关于C#通过HttpWebRequest调用web api 接口post 传参的有关问题

关于C#通过HttpWebRequest调用web api 接口post 传参的问题
场景是 winform程序通过HttpWebRequest 调用web Api 接口
其中参数没传过去,有做过这东西的,给点建议。
下面贴代码;


  
 /// <summary>  
        /// 创建POST方式的HTTP请求  
        /// </summary>  
        /// <param name="url">请求的URL</param>  
        /// <param name="parameters">随同请求POST的参数名称及参数值字典</param>  
        /// <param name="timeout">请求的超时时间</param>  
        /// <param name="userAgent">请求的客户端浏览器信息,可以为空</param>  
        /// <param name="requestEncoding">发送HTTP请求时所用的编码</param>  
        /// <param name="cookies">随同HTTP请求发送的Cookie信息,如果不需要身份验证可以为空</param>  
        /// <returns></returns>  
        public static HttpWebResponse CreatePostHttpResponse(string url, IDictionary<string, string> parameters, int? timeout, string userAgent, Encoding requestEncoding, CookieCollection cookies)
        {
            if (string.IsNullOrEmpty(url))
            {
                throw new ArgumentNullException("url");
            }
            if (requestEncoding == null)
            {
                throw new ArgumentNullException("requestEncoding");
            }
            HttpWebRequest request = null;
            //如果是发送HTTPS请求  
            if (url.StartsWith("https", StringComparison.OrdinalIgnoreCase))
            {
                ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CheckValidationResult);
                request = WebRequest.Create(url) as HttpWebRequest;
                request.ProtocolVersion = HttpVersion.Version10;
            }
            else
            {
                request = WebRequest.Create(url) as HttpWebRequest;
            }
            request.Method = "POST";
            
            request.Headers.Add("X_REG_CODE", "288a633ccc1");
            request.Headers.Add("X_MACHINE_ID", "a306b7c51254cfc5e22c7ac0702cdf87");
            request.Headers.Add("X_REG_SECRET", "de308301cf381bd4a37a184854035475d4c64946");
            request.Headers.Add("X_STORE", "0001");
            request.Headers.Add("X_BAY", "0001-01");
            request.Headers.Add("X-Requested-With", "XMLHttpRequest");
            request.ContentType = "application/x-www-form-urlencoded";
            request.Headers.Add("Accept-Language", "zh-CN");
            request.Headers.Add("Accept-Encoding", "gzip, deflate");
            request.Accept = "*/*";
            
            if (!string.IsNullOrEmpty(userAgent))
            {
                request.UserAgent = userAgent;