public static void AddPersonToPeopleList(List<PersonModel> people, PersonModel person)
        {
            CheckWhiteSpace(person.FirstName, "FirstName");
            CheckWhiteSpace(person.LastName, "LastName");

            people.Add(person);
        }

public static void CheckWhiteSpace(string name, string paramName) {
            if (string.IsNullOrWhiteSpace(name)) {
                throw new ArgumentException("You passed in an invalid parameter", paramName);
            }
        }

[Theory]
        [InlineData("Mick", "", "LastName")]
        [InlineData("", "Adams", "FirstName")]
        public void AddPersonToPeopleList_ShouldFail(string firstName, string lastName, string param)
        {
            PersonModel newPerson = new PersonModel { FirstName = firstName, LastName = lastName };
            List<PersonModel> people = new List<PersonModel>();

            Assert.Throws<ArgumentException>(param, () => DataAccess.AddPersonToPeopleList(people, newPerson));
        }

public static class StringExtensions{
     public static void CheckWhiteSpace(this string name, string paramName){
        if(string.IsNullOrWhiteSpace(name)
            throw new ArgumentException("You passed in an invalid parameter", paramName);
     }
}

person.FirstName.CheckWhiteSpace("FirstName")