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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 class InventoriItemFabric
{
	public static InventoryItem Create()
	{
		return new InventoryItem();
	}
}

public class SuperInventoriItemFabric
{
	public static SuperInventoryItem Create()
	{
		return new SuperInventoryItem();
	}
}

public class Inventory<T>
	where T : InventoryItem
{
	private ObservableCollection<T> _inventory = new ObservableCollection<T>();
	public Inventory()
	{
	}

	public void Add(T item)
	{
		_inventory.Add(item);
	}

	public int Count
	{
		get => _inventory.Count;
	}
}

public class Program
{
	public static void Main()
	{
		var c1 = new Inventory<InventoryItem>();
		c1.Add(InventoriItemFabric.Create());
		c1.Add(SuperInventoriItemFabric.Create());
		
		var c2 = new Inventory<SuperInventoryItem>();
		//c2.Add(InventoriItemFabric.Create());			// Компилятор так сделать не даст
		c2.Add(SuperInventoriItemFabric.Create());
		
		Console.WriteLine(c1.Count);
		Console.WriteLine(c2.Count);
	}
}