Learning .NET Unit Testing the Easy Way

Knowing what you need to know is hard. Sometimes harder than the learning itself.

Many years ago I was getting started with .NET v1 and .NET unit testing, Agile had recently been “invented” and I had a printout of the agile manifesto on the office wall. I was also learning Test Driven Development (TDD), mocking, unit testing frameworks, assertions, data driven testing.

I remember it being a little overwhelming at times, so much to learn with fragments of information scattered around but no clearly defined path to follow to get where I knew I wanted to be: proficient and efficient in writing high quality, tested and testable code.

Today things are a little easier but there can still be the: “I don’t know what I need to know”.

This is where skills paths from Pluralsight can be super helpful. I wish I had had them all those years ago.

A path is a curated collection of courses in a specific order to get you to where you need to be for a specific learning goal.

I’m super proud to have contributed to the C# Unit Testing with NUnit Pluralsight path which at the time of writing you can start to watch for free with a Pluralsight free trial.

While it’s certainly possible that you could find the information and learn the topics yourself, you would also waste so much time in getting the information from disparate sources and trying to “meta learn” what it is you don’t know. Ultimately it depends on how much free time you have and how efficient you want to be at learning. You should always keep the end goal in mind and weigh up the costs/benefits/risks of the different ways of getting to that goal. If you want to learn to “how to write clean, testable code, all the way from writing your first test to mocking out dependencies to developing a pragmatic suite of unit tests for your application” then the C# Unit Testing with NUnit path may be your most efficient approach to get to your goal.


NUnit 3 Quick Tips: Asserting On Collections

When the result you want to check is a collection, you can use NUnit to assert that it has the expected number of items or is empty, that all items are unique, that specific items do/not exist, and that items exist that satisfy some condition or predicate.

Asserting on the Number of Items in a Collection with NUnit Asserts

var names = new[] { "Sarah", "Amrit", "Amanda", "Sarah" };

Assert.That(names, Has.Exactly(4).Items); // pass
Assert.That(names, Is.Empty); // fail
Assert.That(names, Is.Not.Empty); // pass

Asserting That All Items in a Collection are Unique with NUnit Asserts

Assert.That(names, Is.Unique); // fail - 2 Sarah items exist

Asserting That An Item Does or Does Not Exist in a Collection with NUnit Asserts

Assert.That(names, Contains.Item("Sarah")); // pass

// Alternative syntax
Assert.That(names, Does.Contain("Sarah")); // pass
Assert.That(names, Does.Not.Contain("Arnold")); // pass

Asserting That An Item Appears a Specified Number Of Times in a Collection with NUnit Asserts

Assert.That(names, Has.Exactly(1).EqualTo("Sarah")); // fail
Assert.That(names, Has.Exactly(2).EqualTo("Sarah")); // pass
Assert.That(names, Has.Exactly(2).EqualTo("Sarah")
                      .And.Exactly(1).EqualTo("Amrit")); // pass

Asserting That All Items In a Collections Satisfy a Predicate/Condition with NUnit Asserts

Assert.That(names, Is.All.Not.Null); // pass
Assert.That(names, Is.All.Contains("a")); // fail lowercase a
Assert.That(names, Is.All.Contains("a").IgnoreCase); // pass
Assert.That(names, Is.All.Matches<string>(name => name.ToUpperInvariant().Contains("A"))); // pass
Assert.That(names, Is.All.Matches<string>(name => name.Length > 4)); // pass

Asserting That Only One Item In a Collection Satisfies a Predicate with NUnit Asserts

Assert.That(names, Has.Exactly(1).Matches<string>(name => name.Contains("mri"))); // pass
Assert.That(names, Has.Exactly(1).Matches<string>(name => name.Contains("ara"))); // fail (2 Sarah items exist)

To learn more about NUnit 3 check out my Introduction to .NET Testing with NUnit 3 Pluralsight course to learn everything you need to know to get started, including asserts, categories, data-driven tests, customizing NUnit, and reducing duplicate test code.


NUnit 3 Quick Tips: Asserting On Object Reference Equality

When asserting on equality using the EqualConstraint you may not always get the behaviour you want depending on what objects are being asserted on. This can be influenced by whether or not the objects are value or reference types and if the type implements or overrides methods such as IEquatable<T> or object.Equals overrides.

Asserting on Value Type Equality with NUnit

int a = 42;
int b = 42;

Assert.That(a, Is.EqualTo(b)); // pass - values are same, ints are structs with value semantics
Assert.That(a, Is.SameAs(b)); // fail - a and b do not point to the same object in memory

int c = a;

Assert.That(c, Is.EqualTo(a)); // pass - values are same

Asserting on Reference Type Equality with NUnit

By default, 2 instances of a reference type will not pass an equality assert:

class Person
    public string Name { get; set; }
Person p1 = new Person { Name = "Sarah" };
Person p2 = new Person { Name = "Sarah" };

Assert.That(p1, Is.EqualTo(p2)); // fail, Person is class with reference semantics

Asserting That Two References Point to the Same Object with NUnit

If you want to assert that 2 object references point to the same object you can use the SameAsConstraint:

Assert.That(p1, Is.SameAs(p2)); // fail, p1 and p2 point to different objects in memory Person p3 = p1; Assert.That(p3, Is.SameAs(p1)); // pass, p3 and p1 point to same object in memory Assert.That(p3, Is.Not.SameAs(p2)); // pass, p3 and p2 point to different objects in memory

Customizing Equality Asserts with NUnit

There are a number of ways to influence how NUnit performs equality assertions including implementing IEquatable<T>:

class Employee : IEquatable<Employee>
    public string Name { get; set; }

    public bool Equals(Employee other)
        if (other is null)
            return false;

        return Name == other.Name;
Employee e1 = new Employee { Name = "Sarah" };
Employee e2 = new Employee { Name = "Sarah" };

Assert.That(e1, Is.EqualTo(e2)); // pass - IEquatable<Employee>.Equals implementation is used

To learn more about NUnit 3 check out my Introduction to .NET Testing with NUnit 3 Pluralsight course to learn everything you need to know to get started, including asserts, categories, data-driven tests, customizing NUnit, and reducing duplicate test code.


NUnit 3 Quick Tips: Asserting Within Ranges

If you are asserting that a value is equal to something and you want to specify some tolerance you can do so.

Specifying a Range for Values with NUnit Asserts (e.g. int)

var i = 42;

Assert.That(i, Is.EqualTo(40)); // fail

Assert.That(i, Is.EqualTo(40).Within(2)); // pass

Assert.That(i, Is.EqualTo(40).Within(1)); // fail "Expected: 40 +/- 1"

Specifying a Range as a Percentage with NUnit Asserts

In addition to specifying a range tolerance as a fixed value you can also specify it as a percentage:

Assert.That(i, Is.EqualTo(40).Within(5).Percent); // pass

Assert.That(i, Is.EqualTo(40).Within(4).Percent); // fail "Expected: 40 +/- 4 Percent"

Specifying a Range for DateTime Objects with NUnit Asserts

When working with DateTimes you can specify the tolerance as a TimeSpan instance:

var newYearsDay2019 = new DateTime(2019, 1, 1);

Assert.That(newYearsDay2019, Is.EqualTo(new DateTime(2019, 1, 2)).Within(TimeSpan.FromDays(1))); // pass

Or instead of using a TimeSpan you can use one of the convenience modifiers:

Assert.That(newYearsDay2019, Is.EqualTo(new DateTime(2019, 1, 2)).Within(1).Days); // pass

Assert.That(newYearsDay2019, Is.EqualTo(new DateTime(2019, 1, 2)).Within(24).Hours); // pass
Assert.That(newYearsDay2019, Is.EqualTo(new DateTime(2019, 1, 2)).Within(23).Hours); // fail

var numberOfMinutesInADay = 24 * 60;
Assert.That(newYearsDay2019, Is.EqualTo(new DateTime(2019, 1, 2)).Within(numberOfMinutesInADay).Minutes); // pass
Assert.That(newYearsDay2019, Is.EqualTo(new DateTime(2019, 1, 2)).Within(numberOfMinutesInADay - 1).Minutes); // fail "Expected: 2019-01-02 00:00:00 +/- 23:59:00"

// Also Within(n).Seconds .Milliseconds and .Ticks
To learn more about NUnit 3 check out my Introduction to .NET Testing with NUnit 3 Pluralsight course to learn everything you need to know to get started, including asserts, categories, data-driven tests, customizing NUnit, and reducing duplicate test code.


Testing for Thrown Exceptions in NUnit

In a previous post, testing for thrown exceptions using xUnit.net was demonstrated. In this post we’ll see how to do the same with NUnit.

Once again the class being tested is as follows:

public class TemperatureSensor
    bool _isInitialized;

    public void Initialize()
        // Initialize hardware interface
        _isInitialized = true;

    public int ReadCurrentTemperature()
        if (!_isInitialized)
            throw new InvalidOperationException("Cannot read temperature before initializing.");

        // Read hardware temp
        return 42; // Simulate for demo code purposes

The first test can be to test the happy path:

public void ReadTemperature()
    var sut = new TemperatureSensor();


    var temperature = sut.ReadCurrentTemperature();

    Assert.AreEqual(42, temperature);

Next, a test can be written to check that the expected exception is thrown:

public void ErrorIfReadingBeforeInitialized()
    var sut = new TemperatureSensor();

    Assert.Throws<InvalidOperationException>(() => sut.ReadCurrentTemperature());

Notice in the preceding code that any InvalidOperationException thrown will pass the test. To ensure that the thrown exception is correct, it can be captured and further asserts performed against it:

public void ErrorIfReadingBeforeInitialized_CaptureExDemo()
    var sut = new TemperatureSensor();

    var ex = Assert.Throws<InvalidOperationException>(() => sut.ReadCurrentTemperature());

    Assert.AreEqual("Cannot read temperature before initializing.", ex.Message);
    // or:
    Assert.That(ex.Message, Is.EqualTo("Cannot read temperature before initializing."));

There’s also other ways to assert against expected exceptions such as the following:

                .And.Message.EqualTo("Cannot read temperature before initializing."), 
              () => sut.ReadCurrentTemperature());

There’s some personal preference involved when choosing a style, for example the preceding code could be considered more verbose by some and may muddle the distinction between the Act and Assert phases of a test.

To learn more about using exceptions to handle errors in C#, check out my Error Handling in C# with Exceptions Pluralsight course.


New Pluralsight Course - Introduction to .NET Testing with NUnit

If you are just dipping your toe in the water when it comes to testing .NET applications it can be a bit confusing. In addition to learning how and what to write test, you also have to learn a testing framework such as MSTest, xUnit.net, NUnit, etc.

My new beginner Pluralsight course helps you to get started with testing in .NET and how to use the NUnit testing framework.

Watch the course at the above link or get to it from my Pluralsight author page.


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