[I/O]FileStream读取文件长度为0？该怎么解决

                string fullName = openFileDialog.FileName;
                using (FileStream fs = File.OpenRead(fullName)) {
                    int offset = 0;
                    byte[] buffer = new byte[1024];
                    int count = 0;
                    while ((count = fs.Read(buffer, offset, buffer.Length)) > 0) {
                       //进不来这里，因为count总是0？？？
                           offset += count;
                    }
                }