// Exercise 2: avoid the event-handler memory leak (medium)
//
// A long-lived PriceFeed publishes price changes through an event. Short-lived widgets subscribe.
// If they never unsubscribe, the feed keeps every widget alive forever (a classic managed leak).
//
// Implement:
//   - PriceFeed.Publish(price): raise PriceChanged safely (no NullReferenceException when nobody listens).
//   - PriceFeed.SubscriberCount: number of handlers currently attached (use GetInvocationList()).
//   - PriceWidget: subscribes in its constructor, stores the last price it saw, and unsubscribes in Dispose().
//
// Run with: dotnet run (inside a console project).

using System;

class PriceFeed
{
    public event EventHandler<decimal> PriceChanged;
    public void Publish(decimal price)
    {
        // TODO: raise the event safely
    }
    public int SubscriberCount => 0; // TODO
}

class PriceWidget : IDisposable
{
    readonly PriceFeed _feed;
    public decimal Last { get; private set; }
    public PriceWidget(PriceFeed feed)
    {
        _feed = feed;
        // TODO: subscribe
    }
    void OnPrice(object sender, decimal price) => Last = price;
    public void Dispose()
    {
        // TODO: unsubscribe
    }
}

class Program
{

    static void Main()
    {
        int pass = 0, total = 0;
        void Check(string name, bool ok) { total++; if (ok) pass++; Console.WriteLine($"{(ok ? "PASS" : "FAIL")}: {name}"); }

        var feed = new PriceFeed();
        bool threw = false;
        try { feed.Publish(1m); } catch (NullReferenceException) { threw = true; }
        Check("publishing with no subscribers is safe", !threw);

        var a = new PriceWidget(feed);
        var b = new PriceWidget(feed);
        feed.Publish(42.5m);
        Check("widgets receive prices", a.Last == 42.5m && b.Last == 42.5m);
        Check("two subscribers", feed.SubscriberCount == 2);

        a.Dispose();
        feed.Publish(43m);
        Check("disposed widget no longer updated", a.Last == 42.5m && b.Last == 43m);
        Check("unsubscribed on dispose", feed.SubscriberCount == 1);

        for (int i = 0; i < 1000; i++) { using var w = new PriceWidget(feed); }
        Check("no leak after 1000 short-lived widgets", feed.SubscriberCount == 1);

        Console.WriteLine($"--- {pass}/{total} checks passed ---");
    }
}