c# - 添加按钮单击 ="...."导致启动时出现 "Object reference not set to an instance of an object"

Question

这是我用来生成窗口的代码:

<Grid x:Name="LayoutRoot">
    <TreeView DataContext="{Binding ElementName=UserControl, Mode=OneWay}"
        ....
        ItemsSource="{Binding UrlList}" ItemContainerStyle="{DynamicResource TreeViewItemStyle}" >
        <TreeView.Resources>
            <HierarchicalDataTemplate DataType="{x:Type local:UrlList}">
                <Grid Margin="-15,0,0,0">                       
                    <Expander>
                        <Expander.Header>
                            ....
                        </Expander.Header>
                        <StackPanel>
                            ....
                            <DataGrid ItemsSource="{Binding List}" Margin="0,10,0,0" AutoGenerateColumns="False">
                                <DataGrid.Columns>
                                    ....
                                    <DataGridTemplateColumn>
                                        <DataGridTemplateColumn.CellTemplate>
                                            <DataTemplate>
                                                <Button Content="..."  Click="EditUri_Click"/>
                                            </DataTemplate>                                             
                                        </DataGridTemplateColumn.CellTemplate>                                          
                                    </DataGridTemplateColumn>
                                </DataGrid.Columns>
                            </DataGrid>
                        </StackPanel>       
                    </Expander>
                </Grid>
            </HierarchicalDataTemplate>
        </TreeView.Resources>
    </TreeView>
</Grid>

当我使用 <Button Content="..." /> 时，应用程序正常启动，当我使用 <Button Content="..." Click="EditUri_Click"/>我得到以下异常:

    System.NullReferenceException was unhandled
  Message=Object reference not set to an instance of an object.
  Source=MyApp
  StackTrace:
       at UrlListInfoPanel.System.Windows.Markup.IStyleConnector.Connect(Int32 connectionId, Object target) in ...\UrlListInfoPanel.xaml:line 152[this is where the button is]
       at System.Windows.FrameworkTemplate.LoadTemplateXaml(XamlReader templateReader, XamlObjectWriter currentWriter)
       at System.Windows.FrameworkTemplate.LoadTemplateXaml(XamlObjectWriter objectWriter)
       at System.Windows.FrameworkTemplate.LoadOptimizedTemplateContent(DependencyObject container, IComponentConnector componentConnector, IStyleConnector styleConnector, List`1 affectedChildren, UncommonField`1 templatedNonFeChildrenField)
       at System.Windows.FrameworkTemplate.LoadContent(DependencyObject container, List`1 affectedChildren)
       at System.Windows.StyleHelper.ApplyTemplateContent(UncommonField`1 dataField, DependencyObject container, FrameworkElementFactory templateRoot, Int32 lastChildIndex, HybridDictionary childIndexFromChildID, FrameworkTemplate frameworkTemplate)
       at System.Windows.FrameworkTemplate.ApplyTemplateContent(UncommonField`1 templateDataField, FrameworkElement container)
       at System.Windows.FrameworkElement.ApplyTemplate()
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Border.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Grid.MeasureCell(Int32 cell, Boolean forceInfinityV)
       at System.Windows.Controls.Grid.MeasureCellsGroup(Int32 cellsHead, Size referenceSize, Boolean ignoreDesiredSizeU, Boolean forceInfinityV)
       at System.Windows.Controls.Grid.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.TreeViewItem.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.StackPanel.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at MS.Internal.Helper.MeasureElementWithSingleChild(UIElement element, Size constraint)
       at System.Windows.Controls.ItemsPresenter.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at MS.Internal.Helper.MeasureElementWithSingleChild(UIElement element, Size constraint)
       at System.Windows.Controls.ScrollContentPresenter.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Grid.MeasureCell(Int32 cell, Boolean forceInfinityV)
       at System.Windows.Controls.Grid.MeasureCellsGroup(Int32 cellsHead, Size referenceSize, Boolean ignoreDesiredSizeU, Boolean forceInfinityV)
       at System.Windows.Controls.Grid.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.ScrollViewer.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Border.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Control.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Grid.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at MS.Internal.Helper.MeasureElementWithSingleChild(UIElement element, Size constraint)
       at System.Windows.Controls.ContentPresenter.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Border.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Control.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Grid.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Grid.MeasureCell(Int32 cell, Boolean forceInfinityV)
       at System.Windows.Controls.Grid.MeasureCellsGroup(Int32 cellsHead, Size referenceSize, Boolean ignoreDesiredSizeU, Boolean forceInfinityV)
       at System.Windows.Controls.Grid.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at MS.Internal.Helper.MeasureElementWithSingleChild(UIElement element, Size constraint)
       at System.Windows.Controls.ContentPresenter.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Documents.AdornerDecorator.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Grid.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Border.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Controls.Grid.MeasureOverride(Size constraint)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Window.MeasureOverrideHelper(Size constraint)
       at System.Windows.Window.MeasureOverride(Size availableSize)
       at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       at System.Windows.UIElement.Measure(Size availableSize)
       at System.Windows.Interop.HwndSource.SetLayoutSize()
       at System.Windows.Interop.HwndSource.set_RootVisualInternal(Visual value)
       at System.Windows.Window.SetRootVisualAndUpdateSTC()
       at System.Windows.Window.SetupInitialState(Double requestedTop, Double requestedLeft, Double requestedWidth, Double requestedHeight)
       at System.Windows.Window.CreateSourceWindow(Boolean duringShow)
       at System.Windows.Window.ShowHelper(Object booleanBox)
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
       at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
       at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
       at System.Windows.Threading.DispatcherOperation.InvokeImpl()
       at System.Threading.ExecutionContext.runTryCode(Object userData)
       at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext 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 MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
       at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
       at System.Windows.Threading.Dispatcher.InvokeImpl(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.Application.RunInternal(Window window)
       at System.Windows.Application.Run()
       at BookmarkWizV2.App.Main() in App.g.cs:line 0
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

这是从哪里来的，我该如何解决？

Answer 1

我以前遇到过这个问题，WPF 似乎不喜欢向数据模板内的控件添加事件。我通过用完成相同事情的命令替换事件来解决它。

编辑:

也许尝试这样的事情......

<DataGrid ItemsSource="{Binding List}" Margin="0,10,0,0" AutoGenerateColumns="False">
    <DataGrid.CommandBindings>
        <CommandBinding Command="commands:Commands.EditURI" CanExecute="CommandBinding_CanExecute" Executed="CommandBinding_Executed"/>
    </DataGrid.CommandBindings>
    <DataGrid.Columns>
        <DataGridTemplateColumn>
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <Button Content="..." Command="controls:Commands.EditURI"/>
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>
    </DataGrid.Columns>
</DataGrid>

你可以在某处设置一类命令(在uri中引用它，我使用命令:)

public class Commands
{
    public static RoutedCommand EditURI = new RoutedCommand("EditURI", typeof(Commands));
}

然后在您的数据上下文中定义两个被调用的方法...

    private void CommandBinding_CanExecute(object sender, CanExecuteRoutedEventArgs e)
    {
        e.CanExecute = true;
    }

    private void CommandBinding_Executed(object sender, ExecutedRoutedEventArgs e)
    {
         //Your code to execute
    }

您可能需要填补一些漏洞，但这应该是一个关于如何设置它的合适模板。可能有更简单/更简洁的方法，但这是我之前遇到此问题时构建代码的方式。