https://qiita.com/shoheiyokoyama/items/d752834a6a2e208b90ca よりFactory MethodをC#で写経。

Program.cs

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

namespace FactoryMethodPatternTest
{
    public abstract class Product
    {
        public abstract void Use();
    }

    public abstract class Factory
    {
        public Product Create(string owner)
        {
            Product product = createProduct(owner);
            registerProduct(product);
            return product;
        }
        protected abstract Product createProduct(string owner);
        protected abstract void registerProduct(Product product);
    }

    public class AccountFactory : Factory
    {
        private List<string> owners = new List<string>();

        protected override Product createProduct(string owner)
        {
            return new Account(owner);
        }
        protected override void registerProduct(Product product)
        {
            owners.Add(((Account)product).getOwner());
        }
    }

    public class Account : Product
    {
        private string owner;

        public Account(string owner)
        {
            Console.WriteLine("Create account: " + owner);
            this.owner = owner;
        }

        public override void Use()
        {
            Console.WriteLine("Use account: " + owner);
        }
        public string getOwner()
        {
            return owner;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Factory factory = new AccountFactory();
            Product account1 = factory.Create("Ralph");
            Product account2 = factory.Create("RIchard");
            Product account3 = factory.Create("John");
            Product account4 = factory.Create("Erich");

            account1.Use();
            account2.Use();
            account3.Use();
            account4.Use();
        }
    }
}