crash in ComboBox coerce (not my code)

Question

I got the stack trace below reported from a customer. I don't know how to reproduce this. My WPF application has a fair number of ComboBoxes; I'm not sure how to determine which ComboBox failed given the stack trace below. Has anyone else seen this? Can you interpret what was going on from this stack trace? Any ideas?

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Windows.Controls.ComboBox.CoerceIsSelectionBoxHighlighted(Object o, Object value)
   at System.Windows.DependencyObject.ProcessCoerceValue(DependencyProperty dp, PropertyMetadata metadata, EntryIndex& entryIndex, Int32& targetIndex, EffectiveValueEntry& newEntry, EffectiveValueEntry& oldEntry, Object& oldValue, Object baseValue, Object controlValue, CoerceValueCallback coerceValueCallback, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, Boolean skipBaseValueChecks)
   at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
   at System.Windows.DependencyObject.CoerceValue(DependencyProperty dp)
   at System.Windows.Controls.ComboBox.OnIsKeyboardFocusWithinChanged(DependencyPropertyChangedEventArgs e)
   at System.Windows.FocusWithinProperty.FireNotifications(UIElement uie, ContentElement ce, UIElement3D uie3D, Boolean oldValue)
   at System.Windows.ReverseInheritProperty.FirePropertyChangeInAncestry(DependencyObject element, Boolean oldValue, DeferredElementTreeState treeState, Action`2 originChangedAction)
   at System.Windows.ReverseInheritProperty.OnOriginValueChanged(DependencyObject oldOrigin, DependencyObject newOrigin, IList`1 otherOrigins, DeferredElementTreeState& oldTreeState, Action`2 originChangedAction)
   at System.Windows.Input.KeyboardDevice.ChangeFocus(DependencyObject focus, Int32 timestamp)
   at System.Windows.Input.KeyboardDevice.PostProcessInput(Object sender, ProcessInputEventArgs e)
   at System.Windows.Input.InputManager.RaiseProcessInputEventHandlers(ProcessInputEventHandler postProcessInput, ProcessInputEventArgs processInputEventArgs)
   at System.Windows.Input.InputManager.ProcessStagingArea()
   at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
   at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
   at System.Windows.Interop.HwndKeyboardInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawKeyboardActions actions, Int32 scanCode, Boolean isExtendedKey, Boolean isSystemKey, Int32 virtualKey)
   at System.Windows.Interop.HwndKeyboardInputProvider.PossiblyDeactivate(IntPtr hwndFocus)
   at System.Windows.Interop.HwndKeyboardInputProvider.FilterMessage(IntPtr hwnd, WindowMessage message, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at System.Windows.Interop.HwndSource.InputFilterMessage(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 System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

Answer 1

Here's the code (.NET 4.5.2). Either o isn't a ComboBox or HighlightedElement is null.

Personally, my first step would be to distribute the PDBs so you could get line numbers in the stack trace.

private static object CoerceIsSelectionBoxHighlighted(object o, object value)
{
    ComboBox comboBox = (ComboBox)o;
    return (!comboBox.IsDropDownOpen && comboBox.IsKeyboardFocusWithin) ||
           (comboBox.HighlightedInfo != null && comboBox.HighlightedElement.Content == comboBox._clonedElement);
}

private ComboBoxItem HighlightedElement
{
    get { return (_highlightedInfo == null) ? null : _highlightedInfo.Container as ComboBoxItem; }
}

Answer 2

What eventually resolved this for us was to override the event causing the issue:

Protected Overrides Sub OnIsKeyboardFocusWithinChanged(e As DependencyPropertyChangedEventArgs)
    Try
        'GW 2015-09-20 Added this override to prevent windows 10 crashes on comboboxes within forms within datagrids

    Catch ex As Exception
    End Try
End Sub

Answer 3

We had the similar problem on some(didn't have time to get to precise range) versions of runtime and windows.

One of our comboboxes had following style

<ComboBox.Style>
    <Style TargetType="{x:Type ComboBox}" BasedOn="{StaticResource {x:Type ComboBox}}">
        <Setter Property="IsDropDownOpen" Value="False"/>
            <Style.Triggers>
                <Trigger Property="IsKeyboardFocusWithin" Value="True">
                    <Setter Property="IsDropDownOpen" Value="True" />
                </Trigger>
            </Style.Triggers>
    </Style>

It somehow, sometimes messed with selection highlighting and caused exception. Maybe this is helpful.