C# 访问https 未能创建 SSL/TLS 安全通道

C# 访问https请求被中止: 未能创建 SSL/TLS 安全通道(Could not create SSL/TLS secure channel)

一般GetResponse可以直接访问https,如果不行添加回调:

ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(CheckValidationResult);
还不行, 添加: ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;

?

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public string GetHttpWebResponse(string url, string postData, string code = "utf-8") {     ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;     ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(CheckValidationResult);     HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url);     HttpWebResponse response = null;     try     {         response = (HttpWebResponse)request.GetResponse();         using (StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.GetEncoding(code)))         {             return reader.ReadToEnd();         }     }     catch (WebException e)     {         return e.Message;     }     finally     {         if (response != null) response.Close();     } }   public static bool CheckValidationResult(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors) {     return true; }