Test events with nunit

Question

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!

Answer 1

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;

...

Assert.IsTrue(wasCalled);

Due to request - without lambdas:

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

...

Assert.IsTrue(wasCalled);

Answer 2

I prefer to do as follows:

var wait = new AutoResetEvent(false);
foo.MeEvent += (sender, eventArgs) => { wait.Set(); };
Assert.IsTrue(wait.WaitOne(TimeSpan.FromSeconds(5)));

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

Answer 3

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";

Assert.IsTrue(eventRaised);

---

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";

Assert.IsTrue(eventRaised.WaitOne(TIMEOUT));

However, some say testing asynchronous behavior should be avoided.

Answer 4

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 { };
}

[TestFixture]
public class MyTests {
    [Test]
    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");
    }
    [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() {
        this.Reset();
    }

    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 {
        capture.Reset();
        code();
        test(capture);
    }
    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));
        };
    }
}

Answer 5

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:

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

Using Mock Objects in Tests:

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

    myObject.SomeEventTrigger();

    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);

}

Answer 6

I would just use FluentAssertions along with Nunit : https://fluentassertions.com/eventmonitoring/ it works really well. Here is an example from the docs

var subject = new EditCustomerViewModel();
using (var monitoredSubject = subject.Monitor())
{
    subject.Foo();
    monitoredSubject.Should().Raise("NameChangedEvent");
}

Answer 7

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.

Answer 8

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 };

Assert.IsTrue(eventFired);

Answer 9

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

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

Cheers! ;-)