BDD with only XUnit

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xunit;

namespace Specs.Registration
{
    public enum UserStatus
    {
        Pending
    }

    public class UserActivityLog
    {
        public UserActivityLog()
        {
            CreatedAt = DateTime.Now;
            ID = Guid.NewGuid();
        }

        public Guid ID { get; set; }
        public Guid UserID { get; set; }
        public string Subject { get; set; }
        public string Entry { get; set; }
        public DateTime CreatedAt { get; set; }
    }

    public class UserMailerLog
    {
        public UserMailerLog()
        {
            CreatedAt = DateTime.Now;
            ID = Guid.NewGuid();
        }

        public Guid ID { get; set; }
        public Guid UserID { get; set; }
        public string Subject { get; set; }
        public string Body { get; set; }
        public DateTime CreatedAt { get; set; }
    }

    public class User
    {
        public User()
        {
            ID = Guid.NewGuid();
            Status = UserStatus.Pending;
            Logs = new List<UserActivityLog>();
            CreatedAt = DateTime.Now;
        }

        public string Email { get; set; }
        public Guid ID { get; set; }
        public UserStatus Status;
        public ICollection<UserActivityLog> Logs { get; set; }
        public ICollection<UserMailerLog> MailerLogs { get; set; }
        public DateTime CreatedAt { get; set; }
    }

    public class Registrator
    {
        public User ApplyForMembership()
        {
            var user = new User();
            user.Logs.Add(new UserActivityLog { Subject = "Registration", Entry = "User " + user.Email + " succesfully registred", UserID = user.ID });
            return user;
        }
    }

    [Trait("A valid application is submitted", "")]
    public class ValidApplicationReceived
    {
        Registrator _reg;
        User _user;

        public ValidApplicationReceived()
        {
            _reg = new Registrator();
            _user = _reg.ApplyForMembership();
        }

        [Fact]
        public void User_Is_Add_To_System()
        {
            Assert.NotNull(_user);
        }

        [Fact]
        public void User_Status_Set_To_Pending()
        {
            Assert.Equal(UserStatus.Pending, _user.Status);
        }

        [Fact]
        public void Log_Entry_Is_Created_For_Event()
        {
            Assert.Equal(1, _user.Logs.Count);
        }

        [Fact]
        public void Email_Sent_To_Confirm_Address()
        {
            throw new NotImplementedException("Implement me");
        }

        [Fact]
        public void A_Message_Is_Provided_For_User()
        {
            throw new NotImplementedException("Implement me");
        }
    }
}