C# 请求被中止：无法使用Restsharp创建SSL/TLS安全通道

我正在尝试连接服务文件（.svc）。从我的c#代码中，但我得到的信息是

请求被中止：无法创建SSL/TLS安全通道

服务器只接受下面的密码

接受的密码如下所示：

TLSv1.2 128位ECDHE-RSA-AES128-GCM-SHA256曲线p-256 DHE 256

TLSv1.2 256位ECDHE-RSA-AES256-GCM-SHA384曲线p-256 DHE 256

代码：

尝试了以下解决方案

1） 在TLS1.2启用的行下方添加

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(delegate { return true; });

谢谢和问候

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(delegate { return true; });

var client = new RestClient(url);
var request = new RestRequest(Method.POST);
request.AddHeader("Content-Type", "application/xml");
request.AddParameter("undefined", str, ParameterType.RequestBody);
IRestResponse response = client.Execute(request);

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(delegate { return true; });