httpclient cer

X509Certificate2 cer = new X509Certificate2(@"path", "********",
X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable);
            HttpClientHandler handler = new HttpClientHandler();
            handler.UseDefaultCredentials = true;
            handler.ClientCertificateOptions = ClientCertificateOption.Automatic;
            HttpClient httpClient = new HttpClient(handler);
            var stringContent = new StringContent(requestXml, System.Text.Encoding.UTF8);
            var req = await httpClient.PostAsync("https://api.mch.weixin.qq.com/secapi/pay/refund", stringContent);

 private static bool CheckValidationResult(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors)
        {
            return true; //总是接受
        }


 ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CheckValidationResult);
                request = WebRequest.Create(url) as HttpWebRequest;
                request.ProtocolVersion = HttpVersion.Version10;

update: I ran the exact same thing on Windows 7 and it worked exactly as needed.

// using System.Net.Http;
// using System.Security.Authentication;
// using System.Security.Cryptography.X509Certificates;

var handler = new HttpClientHandler();
handler.ClientCertificateOptions = ClientCertificateOption.Manual;
handler.SslProtocols = SslProtocols.Tls12;
handler.ClientCertificates.Add(new X509Certificate2("cert.crt"));
var client = new HttpClient(handler);
var result = client.GetAsync("https://apitest.startssl.com").GetAwaiter().GetResult();

        var clientCertificate = await HttpContext.Connection.GetClientCertificateAsync();

        if(clientCertificate!=null)
            return new ContentResult() { Content = clientCertificate.Subject };

https://blog.pedrofelix.org/2012/12/16/using-httpclient-with-ssltls/

HttpClientHandler and WebRequestHandler.

The first option is to explicitly configure the HttpClient with a HttpClientHandler instance, containing its ClientCertificateOptions property set to Automatic.

var client = new HttpClient(

new HttpClientHandler{
   ClientCertificateOptions = ClientCertificateOption.Automatic
});
// ...

For classical scenarios (e.g. console, WinForms or WPF applications) there is a second option using the WebRequestHandler, which provides more control over the configuration.

var clientHandler = new WebRequestHandler()
clientHandler.ClientCertificates.Add(cert);
var client = new HttpClient(clientHandler)

where cert is a X509Certificate2 instance representing the client certificate.
This instance can be constructed directly from a PFX file or obtained from a Windows certificate store

X509Store store = null;
try
{
  store = new X509Store(StoreName.My, StoreLocation.CurrentUser);
  store.Open(OpenFlags.OpenExistingOnly | OpenFlags.ReadOnly);
  // select the certificate from store.Certificates ...
}
finally
{
  if(store != null) store.Close();
}