C＃列表实例为空

Question

My problem is that I have a List in one class "Menu" containing objects of the class "Pizza". I want to access this list in a third class "Order". I am doing it in console.

class Menu
{
    public List<Pizza> pizzaOnMenu = new List<Pizza>();

    public void CreateElementToMenu()
    { 
        pizzaOnMenu.Add(new Pizza(1, "Magarita", "Tomato, cheese", 49));
        pizzaOnMenu.Add(new Pizza(2, "Hawaii", "Tomato, cheese, ham, pineapple", 55));
        pizzaOnMenu.Add(new Pizza(3, "Cappriciossa", "Tomato, cheese, ham, mushroom", 55));
        pizzaOnMenu.Add(new Pizza(4, "Special", "Tomato, cheese, beef", 60));
    }
}

I have also tried to make this as well

public List<Pizza> pizzaOnMenu(){ get; set; }

Anyway - the method is called from Main():

Menu menu = new Menu();
menu.CreateElementToMenu();

And then I try to access the list in "Order" to check if the pizza exist on the list in "Menu":

class Order
{

    private List<Pizza> orderList = new List<Pizza>();

    public void CreateOrder()
    {
        Console.WriteLine("\nWhich number from the menu do you wish to order?");
        int orderNo = int.Parse(Console.ReadLine());

        Menu menu = new Menu();
        Pizza wantedPizza = null;

        foreach (Pizza p in menu.pizzaOnMenu)
        {
            if (p.Nr == orderNo)
            {
                wantedPizza = p;
                break;
            }
         }

        if (wantedPizza != null)
        {
            orderList.Add(wantedPizza);
            Console.WriteLine("\n" + wantedPizza.PizzaName + " is added to your order");
        }    
        else
        {
            Console.WriteLine("\nThe pizza with no. " + orderNo + " was not found!");
        }
    }

However, when the method CreateOrder() is run from main and I eg choose no. 2 from the menu, it writes "...was not found" instead of "...is added to your order" - it seems that the list (from "Order") is empty. But I am able to print out the list created from Menu :S (if that makes sense?)

Thanks in advance.

Answer 1

Look at the foreach loop over the contents of menukort.pizzaOnMenu : I don't see a closing brace ( } ) where the indentation would indicate it is expected (immediately after the closing brace of the if.

Thus the if (wantedPizza != null) is checked for every possible pizza.

If this is just a cut and paste error when creating the question: edit the question having checked the code you copied in is a true copy, and then make the correction clear.

Answer 2

You probably want to move calling the initialization to constructor of the Menu class, providing that Menukort and Menu are same classes:

class Menu
{
    public Menu()
    {
        CreateElementToMenu();
    }

    public List<Pizza> pizzaOnMenu = new List<Pizza>();

    public void CreateElementToMenu()
    { 
        pizzaOnMenu.Add(new Pizza(1, "Magarita", "Tomato, cheese", 49));
        pizzaOnMenu.Add(new Pizza(2, "Hawaii", "Tomato, cheese, ham, pineapple", 55));
        pizzaOnMenu.Add(new Pizza(3, "Cappriciossa", "Tomato, cheese, ham, mushroom", 55));
        pizzaOnMenu.Add(new Pizza(4, "Special", "Tomato, cheese, beef", 60));
    }
}

Answer 3

Your order class use Menukort object instead of Menu what i don't understand why

You have public void CreateElementToMenu() in Menu class.

Perhaps you should use this code in your Order class

Menu menu = new Menu();
menu.CreateElementToMenu();

Answer 4

使用public static class public class

Answer 5

Just another approach to make it a little more SOLID - see http://en.wikipedia.org/wiki/SOLID_(object-oriented_design :

public class OrderDataEntry
{
    public static void TakeOrder()
    {
        Console.Write("\nWhich number from the menu do you wish to order?");
        int orderNo = int.Parse(Console.ReadLine());

        Menu menukort = new Menu();
        Pizza wantedPizza = menukort.pizzaOnMenu.FirstOrDefault<Pizza>(p => p.OrderNumber.Equals(orderNo));

        if (wantedPizza != null)
        {
            Console.WriteLine("\n" + wantedPizza.PizzaName + " is added to your order");
            Order o = new Order(wantedPizza);
        }
        else
        {
            Console.WriteLine("\nThe pizza with no. " + orderNo + " was not found!");
        }
    }
}

public class Order
{
    private List<Pizza> orderList; 

    public Order(Pizza wantedPizza)
    {
        orderList = new List<Pizza>();
        AddToOrder(wantedPizza);
    }

    public void AddToOrder(Pizza wantedPizza)
    {
        orderList.Add(wantedPizza);
    }
}