WPF

//--------------------------------------------------------------------------------
void CollapseAnimator_Completed(object sender, EventArgs e)
{
    GenericAnimationEnd(false);
}

//--------------------------------------------------------------------------------
void ExpandAnimator_Completed(object sender, EventArgs e)
{
    GenericAnimationEnd(true);
}

//--------------------------------------------------------------------------------
private void GenericAnimationEnd( bool expanding)
{
    double height = this.expandingGrid.ActualHeight;
    this.CollapseAnimator.FillBehavior = FillBehavior.Stop;
    this.ExpandAnimator.FillBehavior = FillBehavior.Stop;
    this.expandingGrid.Height = height;
}

<Window x:Class="AnimateSample.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Width="800" Height="600">
    
    <Window.Resources>
        <Storyboard x:Key="AnimateGridExpand">
            <DoubleAnimation To="300" Duration="0:0:0.2" Storyboard.TargetName="expandingGrid" Storyboard.TargetProperty="Height" />
        </Storyboard>
        <Storyboard x:Key="AnimateGridCollapse">
            <DoubleAnimation To="100" Duration="0:0:0.2" Storyboard.TargetName="expandingGrid" Storyboard.TargetProperty="Height" />
        </Storyboard>

    </Window.Resources>

    <Grid>
        <Button Name="buttonGrowWithAnimation" Width="150" Height="23" Margin="20,5,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Click="buttonGrowWithAnimation_Click">Grow With Animation</Button>
        <Button Name="buttonGrowWithoutAnimation" Width="150" Height="23" Margin="0,5,20,0" HorizontalAlignment="Right" VerticalAlignment="Top" Click="buttonGrowWithoutAnimation_Click">Grow Without Animation</Button>
        <Grid Name="expandingGrid" Margin="0,34,0,0" Height="100" VerticalAlignment="Top">
            <Border Name="border1" BorderThickness="2" CornerRadius="6,6,6,6" Background="LightSlateGray" BorderBrush="Black">
                <CheckBox Height="16" Name="checkBox1" Width="120">CheckBox</CheckBox>
            </Border>
        </Grid>
    </Grid>
    
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

using System.Diagnostics;
using System.Windows.Media.Animation;

namespace AnimateSample
{
	/// <summary>
	/// Interaction logic for Window1.xaml
	/// </summary>
	public partial class Window1 : Window
	{
		public Storyboard   ExpandAnimator    { get; set; }
		public Storyboard   CollapseAnimator  { get; set; }
		public bool         Expanded          { get; set; }
		public int          OriginalZIndex    { get; set; }
		public double       HeightChangeValue { get; set; }


		//--------------------------------------------------------------------------------
		/// <summary>
		/// Costructor
		/// </summary>
		public Window1()
		{
			InitializeComponent();

			// we start out in collapsed mode
			this.Expanded = false;

			// In order to add an event handler (or change other properties), we have to 
			// get an "unfrozen" copy of the storyboard. We have to use the Clone method 
			// to accomplish this.
			this.ExpandAnimator   = ((Storyboard)TryFindResource("AnimateGridExpand")).Clone();
			this.CollapseAnimator = ((Storyboard)TryFindResource("AnimateGridCollapse")).Clone();

			// Retrieve the To value from both storyboards so we can calculate the height 
			// change we will need to make in "manual" (non-animated) mode.
			DoubleAnimation daExpand   = (DoubleAnimation)ExpandAnimator.Children[0];
			DoubleAnimation daCollapse = (DoubleAnimation)CollapseAnimator.Children[0];

			// Here's our manual height change value
			this.HeightChangeValue = (double)daExpand.To - (double)daCollapse.To;

			Debug.WriteLine("\n---------------------------------------------------------------------------------");
			Debug.WriteLine("Application Starts");
			Debug.WriteLine("---------------------------------------------------------------------------------");
			Debug.WriteLine(string.Format("HeightChangeValue = {0:0.0}", this.HeightChangeValue));
		}


		//--------------------------------------------------------------------------------
		/// <summary>
		/// Fired when the user clicks the "Grow With Animation" button
		/// </summary>
		/// <param name="sender"></param>
		/// <param name="e"></param>
		private void buttonGrowWithAnimation_Click(object sender, RoutedEventArgs e)
		{
			Debug.WriteLine("----");
			Debug.WriteLine((this.Expanded)?"Expanded" : "Collapsed");

			// set our animation target
			Debug.WriteLine("Setting animation target to the grid");
			Storyboard.SetTarget(this.ExpandAnimator, this.expandingGrid);
			Storyboard.SetTarget(this.CollapseAnimator, this.expandingGrid);

			// expand or collapse, depending on the current state
			if (this.Expanded)
			{
				Debug.WriteLine("Collapsing with animation");
				this.CollapseAnimator.Begin(this);
			}
			else
			{
				Debug.WriteLine("Expanding with animation");
				this.ExpandAnimator.Begin(this);
			}

			// set our current state to the opposite of what it was
			this.Expanded = !this.Expanded;
			Debug.WriteLine((this.Expanded)?"Expanded" : "Collapsed");
		}


		//--------------------------------------------------------------------------------
		/// <summary>
		/// Fired when the user clicks the "Grow Without Animation" button
		/// </summary>
		/// <param name="sender"></param>
		/// <param name="e"></param>
		private void buttonGrowWithoutAnimation_Click(object sender, RoutedEventArgs e)
		{
			Debug.WriteLine("----");
			Debug.WriteLine((this.Expanded)?"Expanded" : "Collapsed");

			// set our animation targets to null
			Debug.WriteLine("Setting animation target to null");
			Storyboard.SetTarget(this.ExpandAnimator, null);
			Storyboard.SetTarget(this.CollapseAnimator, null);

			// expand or collapse, depending on the current state
			if (this.Expanded)
			{
				Debug.WriteLine("Collapsing WITHOUT animation");
				// Because of the problem I'm having, I have to make sure 
				// the value I'm setting passes a sanity check
				this.expandingGrid.Height = Math.Max(100d, this.expandingGrid.ActualHeight - this.HeightChangeValue);
			}
			else
			{
				Debug.WriteLine("Collapsing WITHOUT animation");
				this.expandingGrid.Height = this.expandingGrid.ActualHeight + this.HeightChangeValue;
			}

			// set our current state to the opposite of what it was
			this.Expanded = !this.Expanded;
			Debug.WriteLine((this.Expanded)?"Expanded" : "Collapsed");
		}
	}
}