More Fun with Generic XAML Errors

Filed under Troubleshooting, Windows Phone 7

Here’s another obscure XAML error I hope no one else stumbles onto.

I have several VisualStates defined for a particular UserControl. One example is:

<VisualState x:Name="Waiting">
    <Storyboard>
        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Visibility)" Storyboard.TargetName="pnlBeforeWakeup">
            <DiscreteObjectKeyFrame KeyTime="00:00:00" Value="0" />
        </ObjectAnimationUsingKeyFrames>

Now, I’d entered that state by hand and it worked just fine in Visual Studio, compiles and runs (at least, on the phone emulator).

But, when I tried to fire up Expression Blend, the control would load initially, but when I tried to view the States tab, crash!

After a few false starts, I just removed all my VisualStates manually and recreated one via Expression Blend.

Presto! Anyone care to guess the “fault”?

<queue Jeopardy theme>

Ok, here’s the version that works fine in both VS and Blend.

<VisualState x:Name="Waiting">
    <Storyboard>
        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Visibility)" Storyboard.TargetName="pnlBeforeWakeup">
            <DiscreteObjectKeyFrame KeyTime="00:00:00">
                <DiscreteObjectKeyFrame.Value>
                    <Visibility>Visible</Visibility>
                </DiscreteObjectKeyFrame.Value>
            </DiscreteObjectKeyFrame>
        </ObjectAnimationUsingKeyFrames>

Notice that instead of Value=”0”, Blend seems to want a specific Value element, with a Visibility sub-element.

And finally, here’s the Expression error that results from using the Value attribute as I did originally.

image

The only clues that helped were the fact that it only happened when I tried clicking on a particular State that made use of the Value=”0” for Visibility, and the mention of “StoryBoard” in the StackTrace above.

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