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Implementing a Simple Consumer

  • Artikkeli

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:

Note

You can use the consumer application described in this section to test the MyProv and Provider sample providers.

Note

To build a consumer application to test MyProv (the same provider described in Enhancing the Simple Read-Only Provider), you must include bookmark support as described in Adding 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.

Note

If you use this consumer application to test the Provider sample provider application, leave out the bookmark support described in this section.

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.

See also

Creating an OLE DB Consumer Using a Wizard