请求被中止: 未能创建 SSL/TLS 安全通道。

访问https网站时出现错误：请求被中止: 未能创建 SSL/TLS 安全通道。

解决方法：在创建HttpWebRequest之前创建ServicePointManager
System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

    public string BrowseRequest(string url)
    {
        string result = "";
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "GET";
        request.Timeout = 5000;
        request.ContentType = "application/x-www-form-urlencoded";
        using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
        {
            using (StreamReader sr = new StreamReader(response.GetResponseStream(), Encoding.UTF8))
            {
                result = sr.ReadToEnd();
            }
        }
        return result;
    }