Question

给出一些上下文：我有一个项目列表，我为每个项目绘制一个图像。它们都有一个黑色边框，但是与给定的“Brush”属性绑定到同一个对象的边框。那个有红色边框显示选择了哪个画笔。

这是代码

<ItemsControl ItemsSource="{Binding}">
<ItemsControl.ItemsPanel>
    <ItemsPanelTemplate>
        <StackPanel Orientation="Horizontal"/>
    </ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemTemplate>
    <DataTemplate>
        <Border BorderBrush="Black"
                BorderThickness="2 2 2 2">
            <Border.Style>
                <Style TargetType="{x:Type Border}">
                    <Style.Triggers>
                        <DataTrigger Value="True">
                            <DataTrigger.Binding>
                                <MultiBinding Converter="{StaticResource ResourceKey=AreEqualConverter}">
                                    <Binding/>
                                    <Binding ElementName="Me" Path="Brush"/>
                                </MultiBinding>
                            </DataTrigger.Binding>
                            <Setter Property="BorderBrush" Value="Red"/>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </Border.Style>
            <Image Source="{Binding}" 
                   Height="{Binding ElementName=TilePreviewSize, Path=Value}"
                   Width="{Binding ElementName=TilePreviewSize, Path=Value}"
                   RenderOptions.BitmapScalingMode="NearestNeighbor"
                   MouseDown="OnBrushMouseDown"/>
        </Border>
    </DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>

我得到以下错误“NullReferenceException：对象引用未设置为对象的实例。”当取消注释以下代码块时。无法弄清楚是什么问题。

<Style TargetType="{x:Type Border}">
    <Style.Triggers>
        <DataTrigger Value="True">
            <DataTrigger.Binding>
                <MultiBinding Converter="{StaticResource ResourceKey=AreEqualConverter}">
                    <Binding/>
                    <Binding ElementName="Me" Path="Brush"/>
                </MultiBinding>
            </DataTrigger.Binding>
            <Setter Property="BorderBrush" Value="Red"/>
        </DataTrigger>
    </Style.Triggers>
</Style>

如果你想查看转换器的代码，这里（非常简单）

public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    if (values.Length < 2)
        throw new NotSupportedException();

    var obj = values[0];

    for (int i = 1; i < values.Length; ++i)
        if (!values[i].Equals(obj))
            return false;

    return true;
}

这是堆栈跟踪或错误：

   at IntegratorUI.Contexts.RoomContext.System.Windows.Markup.IStyleConnector.Connect(Int32 connectionId, Object target) in [...] line 138
   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.StackPanel.MeasureOverride(Size constraint)
   at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
   at System.Windows.UIElement.Measure(Size availableSize)
   at System.Windows.ContextLayoutManager.UpdateLayout()
   at System.Windows.ContextLayoutManager.UpdateLayoutCallback(Object arg)
   at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
   at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)
   at System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget)
   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 IntegratorUI.App.Main() in D:\trollmeme\integrator\IntegratorUI\obj\Debug\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()

感谢您帮助我解决这个问题！

尼克

Answer 1

数组中的某些值可以是null。使用静态Equals方法。检查obj是否为空。

也可能是谨慎的

if (!Object.Equals(values[i], obj))

<强>更新

看到您的堆栈跟踪后，您的问题似乎与this问题有关。当您使用嵌套模板/样式时，似乎。这会影响.NET 4，并且似乎在.NET 4.5中得到修复。建议的解决方法是将样式作为资源引用。

<UserControl.Resources>
   <Style x:Key="MyBorder" TargetType="{x:Type Border}" ...
</UserControl.Resources>

<DataTemplate>
    <Border BorderBrush="Black"
            BorderThickness="2 2 2 2"
            Style="{StaticResource MyBorder}" ...
</DataTemplate>

请注意，如果整个ItemsControl嵌套在另一个模板中，您应该将所有这些模板移动到资源。

Answer 2

您的问题是您已在MouseDown="OnBrushMouseDown";内为DataTemplate定义了处理程序。因此无法正确检索MouseButtonEventHandler。

要解决此问题，请在样式的资源字典中添加新样式：

<Style x:Key="MouseDownHandlerStyle" TargetType="{x:Type Image}">
    <EventSetter Event="MouseDown" Handler="OnBrushMouseDown"/>
</Style>

干杯！

WPF DataTemplate多重绑定导致FrameworkElement.ApplyTemplate（）抛出NullReferenceException

2 个答案: