在post的时候有时也用的到cookie，像登录163发邮件时候就需要发送cookie，所以在外部一个cookie属性随时保存 CookieContainer cookie = new CookieContainer();

！注意：有时候请求会重定向，但我们就需要从重定向url获取东西，像QQ登录成功后获取sid，但上面的会自动根据重定向地址跳转。我们可以用:
request.AllowAutoRedirect = false;设置重定向禁用，你就可以从headers的Location属性中获取重定向地址

Post另一实现：

post 调用时传参，如：

string url = "http://www.baidu.com/";
            string result = string.Empty;

            string param = string.Format("WechatOpenID={0}&Content={1}", webchatOpenID, content);
            result = HttpPostData(url, param);

public string HttpPostData(string url, string param)
        {
            var result = string.Empty;
            //注意提交的编码 这边是需要改变的 这边默认的是Default：系统当前编码
            byte[] postData = Encoding.UTF8.GetBytes(param);

            // 设置提交的相关参数
            HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
            Encoding myEncoding = Encoding.UTF8;
            request.Method = "POST";
            request.KeepAlive = false;
            request.AllowAutoRedirect = true;
            request.ContentType = "application/x-www-form-urlencoded";
            request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 2.0.50727; .NET CLR  3.0.04506.648; .NET CLR 3.5.21022; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)";
            request.ContentLength = postData.Length;

            // 提交请求数据
            System.IO.Stream outputStream = request.GetRequestStream();
            outputStream.Write(postData, 0, postData.Length);
            outputStream.Close();

            HttpWebResponse response;
            Stream responseStream;
            StreamReader reader;
            string srcString;
            response = request.GetResponse() as HttpWebResponse;
            responseStream = response.GetResponseStream();
            reader = new System.IO.StreamReader(responseStream, Encoding.GetEncoding("UTF-8"));
            srcString = reader.ReadToEnd();
            result = srcString;   //返回值赋值
            reader.Close();

            return result;
        }

Post传输图片等时的代码示例:

Post多张图片和其他参数时，用此方法进行调用，调用方法如：

//imgDic是图片接收名称如media,图片本地完整路径或网络完整路径的键值对的集合

imgDesc是图片信息参数及其他参数的集合，图片信息如media1,media2,media3等，这个图片接收名字是由接收方确定的

注意：

FileStream只能用于本地完整路径，如C/xx.jpg等，如果是网路图片则无法使用它。

推荐使用

WebClient wc = new WebClient();
            byte[] buffer = wc.DownloadData(filePath);
            memStream.Write(buffer, 0, buffer.Length);

这种方式，这个类无论是网路图片还是本地图片都可以使用，但路径中要用/，而不是\，否则会出现不支持URI的错误。

if (openIDList != null && openIDList.Any() && imgDesc != null)
            {
                Dictionary<string, string> imgDic = new Dictionary<string, string>();
                foreach (var item in imgDesc)
                {
                    imgDic.Add(item.name, item.path);
                }

                NameValueCollection col = new NameValueCollection();
                col.Add("imgDesc", Newtonsoft.Json.JsonConvert.SerializeObject(imgDesc));
                col.Add("open_ids", string.Join(",", openIDList));
                result = HttpPostData(url, 60000, imgDic, col);
            }

private static string HttpPostData(string url, int timeOut, Dictionary<string, string> imgDic, NameValueCollection stringDict)
        {
            LogEntry entry = new LogEntry("发送图片开始-HttpPostData -- " + stringDict["open_ids"], 1);
            LogStub.Log(entry);

            var firstImg = imgDic.FirstOrDefault();
            string fileKeyName = firstImg.Key;
            string filePath = firstImg.Value;

            string responseContent;
            var memStream = new MemoryStream();
            var webRequest = (HttpWebRequest)WebRequest.Create(url);
            // 边界符
            var boundary = "---------------" + DateTime.Now.Ticks.ToString("x");
            // 边界符
            var beginBoundary = Encoding.ASCII.GetBytes("--" + boundary + "\r\n");
            //var fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
           
            // 最后的结束符
            var endBoundary = Encoding.ASCII.GetBytes("--" + boundary + "--\r\n");

            // 设置属性
            webRequest.Method = "POST";
            webRequest.Timeout = timeOut;
            webRequest.ContentType = "multipart/form-data; boundary=" + boundary;

            // 写入文件
            const string filePartHeader =
                "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\n" +
                 "Content-Type: application/octet-stream\r\n\r\n";
            var header = string.Format(filePartHeader, fileKeyName, filePath);
            var headerbytes = Encoding.UTF8.GetBytes(header);

            memStream.Write(beginBoundary, 0, beginBoundary.Length);
            memStream.Write(headerbytes, 0, headerbytes.Length);

            //var buffer = new byte[1024];
            //int bytesRead; // =0

            //while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0)
            //{
            //    memStream.Write(buffer, 0, bytesRead);
            //}

            WebClient wc = new WebClient();
            byte[] buffer = wc.DownloadData(filePath);
            memStream.Write(buffer, 0, buffer.Length);

            //第二章图片
            //memStream.Write(beginBoundary, 0, beginBoundary.Length);

            //var aaa = Encoding.ASCII.GetBytes("\r\n--" + boundary + "\r\n");
            //memStream.Write(aaa, 0, aaa.Length);

            string imgName = string.Empty;
            string imgPath = string.Empty;
            foreach (var img in imgDic.Where(p => p.Key != fileKeyName))
            {
                imgName = img.Key;
                imgPath = img.Value;

                string nxetFileFormat = "\r\n--" + boundary + "\r\n" + filePartHeader;

                header = string.Format(nxetFileFormat, imgName, imgPath);
                headerbytes = Encoding.UTF8.GetBytes(header);

                memStream.Write(headerbytes, 0, headerbytes.Length);
                //fileStream = new FileStream(imgPath, FileMode.Open, FileAccess.Read);
                //while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0)
                //{
                //    memStream.Write(buffer, 0, bytesRead);
                //}

                buffer = wc.DownloadData(imgPath);
                memStream.Write(buffer, 0, buffer.Length);
            }

            // 写入字符串的Key
            var stringKeyHeader = "\r\n--" + boundary +
                                   "\r\nContent-Disposition: form-data; name=\"{0}\"" +
                                   "\r\n\r\n{1}\r\n";

            foreach (byte[] formitembytes in from string key in stringDict.Keys
                                             select string.Format(stringKeyHeader, key, stringDict[key])
                                                 into formitem
                                                 select Encoding.UTF8.GetBytes(formitem))
            {
                memStream.Write(formitembytes, 0, formitembytes.Length);
            }

            // 写入最后的结束边界符
            memStream.Write(endBoundary, 0, endBoundary.Length);

            webRequest.ContentLength = memStream.Length;

            var requestStream = webRequest.GetRequestStream();

            memStream.Position = 0;
            var tempBuffer = new byte[memStream.Length];
            memStream.Read(tempBuffer, 0, tempBuffer.Length);
            memStream.Close();

            requestStream.Write(tempBuffer, 0, tempBuffer.Length);
            requestStream.Close();

            var httpWebResponse = (HttpWebResponse)webRequest.GetResponse();

            using (var httpStreamReader = new StreamReader(httpWebResponse.GetResponseStream(),
                                                            Encoding.GetEncoding("utf-8")))
            {
                responseContent = httpStreamReader.ReadToEnd();
            }

            fileStream.Close();
            httpWebResponse.Close();
            webRequest.Abort();

            entry = new LogEntry("发送图片结束-HttpPostData -- " + responseContent, 1);
            LogStub.Log(entry);

            return responseContent;
        }

单行图片调用以下方法时，如：

NameValueCollection col = new NameValueCollection();
                //col.Add("media", filePath);
                col.Add("title", title);
                col.Add("content", content);
                col.Add("open_ids", string.Join(",", openIDList));

                result = HttpPostData(url, 60000, "media", filePath, col);

filePath是客户端的图片完整路径，

media是服务端接受图片的参数，这个要看接收端接收图片的参数名，注意此方法是单张图片的post调用

private static string HttpPostData(string url, int timeOut, string fileKeyName,
                                    string filePath, NameValueCollection stringDict)
        {
            string responseContent;
            var memStream = new MemoryStream();
            var webRequest = (HttpWebRequest)WebRequest.Create(url);
            // 边界符
            var boundary = "---------------" + DateTime.Now.Ticks.ToString("x");
            // 边界符
            var beginBoundary = Encoding.ASCII.GetBytes("--" + boundary + "\r\n");
            var fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
            // 最后的结束符
            var endBoundary = Encoding.ASCII.GetBytes("--" + boundary + "--\r\n");

            // 设置属性
            webRequest.Method = "POST";
            webRequest.Timeout = timeOut;
            webRequest.ContentType = "multipart/form-data; boundary=" + boundary;

            // 写入文件
            const string filePartHeader =
                "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\n" +
                 "Content-Type: application/octet-stream\r\n\r\n";
            var header = string.Format(filePartHeader, fileKeyName, filePath);
            var headerbytes = Encoding.UTF8.GetBytes(header);

            memStream.Write(beginBoundary, 0, beginBoundary.Length);
            memStream.Write(headerbytes, 0, headerbytes.Length);

            var buffer = new byte[1024];
            int bytesRead; // =0

            while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0)
            {
                memStream.Write(buffer, 0, bytesRead);
            }

            // 写入字符串的Key
            var stringKeyHeader = "\r\n--" + boundary +
                                   "\r\nContent-Disposition: form-data; name=\"{0}\"" +
                                   "\r\n\r\n{1}\r\n";

            foreach (byte[] formitembytes in from string key in stringDict.Keys
                                             select string.Format(stringKeyHeader, key, stringDict[key])
                                                 into formitem
                                                 select Encoding.UTF8.GetBytes(formitem))
            {
                memStream.Write(formitembytes, 0, formitembytes.Length);
            }

            // 写入最后的结束边界符
            memStream.Write(endBoundary, 0, endBoundary.Length);

            webRequest.ContentLength = memStream.Length;

            var requestStream = webRequest.GetRequestStream();

            memStream.Position = 0;
            var tempBuffer = new byte[memStream.Length];
            memStream.Read(tempBuffer, 0, tempBuffer.Length);
            memStream.Close();

            requestStream.Write(tempBuffer, 0, tempBuffer.Length);
            requestStream.Close();

            var httpWebResponse = (HttpWebResponse)webRequest.GetResponse();

            using (var httpStreamReader = new StreamReader(httpWebResponse.GetResponseStream(),
                                                            Encoding.GetEncoding("utf-8")))
            {
                responseContent = httpStreamReader.ReadToEnd();
            }

            fileStream.Close();
            httpWebResponse.Close();
            webRequest.Abort();

            return responseContent;
        }

 原理简介及简单实现：

WebRequest request = WebRequest.Create("http://www.baidu.com/");//为指定的 URI 方案初始化新的 System.Net.WebRequest 实例
        request.UseDefaultCredentials = false;//获取或设置一个 System.Boolean 值，该值控制 System.Net.CredentialCache.DefaultCredentials
        WebResponse response = request.GetResponse();//返回对 Internet 请求的响应。
        Stream resStream = response.GetResponseStream();//返回从 Internet 资源返回数据流
        StreamReader sr = new StreamReader(resStream, System.Text.Encoding.Default);//实例华一个流的读写器
        ContentHtml.Text = sr.ReadToEnd();//这就是百度首页的HTML哦 ,字符串形式的流的其余部分（从当前位置到末尾）。如果当前位置位于流的末尾，则返回空字符串 ("")
        resStream.Close();//关闭当前流并释放与之关联的所有资源
        sr.Close(); //关闭 System.IO.StreamReader 对象和基础流，并释放与读取器关联的所有系统资源

HttpWebRequest 和 HttpWebResponse 的应用

另一种实现方式：

/// <summary>
        /// Post data到url
        /// </summary>
        /// <param name="data">要post的数据</param>
        /// <param name="url">目标url</param>
        /// <returns>服务器响应</returns>
        static string PostDataToUrl(string data, string url)
        {
            Encoding encoding = Encoding.GetEncoding(sRequestEncoding);
            byte[] bytesToPost = encoding.GetBytes(data);
            return PostDataToUrl(bytesToPost, url);
        }

        /// <summary>
        /// Post data到url
        /// </summary>
        /// <param name="data">要post的数据</param>
        /// <param name="url">目标url</param>
        /// <returns>服务器响应</returns>
        static string PostDataToUrl(byte[] data, string url)
        {
            #region 创建httpWebRequest对象
            WebRequest webRequest = WebRequest.Create(url);
            HttpWebRequest httpRequest = webRequest as HttpWebRequest;
            if (httpRequest == null)
            {
                throw new ApplicationException(
                string.Format("Invalid url string: {0}", url)
                );
            }
            #endregion

            #region 填充httpWebRequest的基本信息
            httpRequest.UserAgent = sUserAgent;
            httpRequest.ContentType = sContentType;
            httpRequest.Method = "POST";
            #endregion

            #region 填充要post的内容
            httpRequest.ContentLength = data.Length;
            Stream requestStream = httpRequest.GetRequestStream();
            requestStream.Write(data, 0, data.Length);
            requestStream.Close();
            #endregion

            #region 发送post请求到服务器并读取服务器返回信息
            Stream responseStream;
            try
            {
                responseStream = httpRequest.GetResponse().GetResponseStream();
            }
            catch (Exception e)
            {
                // log error
                Console.WriteLine(
                string.Format("POST操作发生异常：{0}", e.Message)
                );
                throw e;
            }
            #endregion

            #region 读取服务器返回信息
            string stringResponse = string.Empty;
            using (StreamReader responseReader =
            new StreamReader(responseStream, Encoding.GetEncoding(sResponseEncoding)))
            {
                stringResponse = responseReader.ReadToEnd();
            }
            responseStream.Close();
            #endregion
            return stringResponse;
        }

调用时：

string param = string.Format("time={0}&oid={1}&order={2}&name={3}&area={4}&point={5}&amount={6}&return={7}&hash={8}&version={9}", Time, Oid, OrderID, LoginName, Area, Point, Amount, ReturnUrl, HashMoli, Version);

string result = PostDataToUrl(param, PostUrl);

还有一种实现的例子：

使用httpwebrequest Post数据到网站