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Bypassing the BulletDecorator

October 19, 2007
New York, N.Y.

Got an email this morning from someone trying to make the WPF CheckBox control look a little different. When you set an image or drawing as the content of a CheckBox, the checkmark box is vertically centered, like in the image on the left:

He wanted the checkmark box positioned on the upper left, as in the second image.

The default template for the CheckBox is basically a BulletDecorator; its Bullet property is set to a BulletChrome object (that's the checkmark box) and its Child property is set to a ContentPresenter (that's for the content of the control). After fooling around setting a few alignment properties, I couldn't get that checkmark box to budge. I suspect the vertical positioning is hard-coded in the ArrangeOverride method in BulletDecorator.

So I re-thought the template's visual tree: Rather than a BulletDecorator I began with a Border. I set the Child to a Grid with two auto-width columns. The first column contains a BulletChrome object with its VerticalAlignment property set to Top. The second column is the ContentPresenter. Everything else in the template is identical to the default CheckBox template (which I snagged using the DumpControlTemplate program from Chapter 25 of my book Applications = Code + Markup). This new template doesn't use a BulletDecorator element at all.

Here's the XAML file that produced the screen shot shown above:

UpperLeftCheckBoxTemplate.xaml

The new template is in the Resources section. I cleaned up some of the syntax of the default template and just commented out the old visual tree.


Comments:

I am "someone". I am flattered to see a blog entry about my question. Thank you for your reply.

Actually I did try to 'Grid' it, but I forgot why I gave up keeping going this way.

I have one more question: why the outer Border is needed?

— Vincent Cheng, Fri, 19 Oct 2007 13:15:15 -0400 (EDT)

The original template set the Background of the BulletDecorator to white, presumably because the bullet needs to show up against the background, so I figured I should add something to replace it. I see now I could have left out the Border and just set the Background of the Grid. — Charles

Is there's any way to email subscribe to your wpf blog ?

Guy, Sat, 3 Nov 2007 19:42:49 -0500 (EST)

No, but I have an RSS feed. — Charles


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