   Be certain to select "Use MFC in a shared DLL" when specifying how to link with the MFC Library. If you link in MFC statically, you will observer the following sequence of errors if you use a dbDao function that returns an MFC object (e.g. CDbDatabase.GetConnect() returns a CString). Note: The notation "..." indicates a hex address in the following error messages.
   
   DEBUG build:
   
   
   • A message box displaying: "User breakpoint called from code at 0x..."
   • A trace message stating: "HEAP: Invalid address specified to RtlValidateHeap(...)"
   • An assertion in dbgheap.c
   • An unhandled exception: Access Violation
   RELEASE build:
   • A message box displaying: "The instruction at "0x..." referenced memory at "0x...". The memory could not be "read".
5. For console-based applications, link with the multi-threaded version of the C-Run-time Library. This is done in the project settings dialog box. On the Build menu, click Settings. Then click the C/C++ tab, and in the Code Generation category select "Use run-time library." Failure to do so will generate an unresolved external error on __endthreadex.

Using dbDao in Your Code

For any file using the dbDao classes, use the following three include files:

   #include <afxole.h>
   #include <dbdao.h>
   #include <dbdaoerr.h>

Failure to do so will generate the following errors (among others) in Dbdao.h: When an error is encountered, the dbDao classes will throw an instance of the CdbException class. This is really just a flag, the actual error information is included in the Errors collection of the CdbDBEngine class. The following code demonstrates how to extract this information for each error object in the collection. The collection is refreshed each time dbDao encounters an error.

dbDao may also throw either a class derived from MFC's CException or a win32 exception. The code demonstrates how to catch those as well.

   CdbDBEngine *pEng;

   // Code to instantiate pEng goes here

   try
   {
      // Code to use dbDao in a way that generates an error goes here
   }
   catch( CdbException e )
   {
       for( long l = 0; l < pEng->Errors.GetCount(); l++ )
       {
           TRACE( "\t Error #%ld: #%ld -- %s\n",
                  l,
                  pEng->Errors[l].GetNumber(),
                  pEng->Errors[l].GetDescription() );
       }
   }
   catch(CException *e)
   {
       TRACE( "MFC Exception\n" );
       e->Delete();
   }
   catch(...)
   {
       TRACE( "Win32 Exception\n" );
   }

Information on Redistributing dbDao

The following sources discuss how to redistribute components required by applications using dbDao. These can be found either on your Visual C++ 4.X CD-ROM or in the installed files on your hard disk.

• \Msdev\Redist\Dao\Readme.wri (located on the CD-ROM only)
• \Msdev\Daosdk\Daosdk.hlp
• \Daosdk\Setup\Daosetup.rtf

Supported Platforms

The only supported form of DAO for Visual C++ is a 32-bit in-process DLL requiring either Windows 95 or Windows NT and Visual C++ 4.0 or later. DAO is not available on Win32s. This is documented in the Daosdk.hlp file under the "DAO SDK Files" help topic.