SNI Test Code

Program.cs

using System;
using System.Net;
using System.Net.Security;
using System.Net.Sockets;
using System.Runtime.InteropServices;
using System.Security.Authentication;
using System.Security.Cryptography.X509Certificates;

namespace SNI.Test.Local
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length == 0)
            {
                args = new[] {"https://localhost"};
            }

            Console.WriteLine(RuntimeInformation.FrameworkDescription);
            Console.WriteLine(GetCommonName(new Uri(args[0])));

            Console.WriteLine("Finished.");
            Console.ReadLine();
        }

        private static string certSubject = null;

        private static string GetCommonName(Uri uri)
        {
            var ipAddresses = Dns.GetHostAddressesAsync(uri.Host).GetAwaiter().GetResult();

            foreach (var ipAddress in ipAddresses)
            {
                try
                {
                    var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
                    socket.Connect(ipAddress, uri.Port);

                    using (var stream = new NetworkStream(socket, true))
                    {
                        using (var tlsStream = new SslStream(stream, true, UserCertificateValidationCallback,
                            LocalCertificateSelectionCallback, EncryptionPolicy.RequireEncryption))
                        {
                            tlsStream.AuthenticateAsClientAsync(uri.Host, null, SslProtocols.Tls12, false).GetAwaiter().GetResult();
                        }
                    }

                    return certSubject;
                }
                catch (Exception e)
                {
                    return null;
                }
            }

            return null;
        }

        private static bool UserCertificateValidationCallback(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslpolicyerrors)
        {
            certSubject = certificate.Subject;
            return true;
        }

        private static X509Certificate LocalCertificateSelectionCallback(object sender, string targethost, X509CertificateCollection localcertificates, X509Certificate remotecertificate, string[] acceptableissuers)
        {
            return null;
        }

    }
}

SNI.Test.Local.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>netcoreapp1.0;net452</TargetFrameworks>
  </PropertyGroup>
  
  <ItemGroup>
    <PackageReference Include="System.Runtime.InteropServices.RuntimeInformation" Version="4.0.0" />
  </ItemGroup>

</Project>