Fluent Constructor Assertions

• Introduction
• Getting Started
  • Installing the package
  • Basic example
  • Advanced example

Introduction

Fluent Constructor Assertions is a Fluent API for testing constructors in C#. The goal of this library is to make testing all those pesky ArgumentNullExceptions being thrown around when instantiating classes just a bit easier and less time intensive. It also provides a way to validate the happy path 😊

Getting Started

Installing the package

Install the Fluent.ConstructorAssertions package from nuget using Nuget Package Manager or via the console.

# Install the Fluent.ConstructorAssertions package to the project named MyProject
Install-Package Fluent.ConstructorAssertions -ProjectName MyProject

Basic example

Lets say we want to test a class MyClass that takes IMediator and ILogger<MyClass> in it's constructor.

public class MyClass
{
  private readonly IMediator _mediator;
  private readonly ILogger<MyClass> _logger;

  public MyClass(IMediator mediator, ILogger<MyClass> logger)
  {
    _mediator = mediator ?? throw new ArgumentNullException(nameof(mediator));
    _logger = logger ?? throw new ArgumentNullException(nameof(logger));
  }
}

We can do the below to check that an ArgumentNullException is thrown when IMediator is null - aka the dependency was not registered correctly.

[Fact]
public void GivenNullIMediator_WhenInstantiatingClass_ThenThrowArgumentNullException()
{
  ForConstructorOf<MyClass>
    .WithArgTypes(typeof(IMediator), typeof(ILogger<MyClass>))
    .Throws<ArgumentNullException>("Value cannot be null. (Parameter 'mediator')")
    .ForArgs(null, Substitute.For<ILogger<MyClass>>())
    .Should.BeTrue();
}

Advanced example

We can also use the call chaining functionality built into the API to do the below instead of writing a test for each parameter, and we can even check for the success scenario in the same chain!

[Fact]
public void GivenNullArgs_WhenInstantiatingClass_ThenThrowArgumentNullException()
{
  IMediator mediator = Substitute.For<IMediator>();
  ILogger<MyClass> logger = Substitute.For<ILogger<MyClass>>();

  ForConstructorOf<MyClass>
    .WithArgTypes(typeof(IMediator), typeof(ILogger<MyClass>))
    .Throws<ArgumentNullException>("Value cannot be null. (Parameter 'mediator')")
    .ForArgs(null, logger)
    .And.Throws<ArgumentNullException>("Value cannot be null. (Parameter 'logger')")
    .ForArgs(mediator, null)
    .And.Succeeds("No exception should be thrown")
    .ForArgs(mediator, logger)
    .Should.BeTrue();
}