ILockBytes::ReadAt

ReadAt legge un numero di byte stabilito a partire dall'offset specificato dall'inizio della matrice di byte.

Sintassi

HRESULT ReadAt(   ULARGE_INTEGER ulOffset,   void *pv,   ULONG cb,   ULONG *pcbRead    );

Parametri

Parametro	Descrizione
ulOffset [in]	Specifica il punto da cui avviare la lettura dei dati dall'inizio della matrice di byte
pv [in]	Puntatore al buffer in cui la matrice di byte viene letta.
cb [in]	Specifica il numero di byte dei dati di cui si tenta di eseguire la lettura dalla matrice di byte.
pcbRead [out]	Puntatore al percorso in cui il metodo scrive il numero effettivo di byte letti nella matrice di byte. È possibile impostare il puntatore su NULL per ignorare questo valore. In questo caso, il metodo non indica il numero effettivo di byte letti.

Valore restituito

Termine	Definizione
NOERROR	Il metodo ha avuto esito positivo.
E_UNEXPECTED	Il set di righe è in uno stato zombie.
STG_E_INVALIDPOINTER	Un puntatore passato non è valido.

Osservazioni

Nell'esempio seguente viene descritto come utilizzare il metodo ILockBytes::ReadAt per recuperare i dati di tipo BLOB (Binary Large Object) e vengono letti i dati di tipo image presenti nella colonna "Photo" della tabella Employees.

Esempio

// Declarations
HRESULT            hr;
DBBINDING          rgBindings[1];
DBPROP             dbprop[1];
DBPROPSET          dbpropset[1];
DBOBJECT           dbObject;
BYTE               pData[100];    //buffer for holding the ILockBytes pointer
HROW               rghRows[1]; // Array of row handles obtained from the rowset object
HROW               *prghRows           = rghRows;
BYTE               pBuffer[1000];  //Buffer for holding a chunck of BLOB data
ULARGE_INTEGER     ulOffset;
ULONG              cRows               = 0;
IDBInitialize      *pIDBInitialize     = NULL;        
IDBProperties      *pIDBProperties     = NULL;        
IDBCreateSession   *pIDBCreateSession  = NULL;
IDBCreateCommand   *pIDBCreateCommand  = NULL;
ICommandText       *pICommandText      = NULL;
IAccessor          *pIAccessor         = NULL;
IRowset            *pIRowset           = NULL;
ILockBytes         *pILockBytes        = NULL;
HACCESSOR          hAccessor           = DB_NULL_HACCESSOR;

if (FAILED(hr = CoInitialize(NULL)))
{
    return;
}

VariantInit(&dbprop[0].vValue);        

// Create an instance of the OLE DB Provider
hr = CoCreateInstance(CLSID_SQLSERVERCE_3_0, 0, CLSCTX_INPROC_SERVER,
                IID_IDBInitialize, (void**)&pIDBInitialize);
if(FAILED(hr))
{
    //Send error-specific message and do error handling
    goto Exit;
}

// Initialize a property with name of the database
// Open an exsiting database myDatabase
dbprop[0].dwPropertyID     = DBPROP_INIT_DATASOURCE;
dbprop[0].dwOptions         = DBPROPOPTIONS_REQUIRED;
dbprop[0].vValue.vt         = VT_BSTR;
dbprop[0].vValue.bstrVal = SysAllocString(L"\\windows\\MyDB.sdf");
if(NULL == dbprop[0].vValue.bstrVal)
{
    hr = E_OUTOFMEMORY;
    goto Exit;
}

// Initialize the property set
dbpropset[0].guidPropertySet = DBPROPSET_DBINIT;
dbpropset[0].rgProperties     = dbprop;
dbpropset[0].cProperties     = sizeof(dbprop)/sizeof(dbprop[0]);

//Set initialization properties.
hr = pIDBInitialize->QueryInterface(IID_IDBProperties, 
                (void **)&pIDBProperties);
if(FAILED(hr))
{
    goto Exit;
}

// Sets properties in the data source and initialization property groups
hr = pIDBProperties->SetProperties(1, dbpropset); 
if(FAILED(hr))
{
    goto Exit;
}

// Initializes a data source object 
hr = pIDBInitialize->Initialize();
if(FAILED(hr))
{
    goto Exit;
}

// Get the IDBCreateSession interface
hr = pIDBInitialize->QueryInterface(IID_IDBCreateSession,
    (void**)&pIDBCreateSession);
if (FAILED(hr))
{
    //Send error-specific message and do error handling
    goto Exit;
}
// Get the interface to create a command
hr = pIDBCreateSession->CreateSession(NULL, IID_IDBCreateCommand,
    (IUnknown**) &pIDBCreateCommand);
if (FAILED(hr))
{
    //Send an error-specific message and do error handling.
    goto Exit;
}


// Create the new command that uses parameters.
hr = pIDBCreateCommand->CreateCommand(NULL, IID_ICommandText, (IUnknown**) &pICommandText);
if (FAILED(hr))
{
    //Send an error-specific message and do error handling.
    goto Exit;
}

// Specify the command text using parameter markers in the query syntax.
// We know that this command will return 1 row and one image column (Photo).
hr = pICommandText->SetCommandText(DBGUID_DBSQL, L"SELECT Photo FROM Employees WHERE \"Employee ID\" = 2");
if (FAILED(hr))
{
    //Send an error-specific message and do error handling.
    goto Exit;
}

hr = pICommandText->Execute(NULL, IID_IRowset, NULL, NULL, (IUnknown**) &pIRowset);
if(FAILED(hr))
{
    //Send an error-specific message and do error handling.
    goto Exit;
}

// Create the acessor object and column specific bindings.

hr = pIRowset->QueryInterface(IID_IAccessor, (void**) &pIAccessor);
if (FAILED(hr))
{
    //Send an error-specific message and do error handling.
    goto Exit;
}

// Create the bindings for the photo (BLOB) column
dbObject.dwFlags = STGM_READ;
dbObject.iid     = IID_ILockBytes;
rgBindings[0].iOrdinal = 1;
rgBindings[0].obStatus = 0;
rgBindings[0].obLength = rgBindings[0].obStatus + sizeof(DBSTATUS);
rgBindings[0].obValue = rgBindings[0].obLength + sizeof(ULONG);
rgBindings[0].pTypeInfo = NULL;
rgBindings[0].pObject = &dbObject;
rgBindings[0].pBindExt = NULL;
rgBindings[0].dwPart = DBPART_VALUE | DBPART_LENGTH | DBPART_STATUS;
rgBindings[0].dwMemOwner = DBMEMOWNER_CLIENTOWNED;
rgBindings[0].eParamIO = DBPARAMIO_NOTPARAM;
rgBindings[0].cbMaxLen = sizeof(IUnknown*);
rgBindings[0].dwFlags = 0;
rgBindings[0].wType = DBTYPE_IUNKNOWN;
rgBindings[0].bPrecision = 0;
rgBindings[0].bScale = 0;

// Create accessor.
hr = pIAccessor->CreateAccessor(DBACCESSOR_ROWDATA, 1, rgBindings,
    0, &hAccessor, NULL);
if(FAILED(hr))
{
    //Send an error-specific message and do error handling.
    goto Exit;
}

//Get the first row
hr = pIRowset->GetNextRows(NULL, 0, 1, &cRows, &prghRows);
if(FAILED(hr) || (0 == cRows))
{
    //Send an error-specific message and do error handling.
    goto Exit;
}

//Get the ILockBytes pointer
hr = pIRowset->GetData(rghRows[0], hAccessor, pData);
if(FAILED(hr))
{
    //Send an error-specific message and do error handling.
    goto Exit;
}

if(DBSTATUS_S_OK != *(DBSTATUS*)(pData + rgBindings[0].obStatus))
{
    //Send an error-specific message and do error handling.
    goto Exit;
}

// Get the ILockBytes pointer from the buffer.
pILockBytes = *(ILockBytes**)(pData + rgBindings[0].obValue);

// Initialize Offset. 
ulOffset.QuadPart = 0;

// Retrieve data from the interface in chunks. 
// The infinite loop, for(;;), will terminate when the ReadAt call fails 
// or if less bytes are returned than were requested. 
for(;;)

{

    ULONG cbRead;

    // Read data at the specified offset
    hr = pILockBytes->ReadAt(ulOffset, pBuffer, sizeof(pBuffer)-sizeof(WCHAR), 
    &cbRead);

/////////////////////////////////////////////////////////////////////////
// Do some useful things here with the chunks of data
/////////////////////////////////////////////////////////////////////////

    if(cbRead < sizeof(pBuffer)-sizeof(WCHAR) || FAILED(hr)) break;

    ulOffset.QuadPart += cbRead;
}

//If more rows need to be read, then release the ILockBytes pointer and 
// row handle here

Exit: 

// When finished, clear the properties arrays and release interfaces.
// Uninitialize the environment.

return;

Vedere anche

Riferimenti

ILockBytes::SetSize
ILockBytes::Stat
ILockBytes::WriteAt

Altre risorse

ILockBytes (SQL Server Compact Edition)

Guida in linea e informazioni

Assistenza su SQL Server Compact Edition