Recuperare dati da una colonna FILESTREAM usando ISequentialStream (OLE DB)

Articolo
10/16/2024

Si applica a: SQL Server Database SQL di Azure Istanza gestita di SQL di Azure Azure Synapse Analytics Piattaforma di strumenti analitici (PDW)

Download del driver OLE DB

Questo esempio mostra l'uso di un'interfaccia ISequentialStream in un'interfaccia ICommandText per recuperare un singolo record da una colonna FILESTREAM.

Per altre informazioni sulla funzionalità FILESTREAM, vedere Supporto di FILESTREAM in OLE DB Driver per SQL Server.

Esempi

Assicurarsi che la variabile di ambiente INCLUDE includa la directory che contiene msoledbsql.h.

Per creare la tabella letta in questo esempio, utilizzare uno degli esempi seguenti:

Copiare il primo listato di codice e incollarlo in un file denominato ISSHelper.h.

Copiare il secondo listato di codice e incollarlo in un file denominato ISSHelper.cpp.

Copiare il terzo listato di codice e incollarlo in un file denominato ICommandDownload.cpp.

ICommandDownload.cppCompilare , ISSHelper.cppole32.lib, e oleaut32.lib.

Per eseguire l'esempio, è necessario passare un nome di server (o server\nome_istanza).

// ISSHelper.h: interface for the CISSHelper class.

#if !defined(AFX_ISSHELPER_H__7B88E5F3_263F_11D2_9D1F_00C04F96B8B2__INCLUDED_)
#define AFX_ISSHELPER_H__7B88E5F3_263F_11D2_9D1F_00C04F96B8B2__INCLUDED_

#if _MSC_VER > 1000
#pragma once
#endif // _MSC_VER > 1000

class CISSHelper : public ISequentialStream {
public:

   // Constructor/destructor.
   CISSHelper(__int64 i64LobBytes);
   virtual ~CISSHelper();

   // Helper function to clean up memory.
   virtual void Clear();

   // ISequentialStream interface implementation.
   STDMETHODIMP_(ULONG)AddRef(void);
   STDMETHODIMP_(ULONG)Release(void);
   STDMETHODIMP QueryInterface(REFIID riid, LPVOID *ppv);
   STDMETHODIMP Read(
      /* [out] */ void __RPC_FAR *pv,
      /* [in] */ ULONG cb,
      /* [out] */ ULONG __RPC_FAR *pcbRead);
   STDMETHODIMP Write(
      /* [in] */ const void __RPC_FAR *pv,
      /* [in] */ ULONG cb,
      /* [out] */ ULONG __RPC_FAR *pcbWritten);

public:
   void * m_pBuffer;   // Buffer
   ULONG m_ulLength;   // Total buffer size.
   ULONG m_ulStatus;   // Column status.

   __int64 m_i64BytesUploaded;
   __int64 m_i64LastPrintUploaded;
   __int64 m_i64ThreshHold;
   DWORD m_dwStartTick;
   DWORD m_dwParamOrdinal;

private:
   ULONG m_cRef;  // Reference count (not used).
   ULONG m_iReadPos;   // Current index position for reading from the buffer.
   ULONG m_iWritePos;   // Current index position for writing to the buffer.
};

#endif // !defined(AFX_ISSHELPER_H__7B88E5F3_263F_11D2_9D1F_00C04F96B8B2__INCLUDED_)

// ISSHelper.cpp: implementation of the CISSHelper class.
#pragma once

#define WIN32_LEAN_AND_MEAN// Exclude rarely-used stuff from Windows headers
#include <windows.h>
#include <stdio.h>// printf(...)
#include <stddef.h>// offsetof(...)
#include <conio.h>// _getch()
#include <oledb.h>// OLEDB
#include <oledberr.h>// OLEDB
#include <objbase.h>// CoInitializeEx()
#include <msdasc.h>// IDataInitialize
#include <msdadc.h>// OLEDB conversion library header.
#include <msdaguid.h>// OLEDB conversion library guids.
#include <tchar.h>

// TODO: reference additional headers your program requires here
#define _MSOLEDBSQL_OLEDB_ 1
#include <msoledbsql.h>

#include "ISSHelper.h"

#ifdef _DEBUG
#undef THIS_FILE
static char THIS_FILE[]=__FILE__;
#define new DEBUm_NEW
#endif

// Implementation of ISequentialStream interface
CISSHelper::CISSHelper(__int64 i64LobBytes) {
   m_cRef= 0;
   m_pBuffer= NULL;
   m_ulLength= 0;
   m_ulStatus   = DBSTATUS_S_OK;
   m_iReadPos= 0;
   m_iWritePos= 0;
   m_i64ThreshHold = i64LobBytes;
   m_i64BytesUploaded = 0;
   m_i64LastPrintUploaded = 0;
   m_dwStartTick = 0;
   m_dwParamOrdinal = 0;
}

CISSHelper::~CISSHelper() {
   Clear();
}

void CISSHelper::Clear() {
   CoTaskMemFree( m_pBuffer );
   m_cRef= 0;
   m_pBuffer= NULL;
   m_ulLength= 0;
   m_ulStatus  = DBSTATUS_S_OK;
   m_iReadPos= 0;
   m_iWritePos= 0;
}

ULONG CISSHelper::AddRef() {
   return ++m_cRef;
}

ULONG CISSHelper::Release() {
   return --m_cRef;
}

HRESULT CISSHelper::QueryInterface( REFIID riid, void** ppv ) {
   *ppv = NULL;
   if ( riid == IID_IUnknown ) *ppv = this;
   if ( riid == IID_ISequentialStream ) *ppv = this;
   if ( *ppv )
   {
      ( (IUnknown*) *ppv )->AddRef();
      return S_OK;
   }
   return E_NOINTERFACE;
}

HRESULT CISSHelper::Read( void *pv,ULONG cb, ULONG* pcbRead ) {
   // Check parameters.
   if ( pcbRead ) *pcbRead = 0;
   if ( !pv ) return STG_E_INVALIDPOINTER;
   if ( 0 == cb ) return S_OK;

   // Cut out now if threshold is hit.
   __int64 left = m_i64ThreshHold-m_i64BytesUploaded;
   if (left < cb)
      cb = (ULONG)left;

   if (0 == m_dwStartTick)
      m_dwStartTick = GetTickCount();

   m_i64BytesUploaded += cb;

   if (( m_i64BytesUploaded - m_i64LastPrintUploaded ) >= (1024*1024*4)) {
      __int64 i64Elapsed = (__int64) GetTickCount()-m_dwStartTick;
      // __int64 i64BytesPerSecond = m_i64BytesUploaded * 1000 / (i64Elapsed > 0 ) ? i64Elapsed : 1;
      __int64 i64BytesPerSecond = m_i64BytesUploaded * 1000 / i64Elapsed > 0  ? i64Elapsed : 1;
      printf("Param=%lu TotalBytes=%010I64u ElapsedMS=%010I64u BytesPerSec=%010I64u\n",
         m_dwParamOrdinal, m_i64BytesUploaded, i64Elapsed, i64BytesPerSecond );
      m_i64LastPrintUploaded = m_i64BytesUploaded;
   }

   if ( cb == left ) {
      __int64 i64Elapsed = (__int64) GetTickCount()-m_dwStartTick;
      // __int64 i64BytesPerSecond = m_i64BytesUploaded * 1000 / (i64Elapsed > 0 ) ? i64Elapsed : 1;
      __int64 i64BytesPerSecond = m_i64BytesUploaded * 1000 / i64Elapsed > 0 ? i64Elapsed : 1;
      printf("Last read:\nParam=%lu TotalBytes=%010I64u ElapsedMS=%010I64u BytesPerSec=%010I64u\n",
         m_dwParamOrdinal, m_i64BytesUploaded, i64Elapsed, i64BytesPerSecond );
      m_i64LastPrintUploaded = m_i64BytesUploaded;
   }

   *pcbRead = cb;
   return S_OK;
}

HRESULT CISSHelper::Write( const void *pv, ULONG cb, ULONG* pcbWritten ) {
   // Check parameters.
   if ( !pv ) return STG_E_INVALIDPOINTER;
   if ( pcbWritten ) *pcbWritten = 0;
   if ( 0 == cb ) return S_OK;

   // Enlarge the current buffer.
   m_ulLength += cb;

   // Grow internal buffer to new size.
   m_pBuffer = CoTaskMemRealloc( m_pBuffer, m_ulLength );

   // Check for out of memory situation.
   if ( NULL == m_pBuffer ) {
      Clear();
      return E_OUTOFMEMORY;
   }

   // Copy callers memory to internal buffer and update write position.
   memcpy( (void*)((BYTE*)m_pBuffer + m_iWritePos), pv, cb );
   m_iWritePos += cb;

   // Return bytes written to caller.
   if ( pcbWritten ) *pcbWritten = cb;

   return S_OK;
}

//  ICommandDownload.cpp
#pragma once

#define WIN32_LEAN_AND_MEAN// Exclude rarely-used stuff from Windows headers
#include <windows.h>
#include <stdio.h>// printf(...)
#include <stddef.h>// offsetof(...)
#include <conio.h>// _getch()
#include <oledb.h>// OLEDB
#include <oledberr.h>// OLEDB
#include <objbase.h>// CoInitializeEx()
#include <msdasc.h>// IDataInitialize
#include <msdadc.h>// OLEDB conversion library header.
#include <msdaguid.h>// OLEDB conversion library guids.
#include <tchar.h>

// TODO: reference additional headers your program requires here
#define _MSOLEDBSQL_OLEDB_ 1
#include <msoledbsql.h>

#include "ISSHelper.h"

// Simple data record used by DumpRowset function.
#define MAX_DATA_LENGTH 8096
struct DATA_REC {
   DWORD status;
   DWORD length;
   BYTE Data[MAX_DATA_LENGTH];
};

// Helper macros for COM/OLEDB.
#define SAFE_RELEASE(x)    { if (x) { x->Release(); x = NULL; } }
#define SAFE_SYSFREE(bstr) { if (bstr) { ::SysFreeString(bstr); bstr = NULL; } }
#define CHECK_HR(hr,jump)  { if ( FAILED(hr) ) goto jump; }
#define CHECK_HR_ERR(hr,jump)  { if ( FAILED(hr) ) { DumpErrorRecords(); goto jump; } }

#define CLEAR_PROPS(x) { for ( int i = 0 ; i < x ; i++ ) VariantClear( &InitProperties[i].vValue ); }

#define SET_BSTR_PROP(index,propid,value) { \
   InitProperties[index].dwOptions = DBPROPOPTIONS_REQUIRED; \
   InitProperties[index].colid = DB_NULLID; \
   InitProperties[index].dwStatus = DBPROPSTATUS_OK; \
   InitProperties[index].dwPropertyID = propid; \
   InitProperties[index].vValue.vt = VT_BSTR; \
   InitProperties[index].vValue.bstrVal = SysAllocString(value); }

// Helper function for DumpErrorRecords() to convert a clisd to string representation of clsid.
LPWSTR CLSIDToString( CLSID* pclsid ) {
   static WCHAR wszCLSID[1024];
   if ( NULL == pclsid )
      wcscpy_s(wszCLSID, 1024, L"(Null)");
   else
      ::StringFromGUID2( *pclsid, (OLECHAR*)wszCLSID, sizeof(wszCLSID) );
   return wszCLSID;
}

// Helper function to dump out OLE DB errors to console window.
HRESULT DumpErrorRecords() {
   HRESULT hr = S_OK;
   IErrorRecords* pIErrorRecords = NULL;
   IErrorInfo* pIErrorInfo  = NULL;
   IErrorInfo* pIErrorInfoRecord = NULL;
   BSTR bstrDescription  = NULL;
   BSTR bstrSource  = NULL;
   ULONG i;
   ULONG cRecords = 0;
   LCID lcid = 0;
   ERRORINFO ErrorInfo;

   // Get IErrorInfo pointer from OLE.
   hr = ::GetErrorInfo( 0, &pIErrorInfo );
   if ( FAILED(hr) ) {
      printf( "No error records found.\n" );
      goto DumpErrorRecordsExit;
   }

   // QI for IID_IErrorRecords.
   hr = pIErrorInfo->QueryInterface( IID_IErrorRecords, (void**)&pIErrorRecords );
   if ( FAILED(hr) ) {
      printf( "pIErrorInfo->QueryInterface(IID_IErrorRecords) failed with hr=0x%08x.\n", hr );
      goto DumpErrorRecordsExit;
   }

   // Get error record count.
   hr = pIErrorRecords->GetRecordCount( &cRecords );
   if ( FAILED(hr) ) {
      printf( "pIErrorRecords->GetRecordCount() failed with hr=0x%08x.\n", hr );
      goto DumpErrorRecordsExit;
   }

   // Get system LCID
   lcid = GetSystemDefaultLCID();

   //Loop through the error records.
   for ( i = 0 ; i < cRecords ; i++ ) {
      // Get pIErrorInfo from pIErrorRecords.
      hr = pIErrorRecords->GetErrorInfo( i, lcid, &pIErrorInfoRecord );

      if ( SUCCEEDED(hr) ) {
         // Get error description and source.
         hr = pIErrorInfoRecord->GetDescription( &bstrDescription );
         hr = pIErrorInfoRecord->GetSource( &bstrSource );

         // Retrieve the ErrorInfo structures.
         hr = pIErrorRecords->GetBasicErrorInfo( i, &ErrorInfo );

         // Dump out error.
         printf( "\n" );
         printf( "Dumping error record %lu of %lu:\n", i+1, cRecords );
         printf( "--------------------------------------------------------------------------\n" );
         printf( "   Description = %S\n", bstrDescription );
         printf( "   Source      = %S\n", bstrSource );
         printf( "   ERRORINFO.hrError = 0x%08x\n", ErrorInfo.hrError );
         printf( "   ERRORINFO.dwMinor = %lu\n",ErrorInfo.dwMinor );
         printf( "   ERRORINFO.clsid   = %S\n",CLSIDToString( &ErrorInfo.clsid ) );

         SAFE_RELEASE(pIErrorInfoRecord);
         SAFE_SYSFREE(bstrDescription);
         SAFE_SYSFREE(bstrSource);
      }
   }

DumpErrorRecordsExit:
   SAFE_RELEASE(pIErrorInfoRecord);
   SAFE_RELEASE(pIErrorRecords);
   SAFE_RELEASE(pIErrorInfo);
   SAFE_RELEASE(pIErrorInfo);
   SAFE_SYSFREE(bstrDescription);
   SAFE_SYSFREE(bstrSource);
   return hr;
}

// Helper function to execute a sql command.
HRESULT ExecuteSQL( IDBCreateCommand* pIDBCreateCommand, LPCOLESTR pwszCommand  ) {
   HRESULT hr;
   ICommandText* pICommandText = NULL;
   hr = pIDBCreateCommand->CreateCommand( NULL, IID_ICommandText, (IUnknown**)&pICommandText );
   CHECK_HR(hr,ExecuteSQLCleanup);
   hr = pICommandText->SetCommandText( DBGUID_DBSQL, pwszCommand );
   CHECK_HR(hr,ExecuteSQLCleanup);
   hr = pICommandText->Execute( NULL, IID_NULL, NULL, NULL, NULL );

ExecuteSQLCleanup:
   SAFE_RELEASE(pICommandText);
   return hr;
}

HRESULT OpenSessionSSHelper( IDBProperties*pIDBProperties, IDBCreateCommand** ppIDBCreateCommand, wchar_t* szServer ) {
#define TOTAL_SS_PROPS 3
   HRESULThr;
   IDBInitialize*pIDBInitialize = NULL;
   IDBCreateSession*pIDBCreateSession = NULL;
   DBPROPInitProperties[TOTAL_SS_PROPS];
   DBPROPSETInitPropSet;

   if ( NULL == pIDBProperties )
      return E_INVALIDARG;

   if ( NULL == ppIDBCreateCommand )
      return E_INVALIDARG;

   // NULL our out param in case of failure.
   *ppIDBCreateCommand = NULL;

   // Hard coded this to point to local sql server, change as needed.
   SET_BSTR_PROP( 0, DBPROP_INIT_DATASOURCE, szServer );
   SET_BSTR_PROP( 1, DBPROP_INIT_CATALOG, L"master" );
   SET_BSTR_PROP( 2, DBPROP_AUTH_INTEGRATED, L"SSPI" );

   InitPropSet.guidPropertySet = DBPROPSET_DBINIT;
   InitPropSet.cProperties = TOTAL_SS_PROPS;
   InitPropSet.rgProperties = InitProperties;

   // Set initialization properties.
   hr = pIDBProperties->SetProperties( 1, &InitPropSet );
   CHECK_HR(hr,OpenSessionSSHelperCleanup);

   // Clean up allocated variants.
   CLEAR_PROPS( TOTAL_SS_PROPS );

   // Initialize the DataSource.
   hr = pIDBProperties->QueryInterface( IID_IDBInitialize, (void**) &pIDBInitialize );
   CHECK_HR(hr,OpenSessionSSHelperCleanup);

   hr = pIDBInitialize->Initialize();
   CHECK_HR(hr,OpenSessionSSHelperCleanup);

   // QI for IID_IDBCreateSession and then create session.
   hr = pIDBInitialize->QueryInterface( IID_IDBCreateSession, (void**) &pIDBCreateSession );
   CHECK_HR(hr,OpenSessionSSHelperCleanup);

   hr = pIDBCreateSession->CreateSession( NULL, IID_IDBCreateCommand, (IUnknown**)ppIDBCreateCommand );

OpenSessionSSHelperCleanup:

   SAFE_RELEASE( pIDBInitialize );
   SAFE_RELEASE( pIDBCreateSession );
   return hr;
}

// Helper function to open a SS session.
HRESULT OpenSessionSS( IDBCreateCommand** ppIDBCreateCommand, wchar_t* szServer ) {
   HRESULT hr;
   IDBProperties * pIDBProperties = NULL;

   // Null our out param in case we fail.
   if ( NULL == ppIDBCreateCommand )
      return E_INVALIDARG;

   // Co-create MSOLEDBSQL.
   hr = CoCreateInstance( MSOLEDBSQL_CLSID, NULL, CLSCTX_INPROC_SERVER, IID_IDBProperties,
      (void **) &pIDBProperties );
   CHECK_HR(hr,OpenSessionSSCleanup);

   // Call helper to open session.
   hr = OpenSessionSSHelper( pIDBProperties, ppIDBCreateCommand, szServer );

OpenSessionSSCleanup:
   SAFE_RELEASE( pIDBProperties );
   return hr;
}

void wmain(int argc, wchar_t* argv[]) {
#define PARAM_COUNT 2
   HRESULT hr;
   IDBCreateCommand * pIDBCreateCommand = NULL;
   ICommandText * pICommandText = NULL;
   ICommandPrepare * pICommandPrepare = NULL;
   IAccessor * pIAccessor = NULL;
   IRowset * pIRowset = NULL;
   ICommandProperties * pICommandProperties = NULL;

   HACCESSOR hAccessor;
   DBBINDING dbBind[PARAM_COUNT];
   DBROWCOUNT rows;
   DBPROPSET rgCmdPropSet[1];
   DBPROP rgCmdProperties[1];

   if ( argc < 2 ) {
      wprintf( L"Usage: %s: <server>\n", argv[0] );
      return;
   }

   CoInitialize(NULL);

   // Open connection to SS database.
   hr = OpenSessionSS( &pIDBCreateCommand, argv[1] );
   CHECK_HR_ERR(hr,MainCleanup);

   // Get ICommandText.
   hr = pIDBCreateCommand->CreateCommand( NULL, IID_ICommandText, (IUnknown**)&pICommandText );
   printf( "pIDBCreateCommand->CreateCommand(...) returned 0x%08x.\n", hr );
   CHECK_HR(hr,MainCleanup);

   // Prepare the parameterized execute statement.
   hr = pICommandText->SetCommandText( DBGUID_DBSQL, L"SELECT uid, val FROM [DBFsa]..[FSTRTable]" );

   // Set up parameter value binding.
   ZeroMemory( &dbBind, sizeof(dbBind) );

   struct _DATA_BLOCK {
      long f1;
      long f1_status;
      long f1_length;
      unsigned long imageData_length;
      long imageData_status;
      IUnknown* imageData;
   } DATA_BLOCK;

   dbBind[0].iOrdinal= 1;
   dbBind[0].dwPart= DBPART_VALUE | DBPART_STATUS | DBPART_LENGTH;
   dbBind[0].eParamIO= DBPARAMIO_NOTPARAM;
   dbBind[0].obValue= offsetof( _DATA_BLOCK, f1 );
   dbBind[0].obStatus= offsetof( _DATA_BLOCK, f1_status );
   dbBind[0].obLength= offsetof( _DATA_BLOCK, f1_length );
   dbBind[0].wType= DBTYPE_I4;
   dbBind[0].cbMaxLen= sizeof(int);

   dbBind[1].iOrdinal= 2;
   dbBind[1].dwPart= DBPART_VALUE | DBPART_STATUS;
   dbBind[1].eParamIO= DBPARAMIO_NOTPARAM;
   dbBind[1].obValue= offsetof( _DATA_BLOCK, imageData );
   dbBind[1].obStatus= offsetof( _DATA_BLOCK, imageData_status );
   dbBind[1].obLength  = offsetof( _DATA_BLOCK, imageData_length );
   dbBind[1].dwMemOwner = DBMEMOWNER_CLIENTOWNED;
   dbBind[1].wType= DBTYPE_IUNKNOWN;
   dbBind[1].pObject= new DBOBJECT;
   dbBind[1].pObject->dwFlags = STGM_READ;
   dbBind[1].pObject->iid = IID_ISequentialStream;
   dbBind[1].cbMaxLen= 0;

   hr = pICommandText->QueryInterface(IID_ICommandProperties, (void**) &pICommandProperties);
   CHECK_HR_ERR( hr, MainCleanup );

   rgCmdProperties[0].dwPropertyID = DBPROP_ACCESSORDER;
   rgCmdProperties[0].vValue.vt = VT_I4;
   rgCmdProperties[0].vValue.lVal = DBPROPVAL_AO_SEQUENTIAL;
   rgCmdProperties[0].dwOptions = DBPROPOPTIONS_REQUIRED;
   rgCmdProperties[0].colid = DB_NULLID;

   rgCmdPropSet[0].guidPropertySet = DBPROPSET_ROWSET;
   rgCmdPropSet[0].cProperties = 1;
   rgCmdPropSet[0].rgProperties = rgCmdProperties;

   hr = pICommandProperties->SetProperties(1, rgCmdPropSet);
   CHECK_HR_ERR( hr, MainCleanup );

   // Get an accessor for our binding.
   hr = pICommandText->Execute( NULL, IID_IRowset, NULL, &rows, (IUnknown**) &pIRowset );
   CHECK_HR_ERR(hr, MainCleanup);
   printf( "pICommandText->Execute(...) returned 0x%08x.\n", hr );

   hr = pIRowset->QueryInterface( IID_IAccessor, (void**)&pIAccessor );
   printf( "pIRowset->QueryInterface( IID_IAccessor) returned 0x%08x.\n", hr );
   CHECK_HR_ERR(hr,MainCleanup);

   hr = pIAccessor->CreateAccessor( DBACCESSOR_ROWDATA,
      PARAM_COUNT,
      &dbBind[0],
      sizeof(DATA_BLOCK),
      &hAccessor,
      NULL );
   printf( "pIAccessor->CreateAccessor(...) returned 0x%08x.\n", hr );
   CHECK_HR_ERR(hr,MainCleanup);

   // Set the parameter values and status/length.
   DATA_BLOCK.f1 = 1;
   DATA_BLOCK.f1_status = DBSTATUS_S_OK;
   DATA_BLOCK.f1_length = sizeof(int);
   DATA_BLOCK.imageData = NULL;
   DATA_BLOCK.imageData_status = DBSTATUS_S_OK;
   DATA_BLOCK.imageData_length = 0;

   HROW hRow[1];
   HROW* pRow = hRow;
   DBCOUNTITEM count;
   hr = pIRowset->GetNextRows( DB_NULL_HCHAPTER, 0, 1, &count, &pRow );
   CHECK_HR_ERR(hr, MainCleanup);

   hr = pIRowset->GetData( hRow[0], hAccessor, &DATA_BLOCK );
   CHECK_HR_ERR(hr, MainCleanup);

   // Read and process 5000 bytes at a time.
   if( DATA_BLOCK.imageData_status == DBSTATUS_S_ISNULL ) {
      // Process NULL data.
   }
   else if( DATA_BLOCK.imageData_status == DBSTATUS_S_OK ) {

      ISequentialStream * pISequentialStream;
      BYTE * rgBuffer = new BYTE[1024*1024*4];
      ULONG   cb;
      ULONGLONG total = 0;

      pISequentialStream = (ISequentialStream *) DATA_BLOCK.imageData;
      do {
         cb = sizeof( rgBuffer );
         pISequentialStream->Read(rgBuffer,   // Buffer to write the data
            1024*1024*4,   // Number of bytes to get
            &cb);   // Number of bytes written to buffer
         if (cb > 0) {   // Check to see if we got data
            total += cb;
            printf( "Received %d bytes of data, total = %08I64x\n", cb, total );
         }
      } while (cb >= sizeof(rgBuffer));

      delete rgBuffer;
   }
   else {
      printf( "%08X Problem retrieving varbinary(max)\n", DATA_BLOCK.imageData_status );
   }

MainCleanup:
   SAFE_RELEASE( pICommandProperties );
   SAFE_RELEASE( pIRowset );
   SAFE_RELEASE( pIDBCreateCommand );
   SAFE_RELEASE( pICommandText );
   SAFE_RELEASE( pICommandPrepare );
   SAFE_RELEASE( pIAccessor );

   printf_s( "Test complete.\n" );
}

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Recuperare dati da una colonna FILESTREAM usando ISequentialStream (OLE DB)

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