C# 5.0 Pocket Reference

Events

When using delegates, two emergent roles commonly appear: broadcaster and subscriber. The broadcaster is a type that contains a delegate field. The broadcaster decides when to broadcast, by invoking the delegate. The subscribers are the method target recipients. A subscriber decides when to start and stop listening, by calling += and -= on the broadcaster’s delegate. A subscriber does not know about, or interfere with, other subscribers.

Events are a language feature that formalizes this pattern. An event is a construct that exposes just the subset of delegate features required for the broadcaster/subscriber model. The main purpose of events is to prevent subscribers from interfering with one another.

The easiest way to declare an event is to put the event keyword in front of a delegate member:

public class Broadcaster
{
  public event ProgressReporter Progress;
}

Code within the Broadcaster type has full access to Progress and can treat it as a delegate. Code outside of Broadcaster can only perform += and -= operations on the Progress event.

In the following example, the Stock class fires its PriceChanged event every time the Price of the Stock changes:

public delegate void PriceChangedHandler
 (decimal oldPrice, decimal newPrice);

public class Stock
{
  string symbol; decimal price;

  public Stock (string symbol) { this.symbol = symbol; }

  public event PriceChangedHandler PriceChanged; public decimal Price { get { return price; } set { if (price == value) return; // Fire event if invocation ...