c# - PresentationFramework 中的 NullReference 异常

Question

下面是一个最小的例子，我不可能再减少它了。

我在 ViewModel 中创建一个实时过滤的 CollectionView，如下所示:

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.ComponentModel;
using System.Windows.Data;
using System.Windows;

namespace AntiBonto.ViewModel
{
    [Serializable]
    public class Person
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected void RaisePropertyChanged([CallerMemberName] String propertyName = "")
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
        public string Name { get; set; }
        public override string ToString()
        {
            return Name;
        }

        private int num;
        public int Num
        {
            get { return num; }
            set { num = value; RaisePropertyChanged(); }
        }
    }

    class ObservableCollection2<T> : ObservableCollection<T>
    {
        public ObservableCollection2() : base() { }
        public ObservableCollection2(T[] t) : base(t) { }
        public void AddRange(IEnumerable<T> collection)
        {
            foreach (var i in collection)
            {
                Items.Add(i);
            }
            OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
        }
    }

    class MainWindow: ViewModelBase
    {
        public MainWindow() { }
        private ObservableCollection2<Person> people = new ObservableCollection2<Person>();
        public ObservableCollection2<Person> People
        {
            get
            {
                return people;
            }
            set
            {
                people = value;
                RaisePropertyChanged();
            }
        }
        public ICollectionView Team
        {
            get
            {
                CollectionViewSource cvs = new CollectionViewSource { Source = People, IsLiveFilteringRequested = true, LiveFilteringProperties = { "Num" } };
                cvs.View.Filter = p => ((Person)p).Num != 11;
                return cvs.View;
            }
        }

        public ICollectionView Ujoncok
        {
            get
            {
                CollectionViewSource cvs = new CollectionViewSource { Source = People, IsLiveFilteringRequested = true, LiveFilteringProperties = { "Num" } };
                cvs.View.Filter = p => ((Person)p).Num == 11;
                return cvs.View;
            }
        }
    }
}

GUI 有一个按钮可以修改 People 集合中的 Person 对象:

<Window x:Class="AntiBonto.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:vm="clr-namespace:AntiBonto.ViewModel"
        mc:Ignorable="d"
        Title="AntiBonto" Width="1024" Height="768">
    <Window.DataContext>
        <vm:MainWindow/>
    </Window.DataContext>
    <Window.Resources>
        <FrameworkElement x:Key="DataContextProxy" DataContext="{Binding}"/> <!-- workaround, see http://stackoverflow.com/questions/7660967 -->
    </Window.Resources>
    <TabControl>
        <TabItem Header="Tab2">
            <StackPanel>
                <Button Content="Does" Click="Button_Click"/>
                <ContentControl Visibility="Collapsed" Content="{StaticResource DataContextProxy}"/>
                <!-- workaround part 2 -->
                <DataGrid ItemsSource="{Binding Ujoncok}" CanUserAddRows="False" CanUserDeleteRows="False" AutoGenerateColumns="False">
                    <DataGrid.Columns>
                        <DataGridComboBoxColumn Header="Who" ItemsSource="{Binding DataContext.Team, Source={StaticResource DataContextProxy}, Mode=OneWay}"/>
                    </DataGrid.Columns>
                </DataGrid>
            </StackPanel>
        </TabItem>
    </TabControl>
</Window>

我像这样从 XML 文件加载数据:

using System;
using System.IO;
using System.Linq;
using System.Windows;
using System.Xml.Serialization;

namespace AntiBonto
{
    [Serializable]
    public class AppData
    {
        public Person[] Persons;
    }
    public partial class MainWindow : System.Windows.Window
    {
        public MainWindow()
        {
            InitializeComponent();
            Loaded += MainWindow_Loaded;
        }
        private string filepath = "state.xml";
        private AppData AppData
        {
            get { return new AppData { Persons = viewModel.People.ToArray()}; }
            set { viewModel.People.AddRange(value.Persons);}
        }

        private void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            var xs = new XmlSerializer(typeof(AppData));
            if (File.Exists(filepath))
            {
                using (var file = new StreamReader(filepath))
                {
                    AppData = (AppData)xs.Deserialize(file);
                }
            }
        }     

        private ViewModel.MainWindow viewModel { get { return (ViewModel.MainWindow)DataContext; } }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Person p = viewModel.People.First(q => q.Name == "Ferencz Katalin");
            if (p.Num == 11)
                p.Num = 0;
            else
                p.Num= 11;
        }
    }
}

XML 文件是这样的:

<?xml version="1.0" encoding="utf-8"?>
<AppData xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Persons>
    <Person>
      <Name>Person1</Name>
      <Num>0</Num>
    </Person>
    <Person>
      <Name>Person2</Name>
      <Num>0</Num>
    </Person>
  </Persons>
</AppData>

当我点击按钮一次或两次时，我得到了一个NullReference异常。没有内在的异常(exception)。异常不是出现在我的代码中，而是出现在框架代码中，所以它没有显示来源，我无法找出哪个对象为空以及异常来自哪里。我没有设法设置“进入 .NET 源代码”，它仍然告诉我没有可用的源代码。

这是堆栈跟踪:

at System.Windows.Data.ListCollectionView.RestoreLiveShaping() at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs) at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler) at System.Windows.Threading.DispatcherOperation.InvokeImpl() at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state) at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state) at System.Windows.Threading.DispatcherOperation.Invoke() at System.Windows.Threading.Dispatcher.ProcessQueue() at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o) at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs) at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler) at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs) at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam) at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg) at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame) at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame) at System.Windows.Application.RunDispatcher(Object ignore) at System.Windows.Application.RunInternal(Window window) at System.Windows.Application.Run(Window window) at System.Windows.Application.Run() at AntiBonto.App.Main() in D:\Marci\Programozás\AntiBonto\AntiBonto\obj\Debug\App.g.cs:line 0 at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args) at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args) at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() at System.Threading.ThreadHelper.ThreadStart_Context(Object state) at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadHelper.ThreadStart()

Answer 1

我不知道为什么，但这修复了错误:

public ICollectionView Team
{
    get
    {
        CollectionViewSource cvs = new CollectionViewSource { Source = People, IsLiveFilteringRequested = true, LiveFilteringProperties = { "Num" } };
        cvs.View.Filter = p => ((Person)p).Num != 11;
        cvs.View.CollectionChanged += EmptyEventHandler;
        return cvs.View;
    }
}
private void EmptyEventHandler(object sender, NotifyCollectionChangedEventArgs e) { }

我试图调试异常发生的地方，我想在集合发生变化时设置一个断点。订阅事件使异常消失。