Test events with nunit

I'm just starting with TDD and could solve most of the problems I've faced on my own. But now I'm lost: How can I check if events are fired? I was looking for something like Assert.Raise or Assert.Fire but there's nothing. Google was not very useful, most of the hits were suggestions like foo.myEvent += new EventHandler(bar); Assert.NotNull(foo.myEvent); but that proves nothing.

Thank you!


Checking if events were fired can be done by subscribing to that event and setting a boolean value:

var wasCalled = false;
foo.NyEvent += (o,e) => wasCalled = true;



Due to request - without lambdas:

var wasCalled = false;
foo.NyEvent += delegate(o,e){ wasCalled = true;}



I prefer to do as follows:

var wait = new AutoResetEvent(false);
foo.MeEvent += (sender, eventArgs) => { wait.Set(); };

Advantages: Supports multithreading scenario (if handler is invoked in different thread)

If you know the event will be fired synchronously:

bool eventRaised = false;
Customer customer = new Customer() { Name = "Carl" };
customer.NameChanged += (sender, e) => { eventRaised = true; };

customer.Name = "Sam";


If the event may be fired asynchronously:

ManualResetEvent eventRaised = new ManualResetEvent(false);
Customer customer = new Customer() { Name = "Carl" };
customer.NameChanged += (sender, e) => { eventRaised.Set(); };

customer.Name = "Sam";


However, some say testing asynchronous behavior should be avoided.

I recently had to do this, and below is what I came up with. The reason I did not do what the other posts said, is I do not like the idea of a variable keeping state and having to reset it "manually" between multiple events.

Below is the code of the ClassUnderTest with NameChanged event that is tested in MyTests tests:

public class ClassUnderTest {
    private string name;
    public string Name {
        get { return this.name; }
        set {
            if (value != this.name) {
                this.name = value;
                NameChanged(this, new PropertyChangedEventArgs("Name"));

    public event EventHandler<PropertyChangedEventArgs> NameChanged = delegate { };

public class MyTests {
    public void Test_SameValue() {
        var t = new ClassUnderTest();
        var e = new EventHandlerCapture<PropertyChangedEventArgs>();
        t.NameChanged += e.Handler;

        Event.Assert(e, Event.IsNotRaised<PropertyChangedEventArgs>(), () => t.Name = null);
        t.Name = "test";
        Event.Assert(e, Event.IsNotRaised<PropertyChangedEventArgs>(), () => t.Name = "test");
    public void Test_DifferentValue() {
        var t = new ClassUnderTest();
        var e = new EventHandlerCapture<PropertyChangedEventArgs>();
        t.NameChanged += e.Handler;

        Event.Assert(e, Event.IsPropertyChanged(t, "Name"), () => t.Name = "test");
        Event.Assert(e, Event.IsPropertyChanged(t, "Name"), () => t.Name = null);

The supporting classes are below. The classes can be used with any EventHandler<TEventArgs> or expanded to other delegates. Event tests can be nested.

/// <summary>Class to capture events</summary>
public class EventHandlerCapture<TEventArgs> where TEventArgs : EventArgs {
    public EventHandlerCapture() {

    public object Sender { get; private set; }
    public TEventArgs EventArgs { get; private set; }
    public bool WasRaised { get; private set; }

    public void Reset() {
        this.Sender = null;
        this.EventArgs = null;
        this.WasRaised = false;

    public void Handler(object sender, TEventArgs e) {
        this.WasRaised = true;
        this.Sender = sender;
        this.EventArgs = e;

/// <summary>Contains things that make tests simple</summary>
public static class Event {
    public static void Assert<TEventArgs>(EventHandlerCapture<TEventArgs> capture, Action<EventHandlerCapture<TEventArgs>> test, Action code) where TEventArgs : EventArgs {
    public static Action<EventHandlerCapture<TEventArgs>> IsNotRaised<TEventArgs>() where TEventArgs : EventArgs {
        return (EventHandlerCapture<TEventArgs> test) => {
            NUnit.Framework.Assert.That(test.WasRaised, Is.False);
    public static Action<EventHandlerCapture<PropertyChangedEventArgs>> IsPropertyChanged(object sender, string name) {
        return (EventHandlerCapture<PropertyChangedEventArgs> test) => {
            NUnit.Framework.Assert.That(test.WasRaised, Is.True);
            NUnit.Framework.Assert.That(test.Sender, Is.SameAs(sender));
            NUnit.Framework.Assert.That(test.EventArgs.PropertyName, Is.EqualTo(name));

Not really done this myself, but maybe you could add a dummy event handler to the event you wanna subscribe to and have it update a local boolean variable so that after the method is fired you can check the state of that boolean to see if the event was fired?

Something like:

bool eventFired = false;
foo.MyEvent += (s, e) => { eventFired = true };


@theburningmonk: A ";" is missing. Corrected version is:

bool eventFired = false;
foo.MyEvent += (s, e) => { eventFired = true; };

Cheers! ;-)

Using NUnit and Moq you can do more robust event testing.

Mock Class used to monitor event triggers:

public class AssertEvent { public virtual void Call(string obj) { } }
Mock<AssertEvent> EventMock;
AssertEvent Evt;

Setup for event Triggers:

public void TestInit() {
    EventMock = new Mock<AssertEvent>();
    Evt= EventMock.Object;

Using Mock Objects in Tests:

public void TestMethod() {
    myObject.Event1 += (sender, args) => Evt.Call("Event1Label");
    myObject.Event2 += (sender, args) => Evt.Call("Event2Label");
    myObject.Event3 += (sender, args) => Evt.Call("Event3Label");        


    EventMock.Verify(m => m.Call("Event1Label"), Times.Exactly(1));
    EventMock.Verify(m => m.Call("Event2Label"), Times.Never());
    EventMock.Verify(m => m.Call("Event3Label"), Times.Between(1,3);


You can add your custom event handler which, for example, increments some integer field in test case class. And then check if field was incremented.

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