Binding a Viewbox to a Canvas

Question

I'm trying to bind a Viewbox to Canvas that is created dynamically like so:

        <ListBox.ItemTemplate>
            <DataTemplate>
                <DockPanel>
                    <Viewbox>
                        <ContentPresenter Content="{Binding Canvas}"/>
                    </Viewbox>
                </DockPanel>
            </DataTemplate>
        </ListBox.ItemTemplate>

This works fine as long as the Canvas doesn't have any children, but as soon at the Canvas has children it's not shown. What am I missing here?

Answer 1

How do you know it works? A Canvas is just a Panel with zero width/height. Even if it has any children, its dimensions are still going to be 0,0. You must explicitly set Width and Height to a non-zero value in order for it to appear. Paste the following snippet into XamlPad or just test in your own app. Now, remove either Width or Height and it will vanish.

<Viewbox>
   <ContentPresenter>
      <ContentPresenter.Content>
         <Canvas Background="Red" Width="1" Height="1">
            <TextBlock Canvas.Left="10" Canvas.Top="20" Text="123" />
         </Canvas>
      </ContentPresenter.Content>
   </ContentPresenter>
</Viewbox>

Answer 2

Forget I ever asked :-)

I caused an exception when creating the children of canvas, and this in turn caused the canvas to not be shown. I'm sad to say that it's not the first time I have made this mistake, and it's probably not that last time either:

TextBlock tb = new TextBlock();
tb.SetValue(Canvas.LeftProperty, 5);
tb.SetValue(Canvas.TopProperty, 5);

"5" is not a valid value for 'Left' or 'Top'. It should of course be

TextBlock tb = new TextBlock();
tb.SetValue(Canvas.LeftProperty, 5.0);
tb.SetValue(Canvas.TopProperty, 5.0);

And because it was created as part of data binding, no exception dialog was shown. All in all ... DOOOOH :-) :-)