问为什么CancelIo函数失败？

EN

  public void foo(byte[] bufferToWrite)
  {
      unsafe
      {

            NativeOverlapped overlapped = new NativeOverlapped()
            {
                  EventHandle = eventHandle,
                  OffsetLow   = ( int )( s       & 0xffffffff ),
                  OffsetHigh  = ( int )( s >> 32 & 0xffffffff )
            };

            GCHandle gch = GCHandle.Alloc( bufferToWrite, GCHandleType.Pinned );
            IntPtr ptr = new IntPtr( ( void* )gch.AddrOfPinnedObject() );

            WriteFile( handle, ptr, length, ref bytesWritten, &overlapped );

            dwResult = WaitForSingleObject( EventHandle, 30000 );
            if(dwResult== TIME_OUT)
            {
                   Debug.Assert( CancelIo ( handle ) );

            }
      }
  }