Null Reference Exception in BorderlessWindowBehavior.TopMostChangeNotifierOnValueChanged

[mikeasage]

What steps will reproduce this issue?

I got a Null Reference exception when trying to open my Metro app that contains several metro windows. I really don't have a particular way to re-create this as I typically don't have this error. I'm guessing this is dependent on the execution order of the underlying threads somehow. The fix looks simple I guess, but I'm not sure how to create a test for it (assuming that's required). Here's my stack trace.

---

System.NullReferenceException: Object reference not set to an instance of an object.
at MahApps.Metro.Behaviours.BorderlessWindowBehavior.TopMostChangeNotifierOnValueChanged(Object sender, EventArgs e)
at MahApps.Metro.Controls.PropertyChangeNotifier.OnPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
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.InvalidateProperty(DependencyProperty dp, Boolean preserveCurrentValue)
at System.Windows.Data.BindingExpressionBase.Invalidate(Boolean isASubPropertyChange)
at System.Windows.Data.BindingExpression.TransferValue(Object newValue, Boolean isASubPropertyChange)
at MS.Internal.Data.ClrBindingWorker.NewValueAvailable(Boolean dependencySourcesChanged, Boolean initialValue, Boolean isASubPropertyChange)
at MS.Internal.Data.PropertyPathWorker.UpdateSourceValueState(Int32 k, ICollectionView collectionView, Object newValue, Boolean isASubPropertyChange)
at MS.Internal.Data.PropertyPathWorker.OnDependencyPropertyChanged(DependencyObject d, DependencyProperty dp, Boolean isASubPropertyChange)
at System.Windows.Data.BindingExpression.HandlePropertyInvalidation(DependencyObject d, DependencyPropertyChangedEventArgs args)
at System.Windows.Data.BindingExpressionBase.OnPropertyInvalidation(DependencyObject d, DependencyPropertyChangedEventArgs args)
at System.Windows.Data.BindingExpression.OnPropertyInvalidation(DependencyObject d, DependencyPropertyChangedEventArgs args)
at System.Windows.DependentList.InvalidateDependents(DependencyObject source, DependencyPropertyChangedEventArgs sourceArgs)
at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
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.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType, Boolean isInternal)
at System.Windows.Window.set_Topmost(Boolean value)
at System.Windows.Window.OnContentRendered(EventArgs e)
at System.Windows.Window.b__193_0(Object unused)
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)

---

Expected outcome

Um, it doesn't crash?

Suggested Code Change

Here's the existing code. The only thing that could be null is the AssociatedObject.

    private void TopMostChangeNotifierOnValueChanged(object sender, EventArgs e)
    {
        this.savedTopMost = this.AssociatedObject.Topmost;
    }

So, I think the obvious change should be this, but beyond that, I'm not sure what should happen.

    private void TopMostChangeNotifierOnValueChanged(object sender, EventArgs e)
    {
        if (this.AssociatedObject != null)
            this.savedTopMost = this.AssociatedObject.Topmost;
    }

Environment

• MahApps.Metro v1.3.0
• Windows 7.1
• Visual Studio 2015
• .NET Framework 4.5

[punker76]

@mikeasage That's in my eyes little bt strange and I want to figure out why this can happen. So I see in your stack trace (thx for this) that the TopMost property is changed. Maybe one of your windows do this? Thx.

[mikeasage]

Yes, TopMost is being called in these event handlers for the Metro Window. Perhaps this not the best practice for ensuring that the window is on top?

    ContentRendered="MyWindow_ContentRendered"

    Initialized="MyWindow_Initialized



    private void MyWindow_ContentRendered(object sender, EventArgs e)

    {

        this.Topmost = false;

    }



    private void MyWindow_Initialized(object sender, EventArgs e)

    {

        this.Topmost = true;

    }

From: Jan Karger [mailto:notifications@github.com]
Sent: Thursday, September 29, 2016 2:20 AM
To: MahApps/MahApps.Metro MahApps.Metro@noreply.github.com
Cc: Michael A Sage mikeasage@sbcglobal.net; Mention mention@noreply.github.com
Subject: Re: [MahApps/MahApps.Metro] Null Reference Exception in BorderlessWindowBehavior.TopMostChangeNotifierOnValueChanged (#2674)

@mikeasage https://github.com/mikeasage That's in my eyes little bi strange and I want to figure out why this can happen. So I see in your stack trace (thx for this) that the TopMost property is changed. Maybe one of your windows do this? Thx.

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[punker76]

@mikeasage I merged your changes with some additional changes from me. Thx. But a minor note, you don't need this ContentRendered/Initialized stuff to set the Topmost. Set it directly in Xaml Topmost="True" or in code behind yourWindow.Topmost = true;.