Level Up Tests using a custom Assertion Library: Shouldly

When writing automated tests there is always some form of validation, usually called assertions. Typically the tool or framework you are using usually provides some form this functionality out of the box. For example whenever you create a test project in Visual Studio by default you are going to be using MSTest.

I’ve used MSTest and xUnit but mostly Nunit. I found MSTest harder to use, and more limited in syntax, xUnit had limited documentation, but Nunit well I love NUnit1. It’s very flexible, well documented, has good options for data driven tests, plus a very broad and powerful set of assertions.

After that love fest over Nunit, why wouldn’t you just use that? Why go beyond its greatness?


Nunit has two models for its assertion syntax. The original “classic” style, which follows the convention

Assert.AreEqual(expected, actual, optional_message);

This seems fine at first, but it can get clunky. It also requires you to always remember the correct order of the parameters. Its not a big deal but many people invert the order and it can end up with some confusing failure message.

To address some of the short comings Nunit introduced the “constraint” model.

Assert.That(actual, Is.EqualTo(expected), optional_message);

This style is much more readable but it also switches the order of the parameters, so it’s best to choose one style.

Shouldly steps in and removes the context shift between your domain and the unit test framework. Validations start with “Should” and in most cases are accessed as extension methods which eliminates any expected vs. actual confusion.

public void stringEndWith_Test()
var testor = new TestorTheBarbarian();
var expectedText = "Barbarian";

//Nunit Classic Model
StringAssert.EndsWith(expectedText, testor.Name);

//shouldly syntax

Choose the property you are testing and start typing .Should and Shouldly springs to life.

Failure Messages

The next major boost Should adds to your testing is helpful failure messages. Both nunit assertion models failure message are the same and can leave you a little short of information if you are just looking at the test output trying to resolve failing tests from your nightly build.

public void stringNotNullOrEmpty_Test()
var testor = new TestorTheBarbarian();
testor.EquipWeapon("Fists of Fury");

//Nunit Classic Model
// --> Expected: not null or empty string But was: null

//shouldly syntax
// --> testor.ActiveWeapon should not be null or empty

Shouldly includes the name of the actual input being tested. In the case above the failure message clearly shows that the ActiveWeapon property was the problem. Nunits message only indicates we expected not null but it wasn’t.

This may not be a major issue if your tests are well names and fine grained with a single assert. If your tests have multiple asserts then this is very handy and saves having to include a custom fail message to tell which assertion is the problem.

Shouldly Special Features

Shouldly includes some enhanced assertion methods that just don’t exist in the Nunit standards.
Here are a few examples, you can also check out my AssertionExamples on GitHub, or the Shouldly documentation2 for more.


Shouldly allows a predicate to be applied to all members in an IEnumerable.

public void listAll_test()
var player1 = new Player("Joe Tester");
var teamMember1 = new TestorTheBarbarian();
var teamMember2 = new MasterCodo();

//to fail test
//teamMember2.Health = 0;

//Nunit does not have an equivalent either separate asserts or a boolean which
// will loose detail and have a generic message
Assert.That(player1.Party.All(p => p.Health > 0), Is.True);

//shouldly syntax
player1.Party.ShouldAllBe(p=>p.Health >0);
//-->player1.Party should satisfy the condition (p.Health>0) but
// [TestorTheBarbarianDemo.MasterCodo] but do not

Multiple Assertions

Should also provides a mechanism for having multiple assertions in a single test that will all be executed regardless of failures.

public void multipleAsserts_Test()
var testor = new TestorTheBarbarian();

//change either or both value(s) below to fail test
var expectedText = "testor";
var expectedHealth = 100;

//Nunit does not support this in 2.X but similar functionality is in discussion for v3 (Assert.All)

//shouldly syntax
() => testor.Name.ShouldContain(expectedText),
() => testor.Health.ShouldBe(expectedHealth)
//--> testor should satisfy all the conditions specified, but does not.
// The following errors were found...
//-------------- - Error 1-------------- -
// testor.Name should contain "bugra" but does not
//-------------- - Error 2-------------- -
// testor.Health should be 10 but was 100
//-------------------------------------- -

Timed Tests

You can also assert that methods complete in an expected time.

public void timedResult_Test()
var testor = new TestorTheBarbarian();
//set sleep to >1000 to fail test
var sleepyTime = 500;

//nunit closest option MaxTime attribute on the test method.

() => testor.Rest(sleepyTime), TimeSpan.FromSeconds(1)
//--> Task should complete in 00:00:01 but did not


Framework Lock In

Most frameworks consist of a set of attributes to mark your tests and classes and also special syntax for assertions. It may not happen frequently but changing frameworks (say MSTest to Nunit) is simple a simple find and replace for the test decorators. The assertions however are more complicated. Shouldly provides an abstraction on top of the frameworks syntax.

Easier Testing

Adding Shouldly just makes the test process easier. Easier to investigate failures, and easier to write with simple extension methods. It also methods to make some complex test scenarios easier to write and read.

To see more examples check out my AssertionExamples project on GitHub. There is also a good Pluralsight course by Jason Roberts Better Unit Test Assertions with Shouldly.