Directly answering your question, I would say: No, there is no relation between events and delegates adopting the composite pattern. Delegates design yes, it follows the composite pattern. Events not. (Moreover, notice that you do not need events to take advantage of delegates. (see DelegateBased bellow)) (I will answer your comment about "So what's the point of this line?" at the end as a side note)
Nevertheless, the Delegate type itself follows the composite approach in terms that “The composite pattern describes a group of objects that are treated the same way as a single instance of the same type of object.”.
In turn, as @Flydog57 and @mark-seemann have already mentioned the .NET event model follows the observer pattern.
The relation between Events and Delegates regards the Event declaration that may require a Delegate type (the TypeSpec), as it is stated in the section II.18 Defining events of ECMA-335 (CLI) Partitions I to VI (the standard):
In typical usage, the TypeSpec (if present) identifies a delegate whose signature matches the arguments passed to the event’s fire method.
To make it clear, check the following two equivalent examples where EventBased uses events without a delegate field and DelegateBased uses a delegate field without events. Notice that I say distinctly delegate field or delegate type. They are not the same. Both examples need a delegate type which is declared as the following for this example:
delegate void Observer();
You may run both examples with:
var subject = new DelegateBased(); // replace it with: var subject = new EventBased();
Observer foo = () => Console.Write("Foo");
Observer bar = () => Console.Write("Bar");
subject.RegisterObserver(foo); // subject.Caller += foo;
subject.RegisterObserver(bar); // subject.Caller += bar;
subject.Notify(); // prints: FooBar
Console.WriteLine();
subject.UnregisterObserver(foo); // subject.Caller -= foo;
subject.Notify(); // prints: Bar
Next, the two implementations of EventBased and DelegateBased that use names according to the example of Observer Pattern in Wikipedia
class EventBased {
private List<Observer> observers = new List<Observer>();
public event Observer Caller {
add { RegisterObserver(value); }
remove { UnregisterObserver(value); }
}
public void Notify() { foreach (var caller in observers) caller(); }
public void RegisterObserver(Observer val) { observers.Add(val); }
public void UnregisterObserver(Observer val) { observers.Remove(val); }
}
class DelegateBased {
private Observer observers; // delegate field without events
public void Notify() { observers(); }
public void RegisterObserver(Observer val) {
observers = (Observer) Delegate.Combine(observers, val); // <=> observers += val
}
public void UnregisterObserver(Observer val) {
observers = (Observer) Delegate.Remove(observers, val); // <=> observers -= val
}
}
Regarding your comment about:
EventHandler handler = ThresholdReached; // So what's the point of this line?
handler?.Invoke(this, e);
The reason it is clearly identified by Jeffrey Richter in its masterpiece "Clr via C#" in Chapter 11 - Events at "Raising an Event in a Thread-Safe Way" (see the NewMail as the ThresholdReached of your example) where it states:
The problem with the OnNewMail method is that the thread could see that NewMail is not null, and then, just before invoking NewMail, another thread could remove a delegate from the chain making NewMail null, resulting in a NullReferenceException being thrown.
