c# HttpWebRequest https 提示基础连接已经关闭: 发送时发生错误。

在工作中要获取一个网络api下的内容，因为要auth认证，本以为很简单

string url="https://.....";
string usernamePassword = CustomerID + ":" + CustomerCertificate;
string basic = Convert.ToBase64String(new ASCIIEncoding().GetBytes(usernamePassword));
HttpWebRequest myReq = (HttpWebRequest)WebRequest.Create(url);
myReq.Headers.Add("Authorization", "Basic " + basic);
HttpWebResponse myRes = (HttpWebResponse)myReq.GetResponse();  //<-- 这里就报错了
Stream resStream = myRes.GetResponseStream();
StreamReader strReader = new StreamReader(resStream);
string resStr = strReader.ReadToEnd();
Console.WriteLine(resStr);

结果运行时提示“提示基础连接已经关闭: 发送时发生错误。”
在浏览器下访问网址则正常，刚开始怀疑是basic不对，在浏览器下用同样的值模拟了一下能正常获取，然后把https换成http程序又能正常获取了，定位到问题出在ssl证书上
查了一下资料
添加以下代码

ServicePointManager.Expect100Continue = true;
ServicePointManager.CheckCertificateRevocationList = true;
ServicePointManager.DefaultConnectionLimit = 100;
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls; 
ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };

改回https还是提示原来的异常，换了另外一个https的网址测试却又正常，在浏览器下对比两个https网址的ssl证书，发现能正常访问的测试网址是tls1.0 不能访问的网址是tls1.2
找到问题了，原来在.net4.5以下tls1.2的设置如下

ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072; 
//.net4.5及以上
//ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

再次运行，ok