public void ThrowArgumentNullException_WhenTheFirstMachingValidationRuleIsNull()
{
// Arrange.
IValidatableArgument<string> argument = new ValidatableArgument<string>("argument", null);
argument.IsEqual("asdf").IsNull();
// Act & Assert.
Ensure.ArgumentNullExceptionIsThrown(
() =>
{
argument.Throw();
}, "argument");
}