Events

using System;

public delegate void MyEventHandler<TEventArgs>(Object sender, TEventArgs e) where TEventArgs : EventArgs;

public class MyEventArgs : EventArgs {
  public string Message { get; private set; }

  public MyEventArgs(string message) {
    Message = message;
  }
}

public class Foo {
  public event MyEventHandler<MyEventArgs> Greetings;

  protected virtual void OnGreetings(MyEventArgs e) {
    Greetings?.Invoke(this, e);
  }

  public void Greet(string message) {
    OnGreetings(new MyEventArgs(message));
  }
}

public class Sample {
  public static void Main() {
    var foo = new Foo();
    foo.Greetings += Greet;

    foo.Greet("Hello World!");
  }

  private static void Greet(Object sender, MyEventArgs e) {
    Console.WriteLine(e.Message);
  }
}