相关疑难解决方法(0)

我有一些代码,当它执行时,它抛出一个NullReferenceException,说:

你调用的对象是空的.

这是什么意思,我该怎么做才能解决这个错误？

我正在使用一个ObservableCollection带有两个ICollectionView不同的过滤器.

一种用于按某种类型过滤消息,一种用于计算已检查的消息.正如您所看到的,消息过滤器和消息计数工作正常,但是当我取消检查时,消息将从列表中消失(计数仍然有效).

对于这篇长篇文章,我很抱歉,我想包括所有相关内容.

XAML代码:

<!-- Messages List -->
<DockPanel Grid.Row="1"
           Grid.Column="0"
           Grid.ColumnSpan="3"
           Height="500">
  <ListBox Name="listBoxZone"
           ItemsSource="{Binding filteredMessageList}"
           Background="Transparent"
           BorderThickness="0">
    <ListBox.ItemTemplate>
      <DataTemplate>
        <CheckBox Name="CheckBoxZone"
                  Content="{Binding text}"
                  Tag="{Binding id}"
                  Unchecked="CheckBoxZone_Unchecked"
                  Foreground="WhiteSmoke"
                  Margin="0,5,0,0"
                  IsChecked="{Binding isChecked}" />
      </DataTemplate>
    </ListBox.ItemTemplate>
  </ListBox>
</DockPanel>
<Button Content="Test Add New"
        Grid.Column="2"
        Height="25"
        HorizontalAlignment="Left"
        Margin="34,2,0,0"
        Click="button1_Click" />
<Label Content="{Binding checkedMessageList.Count}"
       Grid.Column="2"
       Height="25"
       Margin="147,2,373,0"
       Width="20"
       Foreground="white" />

截图:

码:

/* ViewModel Class */
public class MainViewModel : INotifyPropertyChanged
{

    // Constructor
    public MainViewModel()
    {
        #region …

我已经编写了带有搜索扩展名的自定义WPF控件,让我们为它命名MyControl.控制是一个ItemsControl 阶级的后代.

所以我像这样将数据源提供给它:

控件本身使用

protected override void OnItemsSourceChanged(System.Collections.IEnumerable oldValue, System.Collections.IEnumerable newValue)
{
    if (newValue != null)
    {
        ICollectionView view = CollectionViewSource.GetDefaultView(newValue);
        view.Filter += this.FilterPredicate;
    }

    if (oldValue != null)
    {
        ICollectionView view = CollectionViewSource.GetDefaultView(oldValue);
        view.Filter -= this.FilterPredicate;
    }

    base.OnItemsSourceChanged(oldValue, newValue);
}

过滤源集合的视图(从而在内部ListBox中显示它).

现在假设我们在XAML中定义了10个具有相同DynamicSource的MyControl.问题是如果其中一个在源集合上应用Filter,它也会影响所有其他实例.

如何更改控件以避免此行为？

下面是一个最小的例子，我不可能再减少它了。

我在 ViewModel 中创建了一个实时过滤的 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(); }
        } …

我的应用程序使用WPF DataGrid.其中一列是模板列,其中包含ComboBox绑定到ObservableCollection为该行提供的实体的一个.当我向其添加值时ObservableCollection,NullReferenceException抛出a.

有谁知道为什么会这样？这是异常的堆栈跟踪:

   at MS.Internal.Data.PropertyPathWorker.DetermineWhetherDBNullIsValid()
   at MS.Internal.Data.PropertyPathWorker.get_IsDBNullValidForUpdate()
   at MS.Internal.Data.ClrBindingWorker.get_IsDBNullValidForUpdate()
   at System.Windows.Data.BindingExpression.ConvertProposedValue(Object value)
   at System.Windows.Data.BindingExpressionBase.UpdateValue()
   at System.Windows.Data.BindingExpression.Update(Boolean synchronous)
   at System.Windows.Data.BindingExpressionBase.Dirty()
   at System.Windows.Data.BindingExpression.SetValue(DependencyObject d, DependencyProperty dp, Object value)
   at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, OperationType operationType, Boolean isInternal)
   at System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
   at System.Windows.Controls.Primitives.Selector.UpdatePublicSelectionProperties()
   at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
   at System.Windows.Controls.Primitives.Selector.OnItemsChanged(NotifyCollectionChangedEventArgs e)
   at System.Windows.Controls.ItemsControl.OnItemCollectionChanged(Object sender, NotifyCollectionChangedEventArgs e)
   at System.Collections.Specialized.NotifyCollectionChangedEventHandler.Invoke(Object sender, NotifyCollectionChangedEventArgs e)
   at System.Windows.Data.CollectionView.OnCollectionChanged(NotifyCollectionChangedEventArgs args)
   at System.Windows.Controls.ItemCollection.System.Windows.IWeakEventListener.ReceiveWeakEvent(Type managerType, Object sender, EventArgs e) …