Test events with nunit

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);

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.

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)

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));
        };
    }
}