// Plain delegate — anyone can invoke or overwrite
public Action<string> OnMessage;   // ❌ dangerous

someObject.OnMessage  = handler;   // wipes all existing handlers!
someObject.OnMessage("test");      // anyone can fire it from outside!

// event — adds two important restrictions
public event Action<string> OnMessage;   // ✅ safe

someObject.OnMessage  = handler;   // ❌ compile error — cannot assign from outside
someObject.OnMessage += handler;   // ✅ only += and -= allowed from outside
someObject.OnMessage("test");      // ❌ compile error — only the class can invoke it

// ── Step 1: Custom EventArgs ──────────────────────────────────
public class TemperatureChangedEventArgs : EventArgs
{
    public double OldTemperature { get; }
    public double NewTemperature { get; }
    public DateTime ChangedAt    { get; } = DateTime.Now;

    public TemperatureChangedEventArgs(double oldTemp, double newTemp)
    {
        OldTemperature = oldTemp;
        NewTemperature = newTemp;
    }
}

// ── Step 2: Publisher ─────────────────────────────────────────
public class Thermostat
{
    private double _temperature;

    // Declare event using standard EventHandler<TEventArgs>
    public event EventHandler<TemperatureChangedEventArgs>? TemperatureChanged;

    public double Temperature
    {
        get => _temperature;
        set
        {
            if (value == _temperature) return;
            var args = new TemperatureChangedEventArgs(_temperature, value);
            _temperature = value;
            // Fire event — ?.Invoke() is null-safe and thread-safe
            TemperatureChanged?.Invoke(this, args);
        }
    }

    // Alternatively — protected virtual method (good for inheritance)
    protected virtual void OnTemperatureChanged(TemperatureChangedEventArgs e)
        => TemperatureChanged?.Invoke(this, e);
}

// ── Step 3: Subscriber ────────────────────────────────────────
public class Alarm
{
    // Event handler method signature must match EventHandler<TEventArgs>
    public void OnTemperatureChanged(object? sender, TemperatureChangedEventArgs e)
    {
        if (e.NewTemperature > 30)
            Console.WriteLine($"🚨 ALARM! Temp rose to {e.NewTemperature}°C");
    }
}

public class Logger
{
    public void OnTemperatureChanged(object? sender, TemperatureChangedEventArgs e)
        => Console.WriteLine($"📊 Log: {e.OldTemperature}° → {e.NewTemperature}° at {e.ChangedAt:HH:mm:ss}");
}

// ── Step 4: Wire up ───────────────────────────────────────────
Thermostat thermostat = new();
Alarm      alarm      = new();
Logger     logger     = new();

thermostat.TemperatureChanged += alarm.OnTemperatureChanged;
thermostat.TemperatureChanged += logger.OnTemperatureChanged;
thermostat.TemperatureChanged += (sender, e) =>   // lambda subscriber
    Console.WriteLine($"UI updated: {e.NewTemperature}°");

// ── Step 5: Fire ──────────────────────────────────────────────
thermostat.Temperature = 22;   // fires — all 3 subscribers notified
thermostat.Temperature = 35;   // fires — alarm also triggers
thermostat.Temperature = 35;   // no fire — value unchanged

// Simple — just data
public class OrderPlacedEventArgs : EventArgs
{
    public int    OrderId   { get; init; }
    public string Customer  { get; init; } = "";
    public decimal Total    { get; init; }
}

// With cancellation support
public class FileDownloadingEventArgs : EventArgs
{
    public string Url       { get; }
    public bool   Cancel    { get; set; }   // subscriber can set this!

    public FileDownloadingEventArgs(string url) => Url = url;
}

// Publisher that checks Cancel
public class Downloader
{
    public event EventHandler<FileDownloadingEventArgs>? Downloading;

    public void Download(string url)
    {
        var args = new FileDownloadingEventArgs(url);
        Downloading?.Invoke(this, args);

        if (args.Cancel)   // subscriber set Cancel = true
        {
            Console.WriteLine("Download cancelled by subscriber");
            return;
        }
        // proceed with download...
    }
}

// Subscriber cancels
downloader.Downloading += (s, e) =>
{
    if (e.Url.Contains("malware"))
        e.Cancel = true;   // cancel the operation
};

Thermostat t = new();

// ── Subscribe with named method ───────────────────────────────
Alarm alarm = new();
t.TemperatureChanged += alarm.OnTemperatureChanged;

// ── Subscribe with lambda ─────────────────────────────────────
t.TemperatureChanged += (sender, e) =>
    Console.WriteLine($"Lambda sees: {e.NewTemperature}");

// ── Subscribe with anonymous method ───────────────────────────
t.TemperatureChanged += delegate(object? s, TemperatureChangedEventArgs e)
{
    Console.WriteLine($"Anon: {e.NewTemperature}");
};

// ── Unsubscribe (named methods only) ─────────────────────────
t.TemperatureChanged -= alarm.OnTemperatureChanged;
// Note: you CANNOT unsubscribe a lambda — save it to a variable first!

// ── How to unsubscribe a lambda ───────────────────────────────
EventHandler<TemperatureChangedEventArgs> handler = (s, e) =>
    Console.WriteLine($"Removable lambda: {e.NewTemperature}");

t.TemperatureChanged += handler;   // add
t.TemperatureChanged -= handler;   // remove ✅

// ── Check if anyone is subscribed ────────────────────────────
// Use null check — event is null when no subscribers
if (t.TemperatureChanged != null)
    Console.WriteLine("Someone is listening");

// When you don't need to pass data
public class GameTimer
{
    public event EventHandler? Tick;    // no custom args

    public void Start()
    {
        for (int i = 0; i < 5; i++)
        {
            Thread.Sleep(1000);
            Tick?.Invoke(this, EventArgs.Empty);   // fire with empty args
        }
    }
}

GameTimer timer = new();
int ticks = 0;
timer.Tick += (s, e) => Console.WriteLine($"Tick #{++ticks}");
timer.Start();
// Tick #1 ... Tick #5

// Full minimal example
public class Button
{
    public event EventHandler? Clicked;
    public void Click() => Clicked?.Invoke(this, EventArgs.Empty);
}

var btn = new Button();
btn.Clicked += (s, e) => Console.WriteLine("Button was clicked!");
btn.Click();   // → "Button was clicked!"