使用HttpWebRequest post数据时要注意UrlEncode

今天在用HttpWebResponse类向一个远程页面post数据时，遇到了一个怪问题：通过对比自己post的参数和服务器接收到的值，发现参数中的一个+号被替换成了空格。

造成这个错误的原因在于+号在url中是特殊字符，远程服务器在接受request的时候，把+转成了空格。同样的，如果想post的数据中有&、%等等，也会被服务器转义，所以我们在post的数据的时候，需要先把数据UrlEncode一下。url  encode在bs开发中本来是一个很常见的问题。

修改后的post数据的示例代码如下：

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 = WebRequest.Create(url) as HttpWebRequest;
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";

            if (!string.IsNullOrEmpty(userAgent))
            {
                request.UserAgent = userAgent;
            }

            if (timeout.HasValue)
            {
                request.Timeout = timeout.Value;
            }
            if (cookies != null)
            {
                request.CookieContainer = new CookieContainer();
                request.CookieContainer.Add(cookies);
            }
            //如果需要POST数据
            if (!(parameters == null || parameters.Count == 0))
            {
                StringBuilder buffer = new StringBuilder();
                int i = 0;
                foreach (string key in parameters.Keys)
                {
                    if (i > 0)
                    {
                        buffer.AppendFormat("&{0}={1}", key, parameters[key]);
                    }
                    else
                    {
                        buffer.AppendFormat("{0}={1}", key, parameters[key]);
                    }
                    i++;
                }
                var postData = string.Join("&", parameters.Select(
                p => string.Format("{0}={1}", p.Key, System.Web.HttpUtility.UrlEncode(p.Value, requestEncoding))).ToArray());
                byte[] data = requestEncoding.GetBytes(postData);
                using (Stream stream = request.GetRequestStream())
                {
                    stream.Write(data, 0, data.Length);
                }
            }
            return request.GetResponse() as HttpWebResponse;
        }