public void Active_Should_Throw_ArgumentNullException_If_Customers_Is_Null()
        {
            IQueryable <Customer> customers = null;

            var ex = Assert.Throws <ArgumentNullException>(() => CustomerExtensions.Active(customers));

            Assert.Equal(nameof(customers), ex.ParamName);
        }
        public void Active_Should_Return_Only_Customers_Not_Deleted()
        {
            var customer1 = Person.Register("firstName", "lastname", Gender.Female, new DateTime(1980, 1, 1));
            var customer2 = Person.Register("firstName2", "lastname2", Gender.Male, new DateTime(1980, 1, 1));
            var customer3 = Company.Register("company", "1234567890");

            customer2.Delete();

            IQueryable <Customer> customers = new Customer[]
            {
                customer1, customer2, customer3
            }.AsQueryable();

            var activeCustomers = CustomerExtensions.Active(customers);

            Assert.True(activeCustomers.All(c => !c.Deleted));
        }