A small set of extensions to make fixtures and assertions more fluent! Wow!
This project was motivated by this article here. In the article, the author demonstrates a way to verify the use of guarded parameters in constructors and methods by asserting that invalid values throw various types of exceptions. A brief overview of my evolution through what would be my initial code and then a refactoring inspired by this article is demonstrated here:
// the Billboard class constructor
public Billboard(string message)
{
if (message is null)
{
throw new ArgumentNullException(nameof(message));
}
this.Message = message;
}
// example test
[Fact]
public void ConstructorGuardsAgainstNullParameters()
{
Action nullMessageAction = () => new Billboard(null);
// shoutout to FluentAssertions for Should() :)
nullMessageAction.Should().Throw<ArgumentNullException>();
}Without completely covering the article, this looks fairly simple right? Well let's take a look at how this test grows as we add a class with multiple constructor parameters. We'll add a Painter who will paint the Billboard.
// the painter class constructor
public Painter(string firstName, string lastName, string socialSecurityNumber)
{
if (firstName is null)
{
throw new ArgumentNullException(nameof(firstName));
}
if (lastName is null)
{
throw new ArgumentNullException(nameof(lastName));
}
if (socialSecurityNumber is null)
{
throw new ArgumentNullException(nameof(socialSecurityNumber));
}
FirstName = firstName;
LastName = lastName;
SocialSecurityNumber = socialSecurityNumber;
}
// another example test
[Theory]
[InlineData("Foo", "Bar", "123-45-6789")]
public void ConstructorGuardsAgainstNullParameters(string firstName, string lastName, string socialSecurityNumber)
{
Action nullFirstNameAction = () => new Painter(null, lastName, socialSecurityNumber);
Action nullLastNameAction = () => new Painter(firstName, null, socialSecurityNumber);
Action nullSocialSecurityNumberAction = () => new Painter(firstName, lastName, null);
nullFirstNameAction.Should().Throw<ArgumentNullException>();
nullLastNameAction.Should().Throw<ArgumentNullException>();
nullSocialSecurityNumberAction.Should().Throw<ArgumentNullException>();
}See how the above tests grow in complexity with more parameters, the article presents a novel method for solving this issue. Demonstrated for reference in the two tests below.
[Fact]
public void BillboardConstructorGuardsAgainstNullParameters()
{
var fixture = new Fixture();
var assertion = new GuardClauseAssertion(fixture);
assertion.Verify(typeof(Billboard).GetConstructors());
}
[Fact]
public void PainterConstructorGuardsAgainstNullParameters()
{
var fixture = new Fixture();
var assertion = new GuardClauseAssertion(fixture);
assertion.Verify(typeof(Painter).GetConstructors());
}While this is a significant improvement over the previous iterations of the tests, I wanted a way to make these tests flow more fluently. This code exercise ultimately lead to a few extension methods demonstrated below.
[Fact]
public void BillboardConstructorGuardsAgainstNullParameters() =>
typeof(Painter)
.Constructors()
.ShouldGuardAgainstNullParameters();
[Fact]
public void PainterConstructorGuardsAgainstNullParameters() =>
typeof(Painter)
.Constructors()
.ShouldGuardAgainstNullParameters();So essentially the inner workings of this extension is to wrap a customized fixture in an extension method and using the GuardClauseAssertion Idiom in a more fluent expression format.
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