using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var customer = new Customer("John", "Doe");

        customer.Save();

        var savedCustomer = Customer.Find(1);

        Console.WriteLine(savedCustomer.Id);

    }

}

​

public class BaseEntity<T>

{

    public int? Id;

    static List<BaseEntity<T>> Entities = new List<BaseEntity<T>>();

    public static BaseEntity<T> Find(int id)

    {

        return Entities.FirstOrDefault(x => x.Id == id);

    }

    public void Save()

    {

        this.Id = (Entities.Max(x => x.Id) ?? 0) + 1;

        Entities.Add(this);

    }

}

​

public class Customer : BaseEntity<Customer>

{

    public string FirstName {get;set;}

    public string LastName {get;set;}

    public Customer(string firstName, string lastName)

    {

        FirstName = firstName;

        LastName = lastName;

    }

}

​