public class GridLengthAnimation : AnimationTimeline
  {
    /// <summary>
    /// Returns the type of object to animate
    /// </summary>
    public override Type TargetPropertyType => typeof(GridLength);
 
    /// <summary>
    /// Creates an instance of the animation object
    /// </summary>
    /// <returns>Returns the instance of the GridLengthAnimation</returns>
    protected override System.Windows.Freezable CreateInstanceCore()
    {
      return new GridLengthAnimation();
    }
 
    /// <summary>
    /// Dependency property for the From property
    /// </summary>
    public static readonly DependencyProperty FromProperty = DependencyProperty.Register("From", typeof(GridLength),
      typeof(GridLengthAnimation));
 
    /// <summary>
    /// CLR Wrapper for the From depenendency property
    /// </summary>
    public GridLength From
    {
      get
      {
        return (GridLength)GetValue(GridLengthAnimation.FromProperty);
      }
      set
      {
        SetValue(GridLengthAnimation.FromProperty, value);
      }
    }
 
    /// <summary>
    /// Dependency property for the To property
    /// </summary>
    public static readonly DependencyProperty ToProperty = DependencyProperty.Register("To", typeof(GridLength),
      typeof(GridLengthAnimation));
 
    /// <summary>
    /// CLR Wrapper for the To property
    /// </summary>
    public GridLength To
    {
      get
      {
        return (GridLength)GetValue(GridLengthAnimation.ToProperty);
      }
      set
      {
        SetValue(GridLengthAnimation.ToProperty, value);
      }
    }
 
    /// <summary>
    /// Animates the grid let set
    /// </summary>
    /// <param name="defaultOriginValue">The original value to animate</param>
    /// <param name="defaultDestinationValue">The final value</param>
    /// <param name="animationClock">The animation clock (timer)</param>
    /// <returns>Returns the new grid length to set</returns>
    public override object GetCurrentValue(object defaultOriginValue,
      object defaultDestinationValue, AnimationClock animationClock)
    {
      double fromVal = ((GridLength)GetValue(GridLengthAnimation.FromProperty)).Value;
 
      double toVal = ((GridLength)GetValue(GridLengthAnimation.ToProperty)).Value;
 
      if (fromVal > toVal)
        return new GridLength((1 - animationClock.CurrentProgress.Value) * (fromVal - toVal) + toVal, GridUnitType.Star);
      else
        return new GridLength(animationClock.CurrentProgress.Value * (toVal - fromVal) + fromVal, GridUnitType.Star);
    }

 /// <summary>
    /// The <see cref="EasingFunction" /> dependency property's name.
    /// </summary>
    public const string EasingFunctionPropertyName = "EasingFunction";
 
    /// <summary>
    /// Gets or sets the value of the <see cref="EasingFunction" />
    /// property. This is a dependency property.
    /// </summary>
    public IEasingFunction EasingFunction
    {
      get
      {
        return (IEasingFunction)GetValue(EasingFunctionProperty);
      }
      set
      {
        SetValue(EasingFunctionProperty, value);
      }
    }
 
    /// <summary>
    /// Identifies the <see cref="EasingFunction" /> dependency property.
    /// </summary>
    public static readonly DependencyProperty EasingFunctionProperty = DependencyProperty.Register(
      EasingFunctionPropertyName,
      typeof(IEasingFunction),
      typeof(GridLengthAnimation),
      new UIPropertyMetadata(null));

public override object GetCurrentValue(object defaultOriginValue,
      object defaultDestinationValue, AnimationClock animationClock)
    {
      double fromVal = ((GridLength)GetValue(FromProperty)).Value;
 
      double toVal = ((GridLength)GetValue(ToProperty)).Value;
 
      //check that from was set from the caller
      //if (fromVal == 1)
      //  //set the from as the actual value
      //  fromVal = ((GridLength)defaultDestinationValue).Value;
 
      double progress = animationClock.CurrentProgress.Value;
 
      IEasingFunction easingFunction = EasingFunction;
      if (easingFunction != null)
      {
        progress = easingFunction.Ease(progress);
      }
 
 
      if (fromVal > toVal)
        return new GridLength((1 - progress) * (fromVal - toVal) + toVal, GridUnitType.Star);
 
        return new GridLength(progress * (toVal - fromVal) + fromVal, GridUnitType.Star);
    }

 <anim:GridLengthAnimation Storyboard.TargetProperty="Width" From="0" To="*" Duration="0:0:0.5"/>