public class HttpHelper
    {
        public static string Post(string url, string paramData)
        {
            return Post(url, paramData, Encoding.UTF8);
        }

        public static string Post(string url, string paramData, Encoding encoding)
        {
            string result;

            if (url.ToLower().IndexOf("https", System.StringComparison.Ordinal) > -1)
            {
                ServicePointManager.ServerCertificateValidationCallback =  new RemoteCertificateValidationCallback((sender, certificate, chain, errors) => { return true; });
            }

            try
            {
                var wc = new WebClient();
                if (string.IsNullOrEmpty(wc.Headers["Content-Type"]))
                {
                    wc.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
                }
                wc.Encoding = encoding;

                result = wc.UploadString(url, "POST", paramData);
            }
            catch (Exception e)
            {
                throw;
            }

            return result;
        }

        public static string Get(string url)
        {
            return Get(url, Encoding.UTF8);
        }

        public static string Get(string url, Encoding encoding)
        {
            try
            {
                var wc = new WebClient { Encoding = encoding };
                var readStream = wc.OpenRead(url);
                using (var sr = new StreamReader(readStream, encoding))
                {
                    var result = sr.ReadToEnd();
                    return result;
                }
            }
            catch (Exception e)
            {
                throw e;
            }
        }

        public static string PostData(string url, NameValueCollection parameters)
        {
            HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create(url);
            Encoding encoding = Encoding.UTF8;
            var parassb = new StringBuilder();
            foreach (string key in parameters.Keys)
            {
                if (parassb.Length > 0)
                {
                    parassb.Append("&");
                }
                parassb.AppendFormat("{0}={1}", key, HttpUtility.UrlEncode(parameters[key]));
            }

            byte[] bs = Encoding.UTF8.GetBytes(parassb.ToString());

            req.Method = "POST";
            req.ContentType = "application/x-www-form-urlencoded";
            req.ContentLength = bs.Length;
            using (Stream reqStream = req.GetRequestStream())
            {
                reqStream.Write(bs, 0, bs.Length);
                reqStream.Close();
            }

            string result = String.Empty;
            using (HttpWebResponse response = (HttpWebResponse)req.GetResponse())
            {
                using (StreamReader reader = new StreamReader(response.GetResponseStream(), encoding))
                {
                    result = reader.ReadToEnd();
                }
                return result;
            }
        }
    }  

使用示例:

      var data = "{\"touser\":\"Rich\" },";
          data += "{\"msgtype\": \"text\"},";
         data += "{\"text\": [{\"content\": \"test\"}]}";
        var json = HttpHelper.Post("https://api.weixin.qq.com/cgi-bin/message/custom/send?access_token=" + accessToken, data);  

var parameters = new NameValueCollection { { "Aid", dModel.Aid }, { "Oid", dModel.Oid }, { "Uid", dModel.Uid }, { "Cid", dModel.Coupon }, { "CidD", dModel.Usedt.ToString() }, { "Info", dModel.Info }, { "Return", dModel.ReturnVal } };
HttpHelper.PostData("http://www.baidu.com", parameters);