WPF Animations

Hi

I am trying to create and start an animation in code instead of using xaml. When I use the Storyboard object(second example) nothing happens(There are no visible results of the animation), if I set animation's properties and start it without using the Storyboard object everything works normaly(first example). Below is the sample code which is put in the event handler for a button which is on the UserControl.

//This works normaly
btn_Click(object sender, RoutedEventArgs e)
{
LinearGradientBrush mask = new LinearGradientBrush();

GradientStop transparentStop = new GradientStop(Colors.Transparent, 0);
GradientStop visibleStop = new GradientStop(Colors.Black, 0);

mask.GradientStops.Add(transparentStop);
mask.GradientStops.Add(visibleStop);

this.OpacityMask = mask;

DoubleAnimation visibleStopAnimation = new DoubleAnimation(0, 1.2, TimeSpan.FromSeconds(1.2), FillBehavior.HoldEnd);
visibleStop.BeginAnimation(GradientStop.OffsetProperty, visibleStopAnimation);
}

//This doesn't work
btn_Click(object sender, RoutedEventArgs e)
{
LinearGradientBrush mask = new LinearGradientBrush();
GradientStop transparentStop = new GradientStop(Colors.Transparent, 0);
GradientStop visibleStop = new GradientStop(Colors.Black, 0);
mask.GradientStops.Add(transparentStop);
mask.GradientStops.Add(visibleStop);
this.OpacityMask = mask;

Storyboard story = new Storyboard();
story.Duration = TimeSpan.FromSeconds(1);
DoubleAnimation visibleStopAnimation = new DoubleAnimation(0, 1, TimeSpan.FromSeconds(1), FillBehavior.HoldEnd);
Storyboard.SetTarget(visibleStopAnimation, visibleStop);
Storyboard.SetTargetProperty(visibleStopAnimation, new PropertyPath("(Offset)"));
story.Children.Add(visibleStopAnimation);
story.Begin();
}

Does anybody has any idea? Any advice will be appreciated!
Uros