Windows Phone 8中的导航错误

Question

我有一个包含多个页面的应用程序；因此，我希望用户通过来回发送的一些数据来回导航到另一个页面。我可以使用

导航到包含一些特定数据的下一页

NavigationService.Navigate(new Uri("/All Files/Product Files/Product List.xaml?pro_cate_id="+(lstb_prod_cate.SelectedItem as ProductsDry).DryCategoryId, UriKind.Relative));

在下一页“Product List.xaml”中，它可以接收发送给它的数据并进行正常查询。但是，在“Product List.xaml”上，我放置了一个按钮并将其事件设置为“NavigationService.GoBack()”，以便用户可以按下以转到上一页。此时，回到上一页时，在

处报错

 private void lstb_prod_cate_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {

            NavigationService.Navigate(new Uri("/All Files/Product Files/Product List.xaml?pro_cate_id="+(lstb_prod_cate.SelectedItem as ProductsDry).DryCategoryId, UriKind.Relative));
        } 

这是它将数据发送到下一页的第一页

产品类别.xmal.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Navigation;
using Microsoft.Phone.Controls;
using Microsoft.Phone.Shell;
using System.Xml.Linq;
using App_Skin_Test_Final_.All_Files.Database_XML;

namespace App_Skin_Test_Final_.All_Files.Product_Files
{
    public partial class Product_Category : PhoneApplicationPage
    {

        public Product_Category()
        {
            InitializeComponent();

        }

        protected override void OnNavigatedTo(NavigationEventArgs e)
        {

            MessageBox.Show("OnNavigateTo");
            base.OnNavigatedTo(e);
            // Do something when the page first loaded
        }


        private void lstb_prod_cate_Loaded(object sender, RoutedEventArgs e)
        {

            XDocument xd = XDocument.Load("All Files/Database XML/ProductsDry.xml");
            var data = from q in xd.Descendants("DryCategory")
                       orderby q.Attribute("DryCategoryName").Value
                       select new ProductsDry
                       {
                           DryCategoryName = q.Attribute("DryCategoryName").Value,
                           DryCategoryId=q.Attribute("DryCategoryId").Value
                       };
            //       MessageBox.Show();
            lstb_prod_cate.DataContext = data;

        }

        private void lstb_prod_cate_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {

            NavigationService.Navigate(new Uri("/All Files/Product Files/Product List.xaml?pro_cate_id="+(lstb_prod_cate.SelectedItem as ProductsDry).DryCategoryId, UriKind.Relative));
        }

    }
}

这是下一页“Product List.xaml.cs”，它将接收数据并可以按按钮返回

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Navigation;
using Microsoft.Phone.Controls;
using Microsoft.Phone.Shell;
using System.Xml.Linq;
using App_Skin_Test_Final_.All_Files.Database_XML;
using System.Windows.Media.Imaging;

namespace App_Skin_Test_Final_.All_Files.Product_Files
{
    public partial class Product_List : PhoneApplicationPage
    {
        string pro_cate_id;
        public Product_List()
        {
            InitializeComponent();

        }
        protected override void OnNavigatedTo(NavigationEventArgs e)
        {

            base.OnNavigatedTo(e);

            if (NavigationContext.QueryString.TryGetValue("pro_cate_id", out pro_cate_id))
            {

            }
        }

        private void lst_product_Loaded(object sender, RoutedEventArgs e)
        {

            MessageBox.Show(pro_cate_id);
            XDocument data = XDocument.Load("All Files/Database XML/ProductsDry.xml");
            var productListData = from q in data.Descendants("DryCategory")
                                  from itemDry in q.Elements("ItemDry")  // mean: itemDry in in DryCategory
                                  where q.Attribute("DryCategoryId").Value == pro_cate_id
                                  select new ProductsDry
                                  {
                                      ItemDryName = itemDry.Attribute("ItemDryName").Value,
                                      ItemDryImage=getImage(itemDry.Attribute("ItemDryImage").Value),
                                      ItemDryId=itemDry.Attribute("ItemDryId").Value
                                  };


            lst_product.DataContext = productListData;

         //   NavigationService.GoBack();
        }

        private System.Windows.Media.ImageSource getImage(string p)
        {
            return new BitmapImage(new Uri(p, UriKind.Relative));

        }


        private void lst_product_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            NavigationService.Navigate(new Uri("/All Files/Product Files/Product Detail.xaml?itemId="+(lst_product.SelectedItem as ProductsDry).ItemDryId,UriKind.Relative));


        }

        private void btnGoBack_Click(object sender, RoutedEventArgs e)
        {


            if (this.NavigationService.CanGoBack)
            {
                this.NavigationService.GoBack();
            }
        }
    }

}

请问，谁能帮我解决这个错误？谢谢

Answer 1

正如您在评论中所说，您怀疑 SelectedItem 是 null。您可以通过进行简单检查来避免这种情况下的异常，并且仅当 SelectedItem 当前不是 null 时才继续您的代码：

private void lstb_prod_cate_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if(lstb_prod_cate.SelectedItem != null)
        NavigationService.Navigate(new Uri("/All Files/Product Files/Product List.xaml?pro_cate_id="+(lstb_prod_cate.SelectedItem as ProductsDry).DryCategoryId, UriKind.Relative));
}