WPF DataTemplate multibinding causes FrameworkElement.ApplyTemplate() to throw NullReferenceException

Question

To give some context : I have a list of items, and i draw an image for each of them. They all have a black border but the one that is bound to the same object than a given "Brush" property. That one has a red border to show which brush is selected.

here is the code

<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>

I get the follow error "NullReferenceException : Object reference not set to an instance of an object." when the following block of code is uncommented. Can't figure out what is the problem.

<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>

If you want to see the converter's code, here it is (quite simple)

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;
}

Here is the stacktrace or the error:

   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()

Thanks for helping me sort this out!

Nic

Answer 1

Your problem is that you've defined your handler for MouseDown="OnBrushMouseDown"; inside of a DataTemplate. It therefore cannot retrieve the MouseButtonEventHandler properly.

To fix the issue, add a new Style in your resource dictionary of the style :

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

Cheers!

Answer 2

Some of the values in the array can be null. Use the static Equals method. It might also be prudent to check that obj is not null.

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

Update

After seeing your stack trace, it seems like your problem is related to this issue. It seems that when you use nested templates/styles. This affects .NET 4, and seems to be fixed in .NET 4.5. The suggested workaround is to reference the style as a resource.

<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>

Note that if your entire ItemsControl is a nested inside another template, you should probably move all those templates to resources.