Implementing a Simple Consumer

The ATL OLE DB Consumer wizard is not available in Visual Studio 2019 and later. You can still add the functionality manually. For more information, see Creating a Consumer Without Using a Wizard.

The following topics show how to edit the files created by the MFC Application Wizard and ATL OLE DB Consumer Wizard to create a simple consumer. This example has the following parts:

• Retrieving Data with the Consumer shows how to implement code in the consumer that reads all the data, row by row, from a database table.

• Adding Bookmark Support to the Consumer shows how to add bookmark support to the consumer.

Retrieving Data with the Consumer

To modify the console application to use the OLE DB consumer

1. In MyCons.cpp, change the main code by inserting the bold text as follows:

   // MyCons.cpp : Defines the entry point for the console application.
   //
   #include "pch.h" // "stdafx.h" in Visual Studio 2017 and earlier
   #include "Products.h"
   ...
   int main(int argc, char* argv[])
   {
      HRESULT hr = CoInitialize(NULL);   // Instantiate rowset
      CProducts rs;
      hr = rs.OpenAll();
      ATLASSERT(SUCCEEDED(hr ) );
      hr = rs.MoveFirst();   // Iterate through the rowset
      while(SUCCEEDED(hr) && hr != DB_S_ENDOFROWSET )   {      // Print out the column information for each row
        printf("Product ID: %d, Name: %s, Unit Price: %d, Quantity per Unit: %d, Units in Stock %d, Reorder Level %d\n",
          rs.m_ProductID, rs.m_ProductName, rs.m_UnitPrice, rs.m_QuantityPerUnit, rs.m_UnitsInStock, rs.m_ReorderLevel );
        hr = rs.MoveNext();   }
      rs.Close();
      rs.ReleaseCommand();
      CoUninitialize();
   
      return 0;
   }
   

Adding Bookmark Support to the Consumer

A bookmark is a column that uniquely identifies rows in the table. Typically it's the key column, but not always; it's provider-specific. This section shows you how to add bookmark support. To do so, you need to do the following steps in the user record class:

• Instantiate the bookmarks. These are objects of type CBookmark.

• Request a bookmark column from the provider by setting the DBPROP_IRowsetLocate property.

• Add a bookmark entry to the column map by using the BOOKMARK_ENTRY macro.

The previous steps give you bookmark support and a bookmark object with which to work. This code example demonstrates a bookmark as follows:

• Open a file for writing.

• Output rowset data to the file row by row.

• Move the rowset cursor to the bookmark by calling MoveToBookmark.

• Output the bookmarked row, appending it to the end of the file.

To instantiate the bookmark

1. The accessor needs to hold an object of type CBookmark. The nSize parameter specifies the size of the bookmark buffer in bytes (typically 4 for 32-bit platforms and 8 for 64-bit platforms). Add the following declaration to the column data members in the user record class:

   //////////////////////////////////////////////////////////////////////
   // Products.h
   class CProductsAccessor
   {
   public:
      CBookmark<4> m_bookmark;   // Add bookmark declaration
      LONG m_ProductID;
      ...
   

To request a bookmark column from the provider

1. Add the following code in the GetRowsetProperties method in the user record class:

   // Set the DBPROP_IRowsetLocate property.
   void GetRowsetProperties(CDBPropSet* pPropSet)
   {
      pPropSet->AddProperty(DBPROP_CANFETCHBACKWARDS, true, DBPROPOPTIONS_OPTIONAL);
      pPropSet->AddProperty(DBPROP_CANSCROLLBACKWARDS, true, DBPROPOPTIONS_OPTIONAL);
      // Add DBPROP_IRowsetLocate property to support bookmarks   pPropSet->AddProperty(DBPROP_IRowsetLocate, true);
   }
   

To add a bookmark entry to the column map

1. Add the following entry to the column map in the user record class:

   // Set a bookmark entry in the column map.
   BEGIN_COLUMN_MAP(CProductsAccessor)
      BOOKMARK_ENTRY(m_bookmark)   // Add bookmark entry
      COLUMN_ENTRY_LENGTH_STATUS(1, m_ProductID, m_dwProductIDLength, m_dwProductIDStatus)
      COLUMN_ENTRY_LENGTH_STATUS(2, m_ProductName, m_dwProductNameLength, m_dwProductNameStatus)
   ...
   END_COLUMN_MAP()
   

To use a bookmark in your main code

1. In the MyCons.cpp file from the console application you previously created, change the main code to read as follows. To use bookmarks, the main code needs to instantiate its own bookmark object (myBookmark); this is a different bookmark from the one in the accessor (m_bookmark).

   ///////////////////////////////////////////////////////////////////////
   // MyCons.cpp : Defines the entry point for the console application.
   //
   
   #include "stdafx.h"
   #include "Products.h"
   #include <iostream>
   #include <fstream>
   using namespace std;
   
   int _tmain(int argc, _TCHAR* argv[])
   {
      HRESULT hr = CoInitialize(NULL);
   
      // Instantiate rowset
      CProducts rs;
   
      hr = rs.OpenAll();
      hr = rs.MoveFirst();
   
      // Cast CURRENCY m_UnitPrice to a long value
      LONGLONG lPrice = rs.m_UnitPrice.int64;
   
      // Open file output.txt for writing in overwrite mode
      ofstream outfile( "C:\\output.txt", ios::out );
   
      if (!outfile)      // Test for invalid file
         return -1;
   
      // Instantiate a bookmark object myBookmark for the main code
      CBookmark<4> myBookmark;
      int nCounter = 0;
   
      // Iterate through the rowset and output column data to output.txt row by row
      // In the file, mark the beginning of this set of data:
      outfile << "initial row dump" << endl;
      while(SUCCEEDED(hr) && hr != DB_S_ENDOFROWSET )
      {
         nCounter++;
         if(nCounter == 5 )
            myBookmark = rs.m_bookmark;
         // Output the column information for each row:
         outfile << rs.m_ProductID << rs.m_ProductName << lPrice << rs.m_QuantityPerUnit << rs.m_UnitsInStock << rs.m_ReorderLevel << endl;
         hr = rs.MoveNext();
      }
   
      // Move cursor to bookmark
      hr = rs.MoveToBookmark(myBookmark);
   
      // Iterate through the rowset and output column data to output.txt row by row
      // In the file, mark the beginning of this set of data:
      outfile << "row dump starting from bookmarked row" << endl;
      while(SUCCEEDED(hr) && hr != DB_S_ENDOFROWSET )
      {
         // Output the column information for each row
         outfile << rs.m_ProductID << rs.m_ProductName << lPrice << rs.m_QuantityPerUnit << rs.m_UnitsInStock << rs.m_ReorderLevel << endl;
         hr = rs.MoveNext();
      }
   
      rs.CloseAll();
      CoUninitialize();
   
      return 0;
   }
   

For more information about bookmarks, see Using Bookmarks. Examples of bookmarks are also shown in Updating Rowsets.