GetAdornerLayer mysteriously returning null

c# wpf nullreferenceexception adorner adornerlayer
devios1 picture devios1 · Jun 14, 2010 · Viewed 9.3k times · Source

I've been using the same bit of code for several versions of my app with no problems, but I'm now mysteriously receiving NullRerefenceExceptions with the following:

this.Loaded += delegate {
    deleteBrush = new DeleteBrushAdorner( background );
    AdornerLayer al = AdornerLayer.GetAdornerLayer( background );
    al.Add( deleteBrush ); // null ref here??
};

background is just a Border element.

My two thoughts on what could be causing it are a) switching to .NET 4.0, and b) placing instances of the above element (which is a UserControl) in an ItemsControl.

Oddly this doesn't happen all the time, and it's hard to predict when it will happen, so it's not reliable.

Answer

Florian Lim picture Florian Lim · Aug 13, 2012

In my case I had a class that is based on Window and GetAdornerLayer() returned null. It turned out that the ControlTemplate for my derived class did not contain the AdornerDecorator. Adding that as the top level in the ControlTemplate solved the issue.

<Style TargetType="my:MyWindow" BasedOn="{StaticResource {x:Type Window}}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="my:MyWindow">
                <AdornerDecorator>
                    <DockPanel ...>
                    </DockPanel>
                </AdornerDecorator>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

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