C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;
					
public class Program
{
	public static void Main()
	{
		var product = new Product
        {
            Id = 1453,
            Price = 1000M,
            Stock = 0
        };

        product.Attach(new ProductPriceObserver());
        product.Attach(new ProductStockObserver());

        product.UpdatePrice(999M);
        product.UpdateStock(10);
	}
}

public record ProductObserverArgs
{
    public int Id { get; set; }
    public int? PreviousStock { get; init; }
    public int? NewStock { get; init; }
    public decimal? PreviousPrice { get; init; }
    public decimal? NewPrice { get; init; }
}

public interface IProductObserver
{
    void Update(ProductObserverArgs args);
}

public class ProductStockObserver : IProductObserver
{
    public void Update(ProductObserverArgs args)
    {
        if (args.PreviousPrice.GetValueOrDefault() > 0 || args.NewStock == 0) return;

        //TODO Inject a service and get users who subscribe to the product.
        Console.WriteLine($"The product is in stock. Stock : {args.NewStock}");
    }
}

public class ProductPriceObserver : IProductObserver
{
    public void Update(ProductObserverArgs args)
    {
        if (args.NewPrice.GetValueOrDefault() >= args.PreviousPrice.GetValueOrDefault()) return;

        //TODO Inject a service and get users who subscribe to the product.
        Console.WriteLine($"The product price dropped. {args.PreviousPrice} -> {args.NewPrice}");
    }
}

public class ObservableProduct
{
    private List<IProductObserver> Observers { get; set; } = new();

    public void Attach(IProductObserver observer)
    {
        Observers.Add(observer);
    }

    public void Detach(IProductObserver observer)
    {
        Observers.Remove(observer);
    }

    protected void Notify(ProductObserverArgs args)
    {
        foreach (var observer in Observers)
        {
            observer.Update(args);
        }
    }
}

public class Product : ObservableProduct
{
    public int Id { get; set; }
    public decimal Price { get; set; }
    public int Stock { get; set; }

    public void UpdateStock(int stock)
    {
        var previousStock = Stock;
        Stock = stock;

        Notify(new ProductObserverArgs
        {
            Id = Id,
            PreviousStock = previousStock,
            NewStock = stock
        });
    }

    public void UpdatePrice(decimal price)
    {
        var previousPrice = Price;
        Price = price;

        Notify(new ProductObserverArgs
        {
            Id = Id,
            PreviousPrice = previousPrice,
            NewPrice = price
        });
    }
}

 
using System;
using System.Collections.Generic;
                    
public class Program
{
    public static void Main()
    {
        var product = new Product
        {
            Id = 1453,
            Price = 1000M,
            Stock = 0
        };
​
        product.Attach(new ProductPriceObserver());
        product.Attach(new ProductStockObserver());
​
        product.UpdatePrice(999M);
        product.UpdateStock(10);
    }
}
​
public record ProductObserverArgs
{