C# 请求被中止: 未能创建 SSL/TLS 安全通道。 设置SecurityProtocol无效

今天为了获取一张图片，用了一段代码：

ServicePointManager.ServerCertificateValidationCallback += new RemoteCertificateValidationCallback((a, b, c, d) => true);

ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

ServicePointManager.Expect100Continue = false;

HttpWebRequest req = (HttpWebRequest)WebRequest.Create(imgUrl);

req.AllowAutoRedirect = true;

req.KeepAlive = true;

req.Method = "GET";

WebResponse resp = req.GetResponse();

Stream stream = resp.GetResponseStream();

里面添加了ServicePointManager.SecurityProtocol，但是一直还是报错："请求被中止: 未能创建 SSL/TLS 安全通道。"

感觉非常奇怪，然后通过DevTools看了一下图片网址的协议版本：

发现版本已经到了TLS 1.3了，因为我本地的版本是.net framework 4.7 ，还没有支持Tls1.3版本，于是修改了ServicePointManager.SecurityProtocol：

ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12 | (SecurityProtocolType)12288;

结果换了新的报错：不支持请求的安全协议。

网上说只有最新的.net 5里面才支持了Tls1.3版本，那.net framework系列就没有解决办法了吗？

C#

.NET40兼容，默认框架越新集成的TLS协议版本就越高，最新最安全的是TLS1.3版本。

[SecurityCritical]

[SecuritySafeCritical]

private static void InitialSecurityProtocol()

{

    try

    {

        ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3

                | SecurityProtocolType.Tls

            | (SecurityProtocolType)0x300 // Tls11

            | (SecurityProtocolType)0xC00 // Tls12

            | (SecurityProtocolType)0x3000; // Tls13

    }

    catch (Exception)

    {

        try

        {

            ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3

                | SecurityProtocolType.Tls

                | (SecurityProtocolType)0x300 // Tls11

                | (SecurityProtocolType)0xC00; // Tls12

        }

        catch (Exception)

        {

            try

            {

                ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3

                    | SecurityProtocolType.Tls

                    | (SecurityProtocolType)0x300; // Tls11

            }

            catch (Exception)

            {

                try

                {

                    ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3

                        | SecurityProtocolType.Tls;

                }

                catch (Exception) { }

            }

        }

    }

}

解决C#中WebClient不能下载https内容问题

在下载代码之前加入如下代码：

// 解决WebClient不能通过https下载内容问题

System.Net.ServicePointManager.ServerCertificateValidationCallback +=

delegate(object sender, System.Security.Cryptography.X509Certificates.X509Certificate certificate,

         System.Security.Cryptography.X509Certificates.X509Chain chain,

         System.Net.Security.SslPolicyErrors sslPolicyErrors)

{

return true; // **** Always accept

};

上述代码主要是为了跳过https的ssl验证。