c# post请求传递中文参数出错求大神解答

/// <summary>
/// 发送post请求
/// </summary>
/// <returns>The post.</returns>
/// <param name="Url">URL.</param>
/// <param name="postDataStr">Post data string.</param>
public string HttpPost(string Url, string postDataStr)
{
CookieContainer cookie = new CookieContainer();
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);
request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";
request.ContentLength = Encoding.UTF8.GetByteCount(postDataStr);
//MessageBox.Show(request.ContentLength.ToString());
request.CookieContainer = cookie;
Stream myRequestStream = request.GetRequestStream();
StreamWriter myStreamWriter = new StreamWriter(myRequestStream, Encoding.GetEncoding("gb2312"));
myStreamWriter.Write(postDataStr);
myStreamWriter.Close();
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
response.Cookies = cookie.GetCookies(response.ResponseUri);
Stream myResponseStream = response.GetResponseStream();
StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.GetEncoding("utf-8"));
string retString = myStreamReader.ReadToEnd();
myStreamReader.Close();
myResponseStream.Close();
return retString;
}
这是百度的一段代码，传递英文参数没问题，传递中文参数出现如下错误，怎么解决，跪求解答