silvrlight 怎么实现与ftp的上传下载 求教之前用wcf可是偏移量是个问题啊

[xiazaiwocaoni]

long offset = file.Offset;
Stream stream = filerequest.GetRequestStream();
stream.Write(file.Data, (int)file.Offset, (int)file.Data_Length);
file.Offset += file.Data_Length;
file.Data = null;