C# HTTP POST（System.Net.WebException：请求已中止：请求已被取消）

一.问题描述

最近在使用c# http post请求时，遇见了这种异常，大概异常提示如下

System.Net.WebException：请求已中止：请求已被取消，在数据写入前关闭了数据流

错误代码如下：

        /// <summary>
        /// 向指定URL发送POST请求
        /// </summary>
        /// <param name="url">发送请求的URL</param>
        /// <param name="jsonData">请求参数，请求参数应该是Json格式字符串的形式</param>
        /// <param name="contenType"><设置响应信息的编码格式，如utf-8/param>
        /// <param name="header">请求头</param>
        /// <returns>所代表远程资源的响应结果</returns>
        public static string SendPost(string url,string jsonData,Encoding encoding,string contenType="application/json",Dictionary<string,string>header=null)
        {
            string result =string.Empty;
            try
            {
                CookieContainer cookie = new CookieContainer();

                HttpWebRequest request= (HttpWebRequest)WebRequest.Create(url);
                request.Method = "POST";
                if(null != header && 0 < header.Count())
                {
                    foreach(var item in header.Keys)
                    {
                        request.Headers.Add(item, header[item]);
                    }
                }
                request.ServicePoint.Expect100Continue = false;
                request.ContentType = contenType;
                request.ContentLength = Encoding.UTF8.GetByteCount(jsonData);
                request.CookieContainer = cookie;
                using (StreamWriter writer = new StreamWriter(request.GetRequestStream(), encoding))
                {
                    writer.Write(jsonData);
                }

                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                response.Cookies = cookie.GetCookies(response.ResponseUri);
                using (Stream responseStream=response.GetResponseStream())
                {
                    using(StreamReader reader= new StreamReader(responseStream,encoding))
                    {
                        result = reader.ReadToEnd();
                        reader.Close();
                    }

                    responseStream.Close();
                }
                response.Close();
                response = null;
                request = null;
            }
            catch (Exception ex)
            {
                string msg = ex.Message;
            }

            return result;
        }

通过代码调试，出问题的大致位置在这里：

 using (StreamWriter writer = new StreamWriter(request.GetRequestStream(), encoding))
 {
     writer.Write(jsonData);
 }

二.问题分析

在Write时，数据流意外关闭了，触发了http数据传输异常，为什么会出现这种情况，使用其它接口时不会出现这种问题，通过分析代码以及查阅资料发现了出问题的原因，我在调用接口函数时是这样的：

SendPost(url, jsonData, Encoding.Default, contenType, header);

这里我指定了编码格式为Encoding.Default，在函数内部计算request.ContentLength 长度的时候却指定了UTF8编码，如下：

request.ContentLength = Encoding.UTF8.GetByteCount(jsonData);

这种情况如果当前默认刚好是UTF8编码，则不会出问题，如果是其它编码，则数据流读取时会出问题（编码解析规则），导致意外结束，分析问题大概是这样

三.问题修改

修改后的代码如下，使用前后一致的编码格式

        /// <summary>
        /// 向指定URL发送POST请求
        /// </summary>
        /// <param name="url">发送请求的URL</param>
        /// <param name="jsonData">请求参数，请求参数应该是Json格式字符串的形式</param>
        /// <param name="contenType"><设置响应信息的编码格式，如utf-8/param>
        /// <param name="header">请求头</param>
        /// <returns>所代表远程资源的响应结果</returns>
        public static string SendPost(string url,string jsonData,Encoding encoding,string contenType="application/json",Dictionary<string,string>header=null)
        {
            string result =string.Empty;
            try
            {
                CookieContainer cookie = new CookieContainer();

                HttpWebRequest request= (HttpWebRequest)WebRequest.Create(url);
                request.Method = "POST";
                if(null != header && 0 < header.Count())
                {
                    foreach(var item in header.Keys)
                    {
                        request.Headers.Add(item, header[item]);
                    }
                }
                request.ServicePoint.Expect100Continue = false;
                request.ContentType = contenType;
                request.ContentLength = encoding.GetByteCount(jsonData);
                request.CookieContainer = cookie;
                using (StreamWriter writer = new StreamWriter(request.GetRequestStream(), encoding))
                {
                    writer.Write(jsonData);
                }

                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                response.Cookies = cookie.GetCookies(response.ResponseUri);
                using (Stream responseStream=response.GetResponseStream())
                {
                    using(StreamReader reader= new StreamReader(responseStream,encoding))
                    {
                        result = reader.ReadToEnd();
                        reader.Close();
                    }

                    responseStream.Close();
                }
                response.Close();
                response = null;
                request = null;
            }
            catch (Exception ex)
            {
                string msg = ex.Message;
            }

            return result;
        }