C# 发送Web服务请求时出错：必须提供请求正文

我已经写了下面的代码-

HttpWebRequest lHttpWebRequest = (HttpWebRequest)WebRequest.Create(url);
      lHttpWebRequest.Method = "POST";
      lHttpWebRequest.Headers.Add("Accept-Encoding", "gzip,deflate");

      lHttpWebRequest.ContentType = "text/xml";
      lHttpWebRequest.KeepAlive = false;

ASCIIEncoding lEnCoding = new ASCIIEncoding();

      byte[] lData = lEnCoding.GetBytes(xmlDocument.InnerXml);
      lHttpWebRequest.ContentLength = lData.Length;
HttpWebResponse lHttpWebResponse = (HttpWebResponse)lHttpWebRequest.GetResponse();

我犯了一个错误-

You must provide a request body if you set ContentLength>0 or SendChunked==true.  Do this by calling [Begin]GetRequestStream before [Begin]GetResponse.

我在最后一行出错了-

  HttpWebResponse lHttpWebResponse = (HttpWebResponse)lHttpWebRequest.GetResponse();

我补充说-

  ASCIIEncoding lEnCoding = new ASCIIEncoding();

  byte[] lData = lEnCoding.GetBytes(xmlDocument.InnerXml);
  lHttpWebRequest.ContentLength = lData.Length;

  using (Stream requestStream = lHttpWebRequest.GetRequestStream())
  {
    requestStream.Write(lData, 0, lData.Length);
    requestStream.Close();
  }

这起作用了