How can I unit test unsubscribing from an event? (C#)

I’ve seen some strange bugs that occur when you don’t properly unsubscribe from events. But it seems I’m always stuck when it comes to unit testing that.

Well, I use Moq for most of my mocking and there are some techniques you can use for testing that your subscribed, namely by Raise(ing) the event in a mock.

But I always struggle when trying to see if I actually unsubscribe in my code. Here is a simple sample that shows how you can do it with very little code.

Here is all the code:

Code Snippet

public interface IHasEvents
{
    event EventHandler BeginRequest;
}
 
public class ClassUnderTest
{
    internal IHasEvents ClassWithEvents { get; set; }
    public ClassUnderTest(IHasEvents classWithEvents)
    {
        ClassWithEvents = classWithEvents;
        ClassWithEvents.BeginRequest += OnBeginRequest;
    }
    public void Dispose()
    {
        // Yes… there are code analysis warnings here… But I want to keep simple.
        ClassWithEvents.BeginRequest -= OnBeginRequest;
    }
 
    public void OnBeginRequest(object sender, EventArgs e) { }
}
 
public class MockHasEvents : IHasEvents
{
    public int BeginRequestSubscriberCount { get; set; }
    public event EventHandler BeginRequest
    {
        add { BeginRequestSubscriberCount++; }
        remove { BeginRequestSubscriberCount–; }
    }
}

And here is the code that tests that the constructor subscribes and the Dispose() unsubscribes.

Code Snippet

[TestClass]
public class ExampleEventMockTest
{
    [TestMethod]
    public void Constructor_Subscribes_To_BeginRequest()
    {
        // Arrange
        var expected = 1;
        var hasEvents = new MockHasEvents();
 
        // Act
        var subject = new ClassUnderTest(hasEvents);
 
        // Assert
        var actual = hasEvents.BeginRequestSubscriberCount;
        Assert.AreEqual(expected, actual);
        //actual.Should().Be(expected); // I usually use FluentAssertions.
    }
 
    [TestMethod]
    public void Dispose_Unsubscribes_From_BeginRequest()
    {
        // Arrange
        var expected = 0;
        var hasEvents = new MockHasEvents();
        var subject = new ClassUnderTest(hasEvents);
 
        // Act
        subject.Dispose();
 
        // Assert
        var actual = hasEvents.BeginRequestSubscriberCount;
        Assert.AreEqual(expected, actual);
    }
}

I hope this helps others.

How can I unit test unsubscribing from an event? (C#)

Comments

Leave a Reply Cancel reply