LearnEvents.cs

using System;

namespace LearnEvents
{
  // Defining delegate type (similar to System.EventHandler)
  delegate void SomeHandler(object sender, EventArgs e);

  class Person
  {
    // Field-like event declared using keyword 'event'
    // allows only for "+=" and "-=" on a hidden instance
    // of a delegate of the type SomeHandler.
    // Removing 'event' makes this into a field holding
    // instance of a delegate.
    public event SomeHandler OnSickness;

    public Person()
    {
      // Explicitly creating an instance of a delegate
      SomeHandler sicknessHandler = 
        delegate (object sender, EventArgs e)
        {
          Console.WriteLine("Person: Vomiting...");
        };
      // Adding above defined instance to invocation list
      OnSickness += sicknessHandler;
    }

    public void EatTerribleFood()
    {
      Console.WriteLine("Person: Eating terrible food...");
      // Invocation of a sickness event
      if (OnSickness != null)
      {
        OnSickness(this, null);
      }
    }
  }

  class Program
  {
    static void Main(string[] args)
    {
      var p = new Person();
      // adding anonymous method by external to Person code
      p.OnSickness += (sender, e) => 
        { Console.WriteLine("External: Calling doctor..."); };

      // COMPILATION ERROR
      // as long as 'event' was used in first line
      // of the Person class. External code cannot invoke sickness.
      // Lines below will compile just fine if you remove 'event'
      p.OnSickness = (sender, e) => 
        { Console.WriteLine("External: Person feels happy..."); };
      p.OnSickness(p, null);

      // For neater design it is responsibility of internals
      // of the Person class to know when to invoke the event
      p.EatTerribleFood();
    }
  }
}