Implementing a Simple Consumer

Article
11/16/2012

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.
"Adding XML Support to the Consumer" shows how to modify the consumer code to output the retrieved rowset data as XML data.

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."

Note

To build a consumer application to test Provider, leave out the bookmark support described in "Adding Bookmark Support to the Consumer" and skip to "Adding XML Support to the Consumer."

Retrieving Data with the Consumer

To modify the console application to use the OLE DB consumer

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 "stdafx.h"
#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 is the key column, but not always; it is provider-specific. This section shows you how to add bookmark support. To do so, you need to do the following 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

The accessor needs to contain 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

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

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

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.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.

Adding XML Support to the Consumer

As discussed in Accessing XML Data, there are two ways to retrieve XML data from a data source: using CStreamRowset or using CXMLAccessor. This example uses CStreamRowset, which is more efficient, but requires you to have SQL Server 2000 running on the computer on which you execute this sample application.

To modify the command class to inherit from CStreamRowset

In the consumer application you previously created, change your CCommand declaration to specify CStreamRowset as the rowset class as follows:
```
class CProducts : public CCommand<CAccessor<CProductsAccessor>, CStreamRowset >
```

To modify the main code to retrieve and output the XML data

In the MyCons.cpp file from the console application you previously created, change the main code to read as follows:

///////////////////////////////////////////////////////////////////////
// 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;

   // Add variable declarations for the Read method to handle sequential stream data
   CHAR buffer[1001];  // Pointer to buffer into which data stream is read
   ULONG cbRead;       // Actual number of bytes read from the data stream

   hr = rs.OpenAll();

   // Open file output.txt for writing in overwrite mode
   ofstream outfile( "C:\\output.txt", ios::out );

   if (!outfile)      // Test for invalid file
      return -1;

   // The following loop reads 1000 bytes of the data stream at a time 
   // until it reaches the end of the data stream
   for (;;)
   {
      // Read sequential stream data into buffer
      HRESULT hr = rs.m_spStream->Read(buffer, 1000, &cbRead);
      if (FAILED (hr))
         break;
      // Output buffer to file
      buffer[cbRead] = 0;
      outfile << buffer;
      // Test for end of data stream
      if (cbRead < 1000)
         break;
   }

   rs.CloseAll();
   CoUninitialize();

   return 0;
}

Implementing a Simple Consumer

Retrieving Data with the Consumer

To modify the console application to use the OLE DB consumer

Adding Bookmark Support to the Consumer

To instantiate the bookmark

To request a bookmark column from the provider

To add a bookmark entry to the column map

To use a bookmark in your main code

Adding XML Support to the Consumer

To modify the command class to inherit from CStreamRowset

To modify the main code to retrieve and output the XML data

See Also

Concepts

Additional resources