ILockBytes::ReadAt

2011-01-17

ReadAt 按指定偏移量从字节数组开始处读取指定数目的字节。

语法

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

参数

参数	说明
ulOffset [in]	指定读取数据的起点，从字节数组开始处算起
pv [in]	指向读入字节数组的缓冲区的指针。
cb [in]	指定试图从字节数组中读取的数据的字节数。
pcbRead [out]	指向该方法将写入实际数目字节（从字节数组中读取）的位置的指针。您可以将该指针设置为 NULL，指示您不需要此值。在这种情况下，该方法不会提供实际数目的读取字节。

返回值

术语	定义
NOERROR	方法成功执行。
E_UNEXPECTED	行集处于“僵停”状态。
STG_E_INVALIDPOINTER	传递的指针无效。

注释

下面的示例显示如何使用 ILockBytes::ReadAt 方法来检索大型二进制对象 (BLOB) 数据。该示例从 Employees 表的“Photo”列中读取 image 数据。

对于 32 位和 64 位平台，已更新下面的示例。64 位兼容类型来自 sqlce_oledb.h 本机头文件。可以在 %ProgramFiles%\Microsoft SQL Server Compact Edition\v3.5\Include 文件夹中找到 sqlce_oledb.h 本机头文件。有关详细信息，请参阅 Microsoft 网站上“OLE DB 编程人员指南”中的“OLE DB 64 位信息”主题。

示例

// Declarations
HRESULT            hr;
DBBINDING          rgBindings[1];
DBPROP             dbprop[1];
DBPROPSET          dbpropset[1];
DBOBJECT           dbObject;
BYTE               pData[100];   // Buffer for holding ILockBytes pointer.
HROW               rghRows[1];   // Array of row handles obtained from rowset object.
HROW               *prghRows           = rghRows;
BYTE               pBuffer[1000];  // Buffer for holding a chunk of BLOB data.
ULARGE_INTEGER     ulOffset;
DBROWCOUNT         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, 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 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 query syntax.
// 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(DBLENGTH);
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 fewer 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, release the ILockBytes pointer  
//   and row handle here.

Exit: 

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

return;

请参阅

其他资源

ILockBytes (SQL Server Compact)

ILockBytes::SetSize

ILockBytes::Stat

ILockBytes::WriteAt

通过

ILockBytes::ReadAt

语法

参数

返回值

注释

示例

请参阅

其他资源

其他资源