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CSharp · Console

/Program.cs

using System;
using System.Collections.ObjectModel;

public class InventoryItem
{
    public string Name { get; set; }
}

public class SuperInventoryItem : InventoryItem { }

public abstract class InventoryItemAbstractFactory<T> where T : InventoryItem
{
    public abstract T Create();
}

public class InventoryItemFactory : InventoryItemAbstractFactory<InventoryItem>
{
    public override InventoryItem Create() => new InventoryItem();
}

public class SuperInventoryItemFactory : InventoryItemAbstractFactory<SuperInventoryItem>
{
    public override SuperInventoryItem Create() => new SuperInventoryItem();
}

public class Inventory<T> where T : InventoryItem
{
    private readonly ObservableCollection<T> _inventory = new ObservableCollection<T>();
    private readonly InventoryItemAbstractFactory<T> _factory;
    public Inventory(InventoryItemAbstractFactory<T> factory) => _factory = factory;
    public void Add() => _inventory.Add(_factory.Create());
    public int Count => _inventory.Count;
}

public class Program
{
    public static void Main()
    {
        var c1 = new Inventory<InventoryItem>(new InventoryItemFactory());
        c1.Add();
        c1.Add();

        var c2 = new Inventory<SuperInventoryItem>(new SuperInventoryItemFactory());
        c2.Add();
        c2.Add();

        Console.WriteLine(c1.Count);
        Console.WriteLine(c2.Count);
    }
}