如果post请求的网站是https协议时需要加入如下代码

ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CheckValidationResult);

ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072; // SecurityProtocolType.Tls1.2;

完整代码如下

class ProgramTest
    {
        static void Main(string[] args)
        {
            string url = "https://www.test.com";
            string result = PostUrl(url, "key=123"); // key=4da4193e-384b-44d8-8a7f-2dd8b076d784
            Console.WriteLine(result);
            Console.WriteLine("OVER");
            Console.ReadLine();
        }

        private static string PostUrl(string url, string postData)
        {
            HttpWebRequest request = null;
            if (url.StartsWith("https", StringComparison.OrdinalIgnoreCase))
            {
                ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CheckValidationResult);
                request = WebRequest.Create(url) as HttpWebRequest;
                request.ProtocolVersion = HttpVersion.Version11;
　　　　　　　　　// 这里设置了协议类型。
                ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;// SecurityProtocolType.Tls1.2; 
                ServicePointManager.CheckCertificateRevocationList = true;
                ServicePointManager.DefaultConnectionLimit = 100;
                ServicePointManager.Expect100Continue = false;
            }
            else
            {
                request = WebRequest.Create(url) as HttpWebRequest;
            }

                request.Method = "GET";
                request.Accept = "*/*";
                request.ContentType = "application/x-www-form-urlencoded";
                request.AllowAutoRedirect = true;
                request.KeepAlive = false;

            byte[] data = Encoding.UTF8.GetBytes(postData);
            Stream newStream = request.GetRequestStream();
            newStream.Write(data, 0, data.Length);
            newStream.Close();

            //获取网页响应结果
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            Stream stream = response.GetResponseStream();
            response.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
            string result = string.Empty;
            using (StreamReader sr = new StreamReader(stream))
            {
                result = sr.ReadToEnd();
            }

            return result;
        }

        private static bool CheckValidationResult(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors)
        {
            return true; //总是接受  
        }
    }