Question

我是c＃的新手并尝试为我的ObsorvableCollection类实现INotifyPropertyChanged

但它给出了错误，数据没有绑定。有人请帮我解决这个问题

发生了System.ArgumentNullException类型的异常 mscorlib.ni.dll但未在用户代码中处理
Additional information: Value cannot be null.

先谢谢。

我的Xaml代码：

<Page 
x:Class="App2.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:App2"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">

<Page.BottomAppBar>
    <CommandBar Background="Orange">
        <AppBarButton Icon="Sort" Label="Sort">
            <AppBarButton.Flyout>
                <MenuFlyout>
                    <MenuFlyoutItem Text="By Upvotes" Click="FilterItem_Click" Tag="name"/>
                    <MenuFlyoutItem Text="By OpenForms" Click="FilterItem_Click" Tag="lname"/>
                    <MenuFlyoutItem Text="By Ideas" Click="FilterItem_Click" Tag="ideas"/>
                </MenuFlyout>
            </AppBarButton.Flyout>
        </AppBarButton>
    </CommandBar>
</Page.BottomAppBar>
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <ListView ItemsSource="{x:Bind Path=person}" Margin="105,130,95,70">
        <ListView.ItemTemplate>
            <DataTemplate x:DataType="local:Person">
                <TextBlock Text="{x:Bind Name}"></TextBlock>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>
   </Grid>
</Page>

我的cs代码：

        using App2.WrittenLibraries;
        using System.Collections.Generic;
        using System.Collections.ObjectModel;
        using System.ComponentModel;
        using System.Linq;
        using System.Threading.Tasks;
        using Windows.UI.Xaml;
        using Windows.UI.Xaml.Controls;

        // The Blank Page item template is documented at http://go.microsoft.com/fwlink/?LinkId=402352&clcid=0x409

        namespace App2
        {
            /// <summary>
            /// An empty page that can be used on its own or navigated to within a Frame.
            /// </summary>
            /// 
            public class Person : INotifyPropertyChanged
            {
                private string name;
                public string Name
                {
                    get { return name;  }
                    set
                    {
                        name = value;
                        OnPropertyChanged("Name");
                    }
                }

                private string lastname;
                private string v1;
                private string v2;

                public string Lastname
                {
                    get { return lastname; }
                    set
                    {
                        lastname = value;
                        OnPropertyChanged("Lastname");
                    }
                }

                public event PropertyChangedEventHandler PropertyChanged;

                private void OnPropertyChanged(string propertyName)
                {
                    if (propertyName != null)
                    {

                        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
                    }
                }


                public Person(string v1, string v2)
                {
                    this.v1 = v1;
                    this.v2 = v2;
                }
            }
            public sealed partial class MainPage : Page
             {
                public ObservableCollection<Person> person = new ObservableCollection<Person>();

                public MainPage()
                {
                    this.InitializeComponent();

                    person.Add(new Person("F1", "L1"));
                    person.Add(new Person("F2", "L2"));
                }


                private void FilterItem_Click(object sender, RoutedEventArgs e)
                {
                    MenuFlyoutItem selectedItem = sender as MenuFlyoutItem;

                    if (selectedItem != null)
                    {
                        if (selectedItem.Tag.ToString() == "name")
                        {
                            Util.debugLog("FILTER BY NAME");
                            person = new ObservableCollection<Person>(person.OrderBy(i => i.Name));

                            //FilterByUpvotes()();
                        }
                        else if (selectedItem.Tag.ToString() == "lname")
                        {
                            Util.debugLog("FILTER BY L_NAME");
                            person = new ObservableCollection<Person>(person.OrderBy(i => i.Lastname));
                            //FilterByOpenForm();
                        }
                        else if (selectedItem.Tag.ToString() == "ideas")
                        {
                            Util.debugLog("FILTER BY IDEAS");
                            //person = new ObservableCollection<Person>(person.OrderBy(i => i));

                            //FilterByIdeas();
                        }
                    }
                }
            }
        }

Answer 1

解决这个问题的第一件事是找到代码的哪一部分生成这个空值，然后尝试理解c＃中的以下代码，这对我来说非常有用，通过检查检索代码来避免空值。将读取我或不是..就像：

if(cmd.ExecuteScalar() == null)
    MessageBox.Show("data is null");
else
    //put ur code here

另外，不要忘记使用try catch block。

Answer 2

您的问题在于使用x：bind绑定name属性。您的名称将始终为null，因为您没有设置任何值。所以绑定失败。你能做的是编辑列表视图模板，如

       <ListView ItemsSource="{x:Bind Path=person}" Margin="105,130,95,70" Loaded="ListView_Loaded">
            <ListView.ItemTemplate>
                <DataTemplate x:DataType="local:Person">
                    <TextBlock Text="{Binding Name}" Foreground="Red"></TextBlock>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>

并在创建人员时在代码内设置名称

 public Person(string v1, string v2)
    {
        this.v1 = v1;
        this.v2 = v2;
        this.name = v1;//set name as v1
    }

Answer 3

您必须弄清楚从何处获取空值并将代码放在 try catch 块中，并为“System.ArgumentNullException”编写 catch 块。

try
{
   `enter code here` // your code enter code here
}

catch (ArgumentNullException ex)
{
   `enter code here` //code specifically for a ArgumentNullException
}

抛出异常：mscorlib.ni.dll中的“System.ArgumentNullException”

3 个答案: