Get DoubleAnimation Reference Object from BeginAnimation - wpf

Question

I want to add a simple double animation with code-behind to apply a fadein to n objects created at runtime:
foreach (var rect in howmanyrect) {
Rectangle bbox = new Rectangle {
Width = rect.Width,
Height = rect.Height,
Stroke = Brushes.BlueViolet,
Opacity = 0D
};
DoubleAnimation da = new DoubleAnimation {
From = 0D,
To = 1D,
Duration = TimeSpan.FromMilliseconds(500D),
RepeatBehavior = new RepeatBehavior(1),
AutoReverse = false
};
GridContainer.Children.Add(bbox);
Canvas.SetLeft(bbox, rect.Left);
Canvas.SetTop(bbox, rect.Top);
bbox.Tag = da; // <- Look HERE
bbox.BeginAnimation(OpacityProperty, da);
After this, when requested, delete the objects collection with fadeout:
foreach (var child in GridContainer.Children) {
Rectangle bbox = (Rectangle) child;
DoubleAnimation da = (DoubleAnimation) bbox.Tag; // <- Look HERE
da.From = 1D;
da.To = 0D;
var childCopy = child; // This copy grants the object reference access for removing inside a forech statement
da.Completed += (obj, arg) => viewerGrid.Children.Remove((UIElement) childCopy);
bbox.BeginAnimation(OpacityProperty, da);
}
This code works perfectly but it's a workaround. In my first revision I created a new Doubleanimation object in the delete method but when I started the animation every object excetutes the first and the second animation before being deleted.
So I decide to pass a reference to the DoubleAnimation instance with the Tag property and change the animation properties.
Is there another way to obtain a reference to the DoubleAnimation object attached with BeginAnimation or to avoid the first animation to be repeated?
Thanks
Lox