该Demo使用是纯C#编写（不建议使用XAML做动画效果，内存开销不可控且不便操作）
效果：速度、启动、暂停、缓动效果、线性渐变

代码如下

using System;
using System.Text.RegularExpressions;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;

namespace MyWPF
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {

        Storyboard sb;//故事板
        Rectangle myRectangle;//故事板主角控件对象
        string rectName = "MyRectangle";//故事板主角控件名
        public MainWindow()
        {
            InitializeComponent();
            this.Loaded += (s,e)=>InitData();
        }

        //初始化数据
        private void InitData()
        {
            sb = new Storyboard();
            myRectangle = new Rectangle()
            {
                Fill = Brushes.Red,
                Height = 200,
                Width = 200,
                HorizontalAlignment = HorizontalAlignment.Left
            };
            myRectangle.Name = rectName;
            this.RegisterName(myRectangle.Name, myRectangle);//使用C#插入控件时要注册控件名
            ui_Grid.Children.Add(myRectangle);

        }

        //启动动画
        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            //故事板动画Clear（等待GC),也可以手动设置为Null
            sb.Children.Clear();
            //边距位移动画
            var marginAnim = new ThicknessAnimation
            {
                From = new Thickness(0, 0, 0, 0),
                To = new Thickness(this.ActualWidth - 200, 0, 0, 0),
                RepeatBehavior = RepeatBehavior.Forever,
                EasingFunction = new BounceEase()
                {
                    Bounces = 0,//反弹次数
                    EasingMode = EasingMode.EaseInOut,//缓动函数In Out
                    Bounciness = 0//反弹大小
                }
            };
            //线性渐变动画
            var colorAnim = new ColorAnimation
            {
                From = Colors.Red,
                To = Colors.Blue,
                RepeatBehavior = RepeatBehavior.Forever//重复播放
            };
            //设置故事板动画目标属性
            Storyboard.SetTargetProperty(marginAnim, new PropertyPath(MarginProperty));
            Storyboard.SetTargetProperty(colorAnim, new PropertyPath("(Fill).(SolidColorBrush.Color)"));
            //将配置完毕的动画插入到故事板
            sb.Children.Add(marginAnim);
            sb.Children.Add(colorAnim);
            if (!Regex.IsMatch(ui_Speed.Text, @"^[+-]?\d*[.]?\d*$"))
                return;
            //故事板运行速度
            sb.SpeedRatio = Convert.ToDouble(ui_Speed.Text);
            //故事板启动
            sb.Begin(myRectangle, true);
        }
        //停止动画
        private void Button_Click_2(object sender, RoutedEventArgs e)
        {
            //故事板停止
            sb.Stop(myRectangle);
        }
    }
}

可能有同学会问new PropertyPath("(Fill).(SolidColorBrush.Color)")为什么要这样写,因为Fill依赖属性本身是Brushes类型。

MSDN上是这样说的：

例如，面板的Background属性是来自主题模板的完整画笔（实际上是SolidColorBrush ）。要完全为Brush设置动画，需要有一个 BrushAnimation（可能每个Brush类型都有一个），并且没有这样的类型。要为画笔设置动画，您可以为特定画笔类型的属性设置动画。您需要从SolidColorBrush到它的Color才能在其中应用ColorAnimation。此示例的属性路径为.Background.Color

For instance, the Background property of a Panel is a complete Brush (actually a SolidColorBrush) that came from a theme template. To animate a Brush completely, there would need to be a BrushAnimation (probably one for every Brush type) and there is no such type. To animate a Brush, you instead animate properties of a particular Brush type. You need to get from SolidColorBrush to its Color to apply a ColorAnimation there. The property path for this example would be Background.Color.

关于PropertyPath XAML语法请看：https://docs.microsoft.com/en-us/dotnet/desktop/wpf/advanced/propertypath-xaml-syntax?view=netframeworkdesktop-4.8

关于更多故事板Storyboard资料请看：https://docs.microsoft.com/en-us/dotnet/desktop/wpf/graphics-multimedia/storyboards-overview?view=netframeworkdesktop-4.8