使用post请求时报出【远程服务器返回错误: (411) 所需的长度。】异常

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情况1：
增加代码：request.ContentLength = 0;

 public string GetCard(string idcard)
        {
            string host = "http://******/doCheckIDCard?cidcode=" + idcard;
            Encoding encoding = Encoding.Default;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(host);
            request.Method = "post";
            request.KeepAlive = true;
            request.ContentLength = 0;
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.Default);
            string result = reader.ReadToEnd();
            return result;
        }

情况2：
增加：request.ContentLength = buffer.Length;

public string getaa()
{
    string token = getBaiduTxtAccessToken();
                string host = "https://******/idcard?access_token=" + token;
                Encoding encoding = Encoding.UTF8;
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(host);
                request.Method = "post";
                request.KeepAlive = true;
                String str = "id_card_side=" + "front" + "&image=" + HttpUtility.UrlEncode(idcardimg.Replace(" ", "+"));
                byte[] buffer = encoding.GetBytes(str);
                request.ContentLength = buffer.Length;
                request.GetRequestStream().Write(buffer, 0, buffer.Length);
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.UTF8);
                string result = reader.ReadToEnd();
}