Rowset.Release( );
  
读操作也完成了，是不是仍然很简单呢？下面我们再来看看最麻烦的二进制数据（text、ntext、image等）的读写。要实现BLOB数据的读写，我们需要一个辅助的类，定义如下：

class CSequentialStream : public ISequentialStream    // BLOB 数据访问类
{
public:
    CSequentialStream( );
    virtual ~CSequentialStream( );
    virtual BOOL Seek( ULONG uPosition );
    virtual BOOL Clear( );
    virtual ULONG GetLength( ) { return m_uBufferUsed; };
    virtual operator void* const( ) { return m_pBuffer; };
    STDMETHODIMP_( ULONG ) AddRef( ) { return ++ m_uRefCount; };
    STDMETHODIMP_( ULONG ) Release( ) { ASSERT( m_uRefCount != 0U ); -- m_uRefCount; if( m_uRefCount == 0U ) { delete this; } return m_uRefCount; };
    STDMETHODIMP QueryInterface( REFIID riid, LPVOID* ppv );
    STDMETHODIMP Read( void __RPC_FAR* pv, ULONG cb, ULONG __RPC_FAR* pcbRead );
    STDMETHODIMP Write( const void __RPC_FAR* pv, ULONG cb, ULONG __RPC_FAR* pcbWritten );
void    ResetPosition( ) { m_uPosition = 0U; };
HRESULT PreAllocBuffer( ULONG uSize );

private:
    ULONG m_uRefCount;     // reference count
    void* m_pBuffer;           // buffer
    ULONG m_uBufferUsed;  // buffer used
    ULONG m_uBufferSize;   // buffer size
    ULONG m_uPosition;       // current index position in the buffer
};

实现如下：

CSequentialStream::CSequentialStream( ) : m_uRefCount( 0U ), m_pBuffer( NULL ), m_uBufferUsed( 0U ), m_uBufferSize( 0U ), m_uPosition( 0U )
{
    AddRef( );
}

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

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

BOOL CSequentialStream::Seek( ULONG uPosition )
{
    ASSERT( uPosition < m_uBufferUsed );
    m_uPosition = uPosition;
    return TRUE;
}

BOOL CSequentialStream::Clear( )
{
    m_uBufferUsed = 0U;
    m_uBufferSize = 0U;
    m_uPosition = 0U;
    ( m_pBuffer != NULL ? CoTaskMemFree( m_pBuffer ) : 0 );
    m_pB