Events & Delegates

Delegates

Type-safe function pointers.

public delegate void ProgressHandler(int percent);

Events

Publisher exposes an event based on a delegate; subscribers attach handlers.

public class Downloader
{
    public event ProgressHandler? ProgressChanged;

    public async Task RunAsync()
    {
        for (int i = 1; i <= 10; i++)
        {
            await Task.Delay(100);
            ProgressChanged?.Invoke(i * 10);
        }
    }
}

Subscriber:

var d = new Downloader();
d.ProgressChanged += p => Console.WriteLine($"Progress: {p}%");
await d.RunAsync();

EventHandler Pattern

Standard signature: (object? sender, EventArgs e) or generic EventHandler<TEventArgs>.

public event EventHandler<UserEventArgs>? UserCreated;

Custom Args

public sealed class UserEventArgs : EventArgs
{
    public required string UserName { get; init; }
    public DateTime CreatedAt { get; init; } = DateTime.UtcNow;
}

Unsubscribing

Important to avoid memory leaks (long-lived publishers). publisher.SomeEvent -= Handler;

Multicast

Events can have multiple subscribers; invocation list runs sequentially.

Async Considerations

Wrap awaits carefully; fire-and-forget tasks may need try/catch logging.

When to Use

UI interaction (button clicks)
Progress and status updates
Domain notifications (e.g., OrderPlaced)

Alternatives

Observer pattern classes
Reactive Extensions (IObservable/IObserver)
Mediator libraries (e.g., MediatR) for larger systems