Unit Test Constructor ArgumentNullException

ArgumentNullExceptionUnitTestHelper.cs

// source: https://mikhail.io/2015/04/unit-testing-null-parameter-checks/

public void ConstructorMustThrowArgumentNullException(Type type)
{
    foreach (var constructor in type.GetConstructors())
    {
        var parameters = constructor.GetParameters();
        var mocks = parameters.Select(
            p =>
                {
                    Type mockType = typeof(Mock<>).MakeGenericType(
                        new[] { p.ParameterType });
                    return (Mock)Activator.CreateInstance(mockType);
                }).ToArray();

        for (int i = 0; i < parameters.Length; i++)
        {
            var mocksCopy = mocks.Select(m => m.Object).ToArray();
            mocksCopy[i] = null;
            try
            {
                constructor.Invoke(mocksCopy);
                Assert.Fail("ArgumentNullException expected for parameter {0} of constructor, but no exception was thrown", parameters[i].Name);
            }
            catch (TargetInvocationException ex)
            {
                Assert.AreEqual(
                    typeof(ArgumentNullException),
                    ex.InnerException.GetType(),
                    string.Format("ArgumentNullException expected for parameter {0} of constructor, but exception of type {1} was thrown", parameters[i].Name, ex.InnerException.GetType()));
            }
        }
    }
}

ConstructorTestingHelper.cs

public static class ConstructorTestingHelper
{
    /// <summary>
    /// Uses reflection to find all constructors and all parameters to each constructor.
    /// Calls each constructor one time for each parameter, passing a null for each parameter one at a time to verify an ArgumentNullException is thrown.
    /// </summary>
    /// <param name="type">The type to be tested.</param>
    public static void VerifyAllConstructorsThrowArgumentNullExceptionsForEveryParameter(Type type)
    {
        if (type == null)
        {
            return;
        }

        foreach (var constructor in type.GetConstructors())
        {
            var parameters = constructor.GetParameters();

            var mocks = parameters
                .Select(p =>
                {
                    var mockType = typeof(Mock<>).MakeGenericType(new[] { p.ParameterType });

                    return (Mock)Activator.CreateInstance(mockType);
                })
                .ToArray();

            for (int i = 0; i < parameters.Length; i++)
            {
                var mockInstances = mocks.Select(m => m.Object).ToArray();

                mockInstances[i] = null;

                try
                {
                    constructor.Invoke(mockInstances);

                    Assert.Fail($"{nameof(ArgumentNullException)} expected for parameter [{parameters[i].Name}] of [{type.FullName}] constructor, but no exception was thrown.");
                }
                catch (TargetInvocationException ex)
                {
                    Assert.AreEqual(
                        typeof(ArgumentNullException),
                        ex.InnerException.GetType(),
                        $"{nameof(ArgumentNullException)} expected for parameter [{parameters[i].Name}] of [{type.FullName}] constructor, but exception of type [{ex.InnerException.GetType().FullName}] was thrown.");
                }
            }
        }
    }
}

original source: https://mikhail.io/2015/04/unit-testing-null-parameter-checks/