how to use storyboards in WPF - wpf

Question

I want to reuse a storyboad defined in a resourcedictionnary and referenced in App.xaml
<Storyboard x:Key="ShowWindowStoryboard">
<DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.RenderTransform).(TransformGroup.Children)[0].(ScaleTransform.ScaleX)"
>
<EasingDoubleKeyFrame KeyTime="0:0:0.3"
Value="1" />
</DoubleAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.RenderTransform).(TransformGroup.Children)[0].(ScaleTransform.ScaleY)"
>
<EasingDoubleKeyFrame KeyTime="0:0:0.3"
Value="1" />
</DoubleAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Opacity)"
>
<EasingDoubleKeyFrame KeyTime="0:0:0.3"
Value="1" />
</DoubleAnimationUsingKeyFrames>
</Storyboard>
and here the code that gave an exception
Storyboard sb = (FindResource("ShowWindowStoryboard") as Storyboard).Clone();
DoubleAnimation da0 = sb.Children[0] as DoubleAnimation;
/* Exception here da0 is null*/
Storyboard.SetTarget(da0, uc);
DoubleAnimation da1 = sb.Children[1] as DoubleAnimation;
Storyboard.SetTarget(da1, uc);
DoubleAnimation da2 = sb.Children[2] as DoubleAnimation;
Storyboard.SetTarget(da2, uc);
sb.Begin();
i have also checked that sb.children.Count == 3 as expected.

Answer 1

You have to add a TransformGroup to the RenderTransform of your uc control. I will be adding a TransformGroup to an Image. The same will work for your control uc.
XAML
<Image Source="untitled.bmp" Name="ImgDemo">
<Image.RenderTransform>
<TransformGroup>
<ScaleTransform/>
</TransformGroup>
</Image.RenderTransform>
</Image>
If you have to add TransformGroup in code behind, you can use following before calling Story.Begin:
TransformGroup transformGroup = new TransformGroup();
transformGroup.Children.Add(new ScaleTransform(1,1));
uc.RenderTransform = transformGroup;
Your Storyboard.TargetProperty should reflect the signature of the object that you want the storyboard to be applied to. This is why you were facing the exception. Also, you might want to change your Storyboard a little to actually see the changes on the object. Like,
<Storyboard x:Key="ShowWindowStoryboard">
<DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.RenderTransform).(TransformGroup.Children)[0].(ScaleTransform.ScaleX)"
>
<EasingDoubleKeyFrame KeyTime="0:0:0.3"
Value="0.5" />
</DoubleAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.RenderTransform).(TransformGroup.Children)[0].(ScaleTransform.ScaleY)"
>
<EasingDoubleKeyFrame KeyTime="0:0:0.3"
Value="0.5" />
</DoubleAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Opacity)"
>
<EasingDoubleKeyFrame KeyTime="0:0:0.3"
Value="0.5" />
</DoubleAnimationUsingKeyFrames>
Note that the values are changed so that the animations will be noticeable. Finally, I do see that your code behind on calling the storyboard does not use proper casting. So you might wanna consider using the following,
Storyboard sb = (FindResource("ShowWindowStoryboard") as Storyboard).Clone();
DoubleAnimationUsingKeyFrames da0 = sb.Children[0] as DoubleAnimationUsingKeyFrames;
Storyboard.SetTarget(da0, ImgDemo);
DoubleAnimationUsingKeyFrames da1 = sb.Children[1] as DoubleAnimationUsingKeyFrames;
Storyboard.SetTarget(da1, ImgDemo);
DoubleAnimationUsingKeyFrames da2 = sb.Children[2] as DoubleAnimationUsingKeyFrames;
Storyboard.SetTarget(da2, ImgDemo);
sb.Begin();
Good luck!