C# longlist选择器don'；t显示项目源

我对“新”的longlistselector控件有问题。我是根据基于toolkit:longlistselector的旧教程制作的

它显示标题，但不显示内容：/

Page1.xaml

Page1.xaml.cs

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
    {
        base.OnNavigatedTo(e);
        if (GroupedList.ItemsSource == null)
        {
            System.Collections.Generic.List<Model.FoodCategory> foodCategories =
                    new System.Collections.Generic.List<Model.FoodCategory>();

            /*---Make burger items---*/
            Model.FoodCategory burgers = new Model.FoodCategory("Burgers");
            burgers.AddFoodItem(new Model.FoodItem("Hamburger"));
            burgers.AddFoodItem(new Model.FoodItem("Chicken burger"));
            burgers.AddFoodItem(new Model.FoodItem("Turkey burger"));
            burgers.AddFoodItem(new Model.FoodItem("Black bean burger"));

            /*---Make fryer items---*/
            Model.FoodCategory fryer = new Model.FoodCategory("Fryer");
            fryer.AddFoodItem(new Model.FoodItem("Fries"));
            fryer.AddFoodItem(new Model.FoodItem("Onion rings"));
            fryer.AddFoodItem(new Model.FoodItem("Tater tots"));
            fryer.AddFoodItem(new Model.FoodItem("Mozzarella sticks"));

            /*---Make fish items---*/
            Model.FoodCategory fish = new Model.FoodCategory("Fish");
            fish.AddFoodItem(new Model.FoodItem("Salmon"));
            fish.AddFoodItem(new Model.FoodItem("Rainbow trout"));
            fish.AddFoodItem(new Model.FoodItem("Grilled tilapia"));

            foodCategories.Add(burgers);
            foodCategories.Add(fryer);
            foodCategories.Add(fish);

            GroupedList.ItemsSource = foodCategories;
        }
    }

class FoodItem
{
    public string Name { get; private set; }

    public FoodItem(string foodName)
    {
        Name = foodName;
    }
}

class FoodCategory
{
    public string Name { get; private set; }
    public System.Collections.Generic.List<FoodItem> Items { get; private set; }

    public FoodCategory(string categoryName)
    {
        Name = categoryName;
        Items = new System.Collections.Generic.List<FoodItem>();
    }

    public void AddFoodItem(FoodItem foodItem)
    {
        Items.Add(foodItem);
    }

    public System.Collections.IEnumerator GetEnumerator()
    {
        return this.Items.GetEnumerator();
    }
}

Model/FoodCategory.cs

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
    {
        base.OnNavigatedTo(e);
        if (GroupedList.ItemsSource == null)
        {
            System.Collections.Generic.List<Model.FoodCategory> foodCategories =
                    new System.Collections.Generic.List<Model.FoodCategory>();

            /*---Make burger items---*/
            Model.FoodCategory burgers = new Model.FoodCategory("Burgers");
            burgers.AddFoodItem(new Model.FoodItem("Hamburger"));
            burgers.AddFoodItem(new Model.FoodItem("Chicken burger"));
            burgers.AddFoodItem(new Model.FoodItem("Turkey burger"));
            burgers.AddFoodItem(new Model.FoodItem("Black bean burger"));

            /*---Make fryer items---*/
            Model.FoodCategory fryer = new Model.FoodCategory("Fryer");
            fryer.AddFoodItem(new Model.FoodItem("Fries"));
            fryer.AddFoodItem(new Model.FoodItem("Onion rings"));
            fryer.AddFoodItem(new Model.FoodItem("Tater tots"));
            fryer.AddFoodItem(new Model.FoodItem("Mozzarella sticks"));

            /*---Make fish items---*/
            Model.FoodCategory fish = new Model.FoodCategory("Fish");
            fish.AddFoodItem(new Model.FoodItem("Salmon"));
            fish.AddFoodItem(new Model.FoodItem("Rainbow trout"));
            fish.AddFoodItem(new Model.FoodItem("Grilled tilapia"));

            foodCategories.Add(burgers);
            foodCategories.Add(fryer);
            foodCategories.Add(fish);

            GroupedList.ItemsSource = foodCategories;
        }
    }

class FoodItem
{
    public string Name { get; private set; }

    public FoodItem(string foodName)
    {
        Name = foodName;
    }
}

class FoodCategory
{
    public string Name { get; private set; }
    public System.Collections.Generic.List<FoodItem> Items { get; private set; }

    public FoodCategory(string categoryName)
    {
        Name = categoryName;
        Items = new System.Collections.Generic.List<FoodItem>();
    }

    public void AddFoodItem(FoodItem foodItem)
    {
        Items.Add(foodItem);
    }

    public System.Collections.IEnumerator GetEnumerator()
    {
        return this.Items.GetEnumerator();
    }
}

class食品分类
{
公共字符串名称{get；private set；}
public System.Collections.Generic.List项{get；private set；}
公共食品类别（字符串类别名称）
{
名称=类别名称；
Items=new System.Collections.Generic.List（）；
}
公共void AddFoodItem（FoodItem FoodItem）
{
添加项目（foodItem）；
}
public System.Collections.IEnumerator GetEnumerator（）
{
返回此.Items.GetEnumerator（）；
}
}

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