HttpWebRequest以UTF-8编码写入内容时发生“Bytes to be written to the stream exceed the

request.ContentLength = Encoding.UTF8.GetByteCount(content);
var writer = new StreamWriter(request.GetRequestStream(), Encoding.UTF8);
writer.Write(content);
writer.Flush();
writer.Close();

原因是直接使用Encoding.UTF8，在StreamWriter中使用这个Encoding会在流的开头写入byte order mark. 解决方法是改用new UTF8Encoding(false)来构造不写入byte order mark的Encoding.