组合框选定项数据绑定空引用异常

Question

我现在对组合框有点失望，希望有人能回答我的问题。问题出在 SelectedItem 上。当我在调试器中运行我的应用程序时，如果我在 ComboBox 中输入与项目中的项目（即 a、b 或 c）匹配的文本，然后删除该文本，它将引发空引用异常。如果我在 ComboBox 中输入文本并且与 Items 中的 Item(ie.. z) 不匹配，然后删除该文本，它不会崩溃。此行为仅在调试器中发生。如果我在外面运行应用程序，我不会崩溃。我正在使用 mvvmlight takeit 但我不认为它与此有关。我的代码在下面

查看：

<ComboBox IsEditable="True"
              VerticalAlignment="Top"
              ItemsSource="{Binding Items}"
              DisplayMemberPath="Name"
              SelectedItem="{Binding Item,Mode=TwoWay}"/>

型号：

public class Item
{
    public string Name { get; set; }
    public int Id { get; set; }
}

虚拟机：

public MainViewModel()
    {
        Items = new List<Item>
          {
            new Item {Name="a", Id=0},
            new Item {Name="b", Id=1},
            new Item {Name="c", Id=2},
          };
    }

    /// <summary>
    /// The <see cref="Items" /> property's name.
    /// </summary>
    public const string ItemsPropertyName = "Items";

    private List<Item> _items;

    /// <summary>
    /// Sets and gets the Items property.
    /// Changes to that property's value raise the PropertyChanged event. 
    /// </summary>
    public List<Item> Items
    {
        get
        {
            return _items;
        }
        set
        {
            Set(ItemsPropertyName, ref _items, value);
        }
    }

    /// <summary>
    /// The <see cref="Item" /> property's name.
    /// </summary>
    public const string ItemPropertyName = "Item";

    private Item _item;

    /// <summary>
    /// Sets and gets the Item property.
    /// Changes to that property's value raise the PropertyChanged event. 
    /// </summary>
    public Item Item
    {
        get
        {
            return _item;
        }
        set
        {
            Set(ItemPropertyName, ref _item, value);
        }
    }

Answer 1

这是 .NET Framework 4（和 .NET 4.5，不在 .NET 3.0 和 .NET 3.5 中）中的错误。

方法PresentationFramework.dll!MS.Internal.Data.PropertyPathWorker.DetermineWhetherDBNullIsValid(object item) 导致问题。

用.NET Reflector查看，其代码如下：

private bool DetermineWhetherDBNullIsValid(object item)
{
    PropertyInfo info;
    PropertyDescriptor descriptor;
    DependencyProperty property;
    DynamicPropertyAccessor accessor;
    this.SetPropertyInfo(this._arySVS[this.Length - 1].info, out info, out descriptor, out property, out accessor);
    string columnName = (descriptor != null) ? descriptor.Name : ((info != null) ? info.Name : null);
    object arg = ((columnName == "Item") && (info != null)) ? this._arySVS[this.Length - 1].args[0] : null;
    return SystemDataHelper.DetermineWhetherDBNullIsValid(item, columnName, arg);
}

问题在以下行：

object arg = ((columnName == "Item") && (info != null)) ? this._arySVS[this.Length - 1].args[0] : null;

代码假定如果columnName 是"Item"，那么属性是索引器并尝试通过args[0] 访问它的第一个参数，这就是NullReferenceException 出现的地方，因为args 是null，因为属性不是索引器。它恰好被命名为"Item"。

.NET 实现者应该在info 上使用PropertyInfo.GetIndexParameters()，如果返回的数组不包含零元素，请确定属性是索引器。或者使用Binding.IndexerName 进行检查（Binding.IndexerName 的值为"Item[]"）。

为什么只在 Visual Studio 调试器中出现问题要微妙得多，它隐藏在以下方法中： PresentationFramework.dll!MS.Internal.Data.PropertyPathWorker.DetermineWhetherDBNullIsValid()。

下面是反汇编代码：

private void DetermineWhetherDBNullIsValid()
{
    bool flag = false;
    object item = this.GetItem(this.Length - 1);
    if ((item != null) && AssemblyHelper.IsLoaded(UncommonAssembly.System_Data))
    {
        flag = this.DetermineWhetherDBNullIsValid(item);
    }
    this._isDBNullValidForUpdate = new bool?(flag);
}

由于item 变量不会为空（它实际上是WeakReference 的实例，其中包含MainViewModel 实例），因此调用失败方法DetermineWhetherDBNullIsValid(item) 的唯一条件是System.Data.dll 程序集已加载，使用 AssemblyHelper.IsLoaded(UncommonAssembly.System_Data) 检查。

Visual Studio 调试器将始终加载 System.Data.dll，因为项目正在引用它，尽管它没有使用它。 在 Visual Studio 调试器之外，System.Data.dll 仅在使用时才被加载，这是永远不会的，这就是应用程序在 Visual Studio 之外不会失败的原因。

您有以下选项可以解决此问题：

1. 将绑定到ComboBox.SelectedItem 的属性重命名为"Item" 以外的其他名称，这样有缺陷的.NET 实现就不再假定该属性是索引器。
2. 从项目引用中删除 System.Data.dll，这样即使在 Visual Studio 调试器中也不会加载它。

我发现选项 2 更脆弱，因为可能会出现 System.Data.dll 必须加载的情况，要么直接由您的应用程序加载，要么由其他加载的程序集间接加载。

所以我会选择选项 1。

Answer 2

我能够在我这边重现这一点。将此添加到您的组合框代码中：

IsTextSearchEnabled="False"

不管怎样，还有谁对这个问题感兴趣，这个异常的stacktrace如下

PresentationFramework.dll!MS.Internal.Data.PropertyPathWorker.DetermineWhetherDBNullIsValid(object item) + 0xc7 bytes
PresentationFramework.dll!MS.Internal.Data.PropertyPathWorker.DetermineWhetherDBNullIsValid() + 0x64 字节
PresentationFramework.dll!MS.Internal.Data.PropertyPathWorker.IsDBNullValidForUpdate.get() + 0x2e 字节 PresentationFramework.dll!MS.Internal.Data.ClrBindingWorker.IsDBNullValidForUpdate.get() + 0xa 字节
PresentationFramework.dll!System.Windows.Data.BindingExpression.ConvertProposedValue(对象值) + 0x177 字节
PresentationFramework.dll!System.Windows.Data.BindingExpressionBase.UpdateValue() + 0x92 字节
PresentationFramework.dll!System.Windows.Data.BindingExpression.UpdateOverride() + 0x3d 字节
PresentationFramework.dll!System.Windows.Data.BindingExpressionBase.Update() + 0x20 字节
PresentationFramework.dll!System.Windows.Data.BindingExpressionBase.ProcessDirty() + 0x2f 字节 PresentationFramework.dll!System.Windows.Data.BindingExpressionBase.Dirty() + 0x40 字节
PresentationFramework.dll!System.Windows.Data.BindingExpressionBase.SetValue(System.Windows.DependencyObject d, System.Windows.DependencyProperty dp, 对象值) + 0x24 字节
WindowsBase.dll!System.Windows.DependencyObject.SetValueCommon（System.Windows.DependencyProperty dp，对象值，System.Windows.PropertyMetadata 元数据，bool coerceWithDeferredReference，bool coerceWithCurrentValue，System.Windows.OperationType operationType，bool isInternal）+ 0x3c4 字节
WindowsBase.dll!System.Windows.DependencyObject.SetCurrentValueInternal(System.Windows.DependencyProperty dp, 对象值) + 0x35 字节
PresentationFramework.dll!System.Windows.Controls.Primitives.Selector.UpdatePublicSelectionProperties() + 0x13f 字节
PresentationFramework.dll!System.Windows.Controls.Primitives.Selector.SelectionChanger.End() + 0x80 字节
PresentationFramework.dll!System.Windows.Controls.Primitives.Selector.SelectionChanger.SelectJustThisItem(System.Windows.Controls.ItemsControl.ItemInfo info, bool assumeInItemsCollection) + 0x145 字节
PresentationFramework.dll!System.Windows.Controls.Primitives.Selector.OnSelectedIndexChanged(System.Windows.DependencyObject d, System.Windows.DependencyPropertyChangedEventArgs e) + 0xd9 字节
WindowsBase.dll!System.Windows.DependencyObject.OnPropertyChanged(System.Windows.DependencyPropertyChangedEventArgs e) + 0x4d 字节 PresentationFramework.dll!System.Windows.FrameworkElement.OnPropertyChanged(System.Windows.DependencyPropertyChangedEventArgs e) + 0x50 字节
WindowsBase.dll!System.Windows.DependencyObject.NotifyPropertyChange(System.Windows.DependencyPropertyChangedEventArgs 参数) + 0x3b 字节
WindowsBase.dll!System.Windows.DependencyObject.UpdateEffectiveValue(System.Windows.EntryIndex entryIndex, System.Windows.DependencyProperty dp, System.Windows.PropertyMetadata 元数据, System.Windows.EffectiveValueEntry oldEntry, ref System.Windows.EffectiveValueEntry newEntry, bool coerceWithDeferredReference , bool coerceWithCurrentValue, System.Windows.OperationType operationType) + 0x757 字节
WindowsBase.dll!System.Windows.DependencyObject.SetValueCommon（System.Windows.DependencyProperty dp，对象值，System.Windows.PropertyMetadata 元数据，bool coerceWithDeferredReference，bool coerceWithCurrentValue，System.Windows.OperationType operationType，bool isInternal）+ 0x2ea 字节
WindowsBase.dll!System.Windows.DependencyObject.SetCurrentValueInternal(System.Windows.DependencyProperty dp, 对象值) + 0x35 字节
PresentationFramework.dll!System.Windows.Controls.ComboBox.TextUpdated(string newText, bool textBoxUpdated) + 0x26e 字节
PresentationFramework.dll!System.Windows.Controls.ComboBox.OnEditableTextBoxTextChanged(object sender, System.Windows.Controls.TextChangedEventArgs e) + 0x2e 字节 PresentationFramework.dll!System.Windows.Controls.TextChangedEventArgs.InvokeEventHandler(System.Delegate genericHandler, object genericTarget) + 0x2c bytes
PresentationCore.dll!System.Windows.RoutedEventArgs.InvokeHandler（System.Delegate 处理程序，对象目标）+ 0x33 字节
PresentationCore.dll!System.Windows.RoutedEventHandlerInfo.InvokeHandler(对象目标，System.Windows.RoutedEventArgs routedEventArgs) + 0x44 字节
PresentationCore.dll!System.Windows.EventRoute.InvokeHandlersImpl（对象源，System.Windows.RoutedEventArgs 参数，bool reRaised）+ 0x1a8 字节
PresentationCore.dll!System.Windows.UIElement.RaiseEventImpl（System.Windows.DependencyObject 发送方，System.Windows.RoutedEventArgs 参数）+ 0x73 字节
PresentationCore.dll!System.Windows.UIElement.RaiseEvent(System.Windows.RoutedEventArgs e) + 0x29 字节 PresentationFramework.dll!System.Windows.Controls.Primitives.TextBoxBase.OnTextChanged(System.Windows.Controls.TextChangedEventArgs e) + 0x5 字节
PresentationFramework.dll!System.Windows.Controls.Primitives.TextBoxBase.OnTextContainerChanged(object sender, System.Windows.Documents.TextContainerChangedEventArgs e) + 0xe0 字节
PresentationFramework.dll!System.Windows.Controls.TextBox.OnTextContainerChanged(object sender, System.Windows.Documents.TextContainerChangedEventArgs e) + 0x17d 字节 PresentationFramework.dll!System.Windows.Documents.TextContainer.EndChange(bool skipEvents) + 0xb6 字节
PresentationFramework.dll!System.Windows.Documents.TextContainer.System.Windows.Documents.ITextContainer.EndChange(bool skipEvents) + 0xb 字节 PresentationFramework.dll!System.Windows.Documents.TextRangeBase.EndChange(System.Windows.Documents.ITextRange thisRange, bool disableScroll, bool skipEvents) + 0x59 字节 PresentationFramework.dll!System.Windows.Documents.TextRange.System.Windows.Documents.ITextRange.EndChange(bool disableScroll, bool skipEvents) + 0x11 字节
PresentationFramework.dll!System.Windows.Documents.TextRange.ChangeBlock.System.IDisposable.Dispose() + 0x15 字节
PresentationFramework.dll!System.Windows.Documents.TextEditorTyping.OnDelete(对象发送者，System.Windows.Input.ExecutedRoutedEventArgs args) + 0x1a7 字节
PresentationCore.dll!System.Windows.Input.CommandBinding.OnExecuted(object sender, System.Windows.Input.ExecutedRoutedEventArgs e) + 0x65 bytes PresentationCore.dll!System.Windows.Input.CommandManager.ExecuteCommandBinding(object sender, System.Windows.Input.ExecutedRoutedEventArgs e, System.Windows.Input.CommandBinding commandBinding) + 0x92 字节
PresentationCore.dll!System.Windows.Input.CommandManager.FindCommandBinding（System.Windows.Input.CommandBindingCollection commandBindings，对象发送者，System.Windows.RoutedEventArgs e，System.Windows.Input.ICommand 命令，布尔执行）+ 0x105 字节
PresentationCore.dll!System.Windows.Input.CommandManager.FindCommandBinding(object sender, System.Windows.RoutedEventArgs e, System.Windows.Input.ICommand command, bool execute) + 0x15e bytes PresentationCore.dll!System.Windows.Input.CommandManager.OnExecuted(object sender, System.Windows.Input.ExecutedRoutedEventArgs e) + 0x25 字节 PresentationCore.dll!System.Windows.UIElement.OnExecutedThunk(object sender, System.Windows.Input.ExecutedRoutedEventArgs e) + 0x46 字节
PresentationCore.dll!System.Windows.Input.ExecutedRoutedEventArgs.InvokeEventHandler(System.Delegate genericHandler, object target) + 0x3c bytes
PresentationCore.dll!System.Windows.RoutedEventArgs.InvokeHandler（System.Delegate 处理程序，对象目标）+ 0x33 字节
PresentationCore.dll!System.Windows.RoutedEventHandlerInfo.InvokeHandler(对象目标，System.Windows.RoutedEventArgs routedEventArgs) + 0x44 字节
PresentationCore.dll!System.Windows.EventRoute.InvokeHandlersImpl（对象源，System.Windows.RoutedEventArgs 参数，bool reRaised）+ 0x1a8 字节
PresentationCore.dll!System.Windows.UIElement.RaiseEventImpl（System.Windows.DependencyObject 发送方，System.Windows.RoutedEventArgs 参数）+ 0x73 字节
PresentationCore.dll!System.Windows.UIElement.RaiseTrustedEvent(System.Windows.RoutedEventArgs args) + 0x3d 字节
PresentationCore.dll!System.Windows.UIElement.RaiseEvent(System.Windows.RoutedEventArgs args, bool trust) + 0x40 字节
PresentationCore.dll!System.Windows.Input.RoutedCommand.ExecuteImpl（对象参数，System.Windows.IInputElement 目标，bool userInitiated）+ 0x105 字节
PresentationCore.dll!System.Windows.Input.RoutedCommand.ExecuteCore(object parameter, System.Windows.IInputElement target, bool userInitiated) + 0x59 bytes PresentationCore.dll!System.Windows.Input.CommandManager.TranslateInput(System.Windows.IInputElement targetElement, System.Windows.Input.InputEventArgs inputEventArgs) + 0x59b 字节
PresentationCore.dll!System.Windows.UIElement.OnKeyDownThunk(object sender, System.Windows.Input.KeyEventArgs e) + 0x52 字节
PresentationCore.dll!System.Windows.Input.KeyEventArgs.InvokeEventHandler(System.Delegate genericHandler, object genericTarget) + 0x2c 字节
PresentationCore.dll!System.Windows.RoutedEventArgs.InvokeHandler（System.Delegate 处理程序，对象目标）+ 0x33 字节
PresentationCore.dll!System.Windows.RoutedEventHandlerInfo.InvokeHandler(对象目标，System.Windows.RoutedEventArgs routedEventArgs) + 0x44 字节
PresentationCore.dll!System.Windows.EventRoute.InvokeHandlersImpl（对象源，System.Windows.RoutedEventArgs 参数，bool reRaised）+ 0x1a8 字节
PresentationCore.dll!System.Windows.UIElement.RaiseEventImpl（System.Windows.DependencyObject 发送方，System.Windows.RoutedEventArgs 参数）+ 0x73 字节
PresentationCore.dll!System.Windows.UIElement.RaiseTrustedEvent(System.Windows.RoutedEventArgs args) + 0x3d 字节
PresentationCore.dll!System.Windows.UIElement.RaiseEvent(System.Windows.RoutedEventArgs args, bool trust) + 0x40 字节
PresentationCore.dll!System.Windows.Input.InputManager.ProcessStagingArea() + 0x1f8 字节
PresentationCore.dll!System.Windows.Input.InputManager.ProcessInput(System.Windows.Input.InputEventArgs 输入) + 0x45 字节 PresentationCore.dll!System.Windows.Input.InputProviderSite.ReportInput(System.Windows.Input.InputReport inputReport) + 0x62 字节
PresentationCore.dll!System.Windows.Interop.HwndKeyboardInputProvider.ReportInput(System.IntPtr hwnd, System.Windows.Input.InputMode 模式, int 时间戳, System.Windows.Input.RawKeyboardActions 动作, int scanCode, bool isExtendedKey, bool isSystemKey, int virtualKey) + 0xee 字节 PresentationCore.dll!System.Windows.Interop.HwndKeyboardInputProvider.ProcessKeyAction（参考 System.Windows.Interop.MSG 味精，参考布尔处理）+ 0xac 字节
PresentationCore.dll!System.Windows.Interop.HwndSource.CriticalTranslateAccelerator（参考 System.Windows.Interop.MSG msg，System.Windows.Input.ModifierKeys 修饰符）+ 0x94 字节
PresentationCore.dll!System.Windows.Interop.HwndSource.OnPreprocessMessage(object param) + 0x12c bytes
WindowsBase.dll!System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate 回调，对象 args，int numArgs) + 0x56 字节 WindowsBase.dll!MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen（对象源，System.Delegate 方法，对象 args，int numArgs，System.Delegate catchHandler）+ 0x3a 字节
WindowsBase.dll!System.Windows.Threading.Dispatcher.LegacyInvokeImpl（System.Windows.Threading.DispatcherPriority 优先级，System.TimeSpan 超时，System.Delegate 方法，对象参数，int numArgs）+ 0x10e 字节 WindowsBase.dll!System.Windows.Threading.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority 优先级，System.Delegate 方法，对象 arg) + 0x3e 字节
PresentationCore.dll!System.Windows.Interop.HwndSource.OnPreprocessMessageThunk(ref System.Windows.Interop.MSG msg, ref bool 处理) + 0x93 字节
PresentationCore.dll!System.Windows.Interop.HwndSource.WeakEventPreprocessMessage.OnPreprocessMessage（参考 System.Windows.Interop.MSG 味精，参考布尔处理）+ 0x33 字节
WindowsBase.dll!System.Windows.Interop.ComponentDispatcherThread.RaiseThreadMessage(ref System.Windows.Interop.MSG msg) + 0x3c 字节
WindowsBase.dll!System.Windows.Threading.Dispatcher.PushFrameImpl(System.Windows.Threading.DispatcherFrame 帧) + 0x9a 字节
WindowsBase.dll!System.Windows.Threading.Dispatcher.PushFrame(System.Windows.Threading.DispatcherFrame 帧) + 0x49 字节
WindowsBase.dll!System.Windows.Threading.Dispatcher.Run() + 0x4b 字节
PresentationFramework.dll!System.Windows.Application.RunDispatcher(对象忽略) + 0x17 字节
PresentationFramework.dll!System.Windows.Application.RunInternal(System.Windows.Window 窗口) + 0x6f 字节 PresentationFramework.dll!System.Windows.Application.Run(System.Windows.Window 窗口) + 0x26 字节 PresentationFramework.dll!System.Windows.Application.Run() + 0x1b 字节 WpfApplication1.exe!WpfApplication1.App.Main() + 0x59 字节 C# [本机到托管转换]
[托管到本地转换]
mscorlib.dll!System.AppDomain.ExecuteAssembly(string assemblyFile, System.Security.Policy.Evidence assemblySecurity, string[] args) + 0x6b 字节
Microsoft.VisualStudio.HostingProcess.Utilities.dll!Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() + 0x27 字节
mscorlib.dll!System.Threading.ThreadHelper.ThreadStart_Context(对象状态) + 0x6f 字节
mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback 回调, 对象状态, bool preserveSyncCtx) + 0xa7 字节
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback 回调, 对象状态, bool preserveSyncCtx) + 0x16 bytes
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback 回调, 对象状态) + 0x41 字节
mscorlib.dll!System.Threading.ThreadHelper.ThreadStart() + 0x44 字节
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Answer 3

试试这个：

1. 写一个转换器

   public class NullToItemConverter : IValueConverter
   {
       public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
       {
           return value;
       }
   
       public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
       {
           if (value == null)
               return new Item();
           else
               return value;
       }
   }
   

2. 在 XAML 中

   <Window.Resources>
       <local:NullToItemConverter x:Key="nullToItemConverter"/>
   </Window.Resources
   

   ...

   <ComboBox IsEditable="True"
         VerticalAlignment="Top"
         ItemsSource="{Binding Items}"
         DisplayMemberPath="Name"
         SelectedItem="{Binding Item, Mode=TwoWay , Converter={StaticResource nullToItemConverter}}"/>
   

Answer 4

我通过将访问修饰符设为public解决了这个问题，这样它就可以在双向中绑定。