为什么我在ComboBox中获得异常（'System.NullReferenceException'）

时间：2012-11-19 14:11:07

标签： wpf data-binding combobox

如果我将一个eventhandler添加到combobox，SelectionChanged为examle，我得到一个异常。任何人都可以告诉我原因。

奇怪的行为，因为在我的笔记本电脑上安装了带有VS2010的win7，它运行成功。

例外：

A first chance exception of type 'System.NullReferenceException' occurred in    wpfapplication.exe
Additional information: Object reference not set to an instance of an object.

异常callstack：

  

PresentationFramework.dll！System.Windows.FrameworkTemplate.LoadTemplateXaml（System.Xaml.XamlReader templateReader，System.Xaml.XamlObjectWriter currentWriter）+ 0x247 bytes
      PresentationFramework.dll！System.Windows.FrameworkTemplate.LoadTemplateXaml（System.Xaml.XamlObjectWriter objectWriter）+ 0x66 bytes       PresentationFramework.dll！System.Windows.FrameworkTemplate.LoadOptimizedTemplateContent（System.Windows.DependencyObject container，System.Windows.Markup.IComponentConnector componentConnector，System.Windows.Markup.IStyleConnector styleConnector，System.Collections.Generic.List affectedChildren，System.Windows。 UncommonField templatedNonFeChildrenField）+ 0x26a字节       PresentationFramework.dll！System.Windows.FrameworkTemplate.LoadContent（System.Windows.DependencyObject container，System.Collections.Generic.List affectedChildren）+ 0x76 bytes
      PresentationFramework.dll！System.Windows.StyleHelper.ApplyTemplateContent（System.Windows.UncommonField dataField，System.Windows.DependencyObject container，System.Windows.FrameworkElementFactory templateRoot，int lastChildIndex，System.Collections.Specialized.HybridDictionary childIndexFromChildID，System.Windows.FrameworkTemplate frameworkTemplate）+ 0xc4字节
      PresentationFramework.dll！System.Windows.FrameworkTemplate.ApplyTemplateContent（System.Windows.UncommonField templateDataField，System.Windows.FrameworkElement container）+ 0x3b bytes
      PresentationFramework.dll！System.Windows.FrameworkElement.ApplyTemplate（）+ 0x7b bytes
      PresentationFramework.dll！System.Windows.FrameworkElement.MeasureCore（System.Windows.Size availableSize）+ 0x40 bytes
      PresentationCore.dll！System.Windows.UIElement.Measure（System.Windows.Size availableSize）+ 0x207 bytes

的Xaml：

<DataGrid Margin="80,0,0,0"  GridLinesVisibility="None"  
                                              RowDetailsVisibilityMode="Collapsed" 
                                              ColumnHeaderStyle="{StaticResource ColumnHeaderStyle}"  
                                              RowStyle="{StaticResource DataGridRowStyleAlternation}" 
                                              AutoGenerateColumns="False" SelectionMode="Single" 
                                              AlternationCount="2" Background="Transparent" HeadersVisibility="Column" 
                                              x:Name="detailsDatagrid" 
                                              Initialized="scenesDetailsDataGrid_Initialized"
                                              ItemsSource="{Binding Source={StaticResource scenesRequisitesViewSource}}" 
                                              ColumnDisplayIndexChanged="detailsDatagrid_ColumnDisplayIndexChanged" >
                                        <DataGrid.Columns>
                                            <DataGridTemplateColumn Header="Fahrzeug" Width="SizeToCells">

                                                <DataGridTemplateColumn.CellTemplate>
                                                    <DataTemplate>
                                                        <ComboBox DisplayMemberPath="Name"
                                                                   ItemsSource="{Binding Path=ProjectsRequisites, Source={StaticResource projectsViewSource}, UpdateSourceTrigger=PropertyChanged}" 
                                                                   SelectedValue="{Binding Path=RequisiteId,Mode=TwoWay}"
                                                                   SelectedValuePath="ID"     
                                                                  SelectionChanged="ComboboxRequisite_SelectionChanged"
                                                                   Tag="{Binding}" />

                                                    </DataTemplate>
                                                </DataGridTemplateColumn.CellTemplate>
                                            </DataGridTemplateColumn>
                                            <DataGridTextColumn IsReadOnly="True"  Binding="{Binding Path=Costs,StringFormat={}{0:C}, ConverterCulture={x:Static gl:CultureInfo.CurrentCulture}}" Header="Kosten" Width="SizeToCells" />
                                            <DataGridTextColumn Binding="{Binding Path=Count}" Header="Anzahl" Width="SizeToHeader" />
                                        </DataGrid.Columns>
                                    </DataGrid>

3 个答案:

答案 0 :(得分：1)

当我剥离xaml中依赖于你的背景代码的东西时，我无法重现这个。这个，以及你的错误的性质告诉我，你的xaml绑定的对象之一有问题。它们都是新的吗？

ColumnDisplayIndexChanged="detailsDatagrid_ColumnDisplayIndexChanged" >
ItemsSource="{Binding Source={StaticResource scenesRequisitesViewSource}}" 
ColumnHeaderStyle="{StaticResource ColumnHeaderStyle}"  
RowStyle="{StaticResource DataGridRowStyleAlternation}"
Initialized="scenesDetailsDataGrid_Initialized"
Source={StaticResource projectsViewSource},
Binding="{Binding Path=Costs,StringFormat={}{0:C}, ConverterCulture={x:Static gl:CultureInfo.CurrentCulture}}"

这是我为了让它运行而取出的，大部分原因只是因为我缺少你的代码。其中一个必定是有问题的。

你的处理程序更有可能是错误的，它里面有什么？

答案 1 :(得分：0)

我认为问题是你的ComboBox上的Tag属性 - ＆gt; “变量=”{结合} “”。尝试删除它，问题应该消失。

我在其他控件中有这种奇怪的行为：Grid，TextBlock。所有安装了Visual Studio 2010的计算机上都可以使用，但在其他计算机上却没有。当我删除Tag属性时，我的程序开始在其他机器上工作。

我的事件查看器错误是：

  Application: PragmaGRP.exe
    Framework Version: v4.0.30319
    Description: The process was terminated due to an unhandled exception.
    Exception Info: System.NullReferenceException
    Stack:
       at System.Windows.FrameworkTemplate.LoadTemplateXaml(System.Xaml.XamlReader, System.Xaml.XamlObjectWriter)
       at System.Windows.FrameworkTemplate.LoadTemplateXaml(System.Xaml.XamlObjectWriter)
       at System.Windows.FrameworkTemplate.LoadOptimizedTemplateContent(System.Windows.DependencyObject, System.Windows.Markup.IComponentConnector, System.Windows.Markup.IStyleConnector, System.Collections.Generic.List`1<System.Windows.DependencyObject>, System.Windows.UncommonField`1<System.Collections.Hashtable>)
       at System.Windows.FrameworkTemplate.LoadContent(System.Windows.DependencyObject, System.Collections.Generic.List`1<System.Windows.DependencyObject>)
       at System.Windows.StyleHelper.ApplyTemplateContent(System.Windows.UncommonField`1<System.Collections.Specialized.HybridDictionary[]>, System.Windows.DependencyObject, System.Windows.FrameworkElementFactory, Int32, System.Collections.Specialized.HybridDictionary, System.Windows.FrameworkTemplate)
       at System.Windows.FrameworkTemplate.ApplyTemplateContent(System.Windows.UncommonField`1<System.Collections.Specialized.HybridDictionary[]>, System.Windows.FrameworkElement)
       at System.Windows.FrameworkElement.ApplyTemplate()
       at System.Windows.FrameworkElement.MeasureCore(System.Windows.Size)
       at System.Windows.UIElement.Measure(System.Windows.Size)
       at System.Windows.Controls.Grid.MeasureCell(Int32, Boolean)
       at System.Windows.Controls.Grid.MeasureCellsGroup(Int32, System.Windows.Size, Boolean, Boolean)
       at System.Windows.Controls.Grid.MeasureOverride(System.Windows.Size)
       at System.Windows.FrameworkElement.MeasureCore(System.Windows.Size)
       at System.Windows.UIElement.Measure(System.Windows.Size)
       at System.Windows.Controls.Control.MeasureOverride(System.Windows.Size)
       at System.Windows.FrameworkElement.MeasureCore(System.Windows.Size)
       at System.Windows.UIElement.Measure(System.Windows.Size)
       at System.Windows.Controls.Grid.MeasureCell(Int32, Boolean)
       at System.Windows.Controls.Grid.MeasureCellsGroup(Int32, System.Windows.Size, Boolean, Boolean)
       at System.Windows.Controls.Grid.MeasureOverride(System.Windows.Size)
       at System.Windows.FrameworkElement.MeasureCore(System.Windows.Size)
       at System.Windows.UIElement.Measure(System.Windows.Size)
       at MS.Internal.Helper.MeasureElementWithSingleChild(System.Windows.UIElement, System.Windows.Size)
       at System.Windows.Controls.ContentPresenter.MeasureOverride(System.Windows.Size)
       at System.Windows.FrameworkElement.MeasureCore(System.Windows.Size)
       at System.Windows.UIElement.Measure(System.Windows.Size)
       at System.Windows.Controls.Border.MeasureOverride(System.Windows.Size)
       at System.Windows.FrameworkElement.MeasureCore(System.Windows.Size)
       at System.Windows.UIElement.Measure(System.Windows.Size)
       at System.Windows.Controls.Control.MeasureOverride(System.Windows.Size)
       at System.Windows.FrameworkElement.MeasureCore(System.Windows.Size)
       at System.Windows.UIElement.Measure(System.Windows.Size)
       at System.Windows.Controls.Grid.MeasureOverride(System.Windows.Size)
       at System.Windows.FrameworkElement.MeasureCore(System.Windows.Size)
       at System.Windows.UIElement.Measure(System.Windows.Size)
       at System.Windows.Controls.Grid.MeasureOverride(System.Windows.Size)
       at System.Windows.FrameworkElement.MeasureCore(System.Windows.Size)
       at System.Windows.UIElement.Measure(System.Windows.Size)
       at System.Windows.ContextLayoutManager.UpdateLayout()
       at System.Windows.ContextLayoutManager.UpdateLayoutCallback(System.Object)
       at System.Windows.Media.MediaContext+InvokeOnRenderCallback.DoWork()
       at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
       at System.Windows.Media.MediaContext.RenderMessageHandlerCore(System.Object)
       at System.Windows.Media.MediaContext.RenderMessageHandler(System.Object)
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate, System.Object, Int32)
       at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(System.Object, System.Delegate, System.Object, Int32, System.Delegate)
       at System.Windows.Threading.DispatcherOperation.InvokeImpl()
       at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(System.Object)
       at System.Threading.ExecutionContext.runTryCode(System.Object)
       at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode, CleanupCode, System.Object)
       at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
       at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
       at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
       at System.Windows.Threading.DispatcherOperation.Invoke()
       at System.Windows.Threading.Dispatcher.ProcessQueue()
       at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr, Int32, IntPtr, IntPtr, Boolean ByRef)
       at MS.Win32.HwndWrapper.WndProc(IntPtr, Int32, IntPtr, IntPtr, Boolean ByRef)
       at MS.Win32.HwndSubclass.DispatcherCallbackOperation(System.Object)
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate, System.Object, Int32)
       at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(System.Object, System.Delegate, System.Object, Int32, System.Delegate)
       at System.Windows.Threading.Dispatcher.InvokeImpl(System.Windows.Threading.DispatcherPriority, System.TimeSpan, System.Delegate, System.Object, Int32)
       at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr, Int32, IntPtr, IntPtr)
       at MS.Win32.UnsafeNativeMethods.DispatchMessage(System.Windows.Interop.MSG ByRef)
       at System.Windows.Threading.Dispatcher.PushFrameImpl(System.Windows.Threading.DispatcherFrame)
       at System.Windows.Threading.Dispatcher.PushFrame(System.Windows.Threading.DispatcherFrame)
       at System.Windows.Application.RunDispatcher(System.Object)
       at System.Windows.Application.RunInternal(System.Windows.Window)
       at System.Windows.Application.Run(System.Windows.Window)
       at PragmaGRP.App.Main()

答案 2 :(得分：0)

我也遇到了这个错误。就我而言，我在WinForms应用程序中托管了一些WPF用户控件，错误发生在某些机器上，而不是其他机器上。

此外，当我使用AppDomain.CurrentDomain.UnhandledException捕获异常时，我得到的堆栈跟踪甚至比LoadTemplateXaml更深：

Object reference not set to an instance of an object.
Stack:
   at System.Windows.DeferredAppResourceReference.GetValue(BaseValueSourceInternal valueSource)
   at System.Windows.StaticResourceExtension.TryProvideValueInternal(IServiceProvider serviceProvider, Boolean allowDeferredReference, Boolean mustReturnDeferredResourceReference)
   at System.Windows.StaticResourceExtension.ProvideValueInternal(IServiceProvider serviceProvider, Boolean allowDeferredReference)
   at System.Windows.StaticResourceExtension.ProvideValue(IServiceProvider serviceProvider)
   at MS.Internal.Xaml.Runtime.ClrObjectRuntime.CallProvideValue(MarkupExtension me, IServiceProvider serviceProvider)
   at MS.Internal.Xaml.Runtime.PartialTrustTolerantRuntime.CallProvideValue(MarkupExtension me, IServiceProvider serviceProvider)
   at System.Xaml.XamlObjectWriter.Logic_ProvideValue(ObjectWriterContext ctx)
   at System.Xaml.XamlObjectWriter.Logic_AssignProvidedValue(ObjectWriterContext ctx)
   at System.Xaml.XamlObjectWriter.WriteEndMember()
   at System.Xaml.XamlWriter.WriteNode(XamlReader reader)
   at System.Windows.FrameworkTemplate.LoadTemplateXaml(XamlReader templateReader, XamlObjectWriter currentWriter)
   at System.Windows.FrameworkTemplate.LoadTemplateXaml(XamlObjectWriter objectWriter)
   ...

原来问题是，因为我在WinForms应用程序中托管，Application.Current为空。我通过在创建UserControl之前实例化WPF应用程序来解决问题：

if (Application.Current == null)
    new Application { ShutdownMode = ShutdownMode.OnExplicitShutdown };
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