Easy Way To Test All Constructor Null Argument Checks

Often C# classes will have several dependencies passed into the constructor and some (maybe all) of these will be mandatory for the class to function. In this case, ordinarily you'd add a null argument check in the ctor and if something is null throw an "ArgumentNullException". This is pretty boiler plate and is usually auto-generated code, but still it needs to be tested in your unit tests to assert that indeed all mandatory options have been checked (and conversely that all optional dependencies can indeed be null). This can be quite tedious and repetitive and later changing the signature of the ctor can result in many tests that requiring fixing up. To make these tests smaller and more concise I've come up with a new strategy using test cases and nullable mocks, as follows:

// SomeClass.cs
public class SomeClass
{
	private readonly IDependency1 _dependency1;
	private readonly IDependency2 _dependency2;
	private readonly IDependency3 _dependency3;

	public SomeClass(IDependency1 dependency1, IDependency2 dependency2, IDependency3 dependency3)
	{
		_dependency1 = dependency1 ?? throw new ArgumentNullException(nameof(dependency1));
		_dependency2 = dependency2 ?? throw new ArgumentNullException(nameof(dependency2));
		_dependency3 = dependency3 ?? throw new ArgumentNullException(nameof(dependency3));
	}
}

// SomeClass.tests.cs
[TestFixture]
public class SomeClassTests
{
	private Mock<IDependency1> _dependency1Mock;
	private Mock<IDependency2> _dependency2Mock;
	private Mock<IDependency3> _dependency3Mock;

	[SetUp]
	public void SetUp()
	{
		_dependency1Mock = new Mock<IDependency1>();
		_dependency2Mock = new Mock<IDependency2>();
		_dependency3Mock = new Mock<IDependency3>();
	}

	[TestCase("dependency1")]
	[TestCase("dependency2")]
	[TestCase("dependency3")]
	public void Ctor_RequiredDependencyNull_ThrowsException(string dependency)
	{
		var setup = new Dictionary<string, Action>
		{
			{"dependency1", () => _dependency1Mock = null },
			{"dependency2", () => _dependency2Mock = null },
			{"dependency3", () => _dependency3Mock = null }
		};
		setup[dependency]();

		Func<SomeClass> act = GetDefaultSut;

		act.Should().Throw<ArgumentNullException>().And.ParamName.Should().Be(dependency);
	}

   private SomeClass GetDefaultSut()
	{
		return new SomeClass(_dependency1Mock?.Object, _dependency2Mock?.Object, _dependency3Mock?.Object);
	}
}

My above example is for NUnit, using FluentAssertions and Moq but can be converted to your testing tools of choice. An even easier option, when all your dependencies are required is to use the "GuardClauseAssertion" from "AutoFixture.Idioms"

private readonly Fixture _fixture;

public SomeClassTest(){
     _fixture = new Fixture();
     _fixture.Customize(new AutoMoqCustomization());
}

[Test]
public void Ctor_NullDependency_Throws()
{
        new GuardClauseAssertion(_fixture)
                .Verify(typeof(SomeClass).GetConstructors());
}