OrderServiceTest.cs

[TestClass]
public class OrderServiceTest
{
    [TestMethod]
    public void CreateOrders()
    {
        // Arrange
        OrderService orderService = new OrderService();
        List<string> names = new List<string> { "Book", "Table", "Pen", "Mouse" };

        // Act
        orderService.CreateOrdersByNames(names);

        // Assert order.Save() has been called for 4 times
    }
}

public class OrderService
{
    public List<Order> CreateOrdersByNames(List<string> names)
    {
        List<Order> orders = new List<Order>();
        foreach(string name in names)
        {
            Order order = new Order(name);
            order.Save();
            orders.Add(order);
        }
        return orders;
    }
}

public class Order : IRepository
{
    public Order(string name)
    {
        this.Name = name;
    }
    public string Name { get; set; }
    public void Save()
    {
        // Write to DB or File
    }
}

public interface IRepository
{
    void Save();
}