Event bubbling in C#

Published by Carlos Blé on 08/04/2016

How to propagate an event from a low level class to a top level one:

	public class TopLevel{
	public bool Bubbled { get; private set; }
	private MiddleLevel observable;
	public TopLevel(MiddleLevel observable){
	this.observable = observable;
	observable.Triggered += (s, e) => {
	Bubbled = true;
	};
	}
	}

	public class MiddleLevel{
	public event EventHandler Triggered;
	private BottomLevel observable;
	public MiddleLevel(BottomLevel observable){
	this.observable = observable;
	//One may be tempted to bubble like this:
	//observable.Triggered += Triggered;
	//However, Triggered is null unless there is already
	//a subscriber. This is a better approach:

	observable.Triggered += (s, e) => {
	Triggered(s, e);
	};
	}
	}

	public class BottomLevel{
	public event EventHandler Triggered;

	public void DoSomething(){
	Triggered(this, EventArgs.Empty);
	}
	}

	[TestFixture]
	public class TestingEventBubbling {
	[Test]
	public void Bubbling(){
	var bottom = new BottomLevel();
	var middle = new MiddleLevel(bottom);
	var top = new TopLevel(middle);

	bottom.DoSomething();

	top.Bubbled.Should().BeTrue();
	}
	}

view raw codesai_carlos_event_bubbling_1.cs hosted with ❤ by GitHub

Events can only be raised from within the declaring type. Unfortunately they can’t be be passed in as arguments to methods. Only += and -= operators are allowed out of the declaring type. One way to stub out the event could be through inheritance:

	public class BottomLevel{
	public virtual event EventHandler Triggered;

	public void DoSomething(){
	Triggered(this, EventArgs.Empty);
	}
	}

	public class StubbedBottomLevel : BottomLevel {
	public override event EventHandler Triggered;

	public void RaiseEvent(){
	Triggered(this, EventArgs.Empty);
	}
	}

	[TestFixture]
	public class TestingEventBubbling {
	[Test]
	public void BubblingWithStub(){
	var bottom = new StubbedBottomLevel();
	var middle = new MiddleLevel(bottom);
	var top = new TopLevel(middle);

	bottom.RaiseEvent();
	//bottom.DoSomething(); will not throw the event!

	top.Bubbled.Should().BeTrue();
	}

	}

view raw codesai_carlos_event_bubbling_2.cs hosted with ❤ by GitHub

But declaring the event as virtual and then overriding it, is very tricky: replacing the call to RaiseEvent to DoSomething, makes the test fail! Looks like events where not designed to be overridden.

A better approach would be:

	public class BottomLevel{
	public event EventHandler Triggered;

	public virtual void DoSomething(){
	//SomeLogic would go here...
	Raise(EventArgs.Empty);
	}
	protected virtual void Raise(EventArgs args){
	Triggered(this, args);
	}
	}

	public class StubbedBottomLevel : BottomLevel {
	public override void DoSomething(){
	Raise(EventArgs.Empty);
	}
	}

view raw codesai_carlos_event_bubbling_3.cs hosted with ❤ by GitHub

Originally published in Carlos Blé's blog.

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