流.read方法接受长度作为整数类型???作为

把它转换成整数。。。

FileByteStream.Read(buffer, 0, Convert.ToInt32(outputMessage.FileByteStream.Length))

如果您正在读取的数据大小不合理,您可能需要考虑循环将数据读入缓冲区(例如 MSDN docs ):

//s is the stream that I'm working with...
byte[] bytes = new byte[s.Length];
int numBytesToRead = (int) s.Length;
int numBytesRead = 0;
while (numBytesToRead > 0)
{
    // Read may return anything from 0 to 10.
    int n = s.Read(bytes, numBytesRead, 10);
    // The end of the file is reached.
    if (n == 0)
    {
        break;
    }
    numBytesRead += n;
    numBytesToRead -= n;
}

这样就不需要强制转换,如果选择一个相当大的数字读入缓冲区,则只需通过 while 循环一次。