Animate height of groupbox from 0 to auto

I have groupboxes acting like expanders in my application. When I need to colapse a groupbox I set its height equal to 0. when I need to expand it I set it's height equal to auto (double.Nan) is it posible to do this with a storyboard. How could I know the auto height in advance. Expression blend does not enable me to animate to an auto.

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Solution

You can use ScaleTransform for this

<GroupBox Header="GroupBox">
    <GroupBox.RenderTransform>
        <ScaleTransform ScaleY="1"/>
    </GroupBox.RenderTransform>
</GroupBox>

When collapse a groupbox set ScaleTransform.ScaleX to 0. And when expand set to 1.