一、使矩形边框产生动画需要使用ColorAnimation，并且要注意，要给需要添加动画的属性（这里是Stroke.Color）设置初始值，不然会触发异常。

MainWindow.xaml文件代码（按键盘→键可以看到右侧被挡住的代码）

<Window x:Class="WpfApp3.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp3"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <ResourceDictionary>
            <Style TargetType="Rectangle">
                <Setter Property="Stroke" Value="Black"/>
                <Setter Property="Fill" Value="AliceBlue"/>
                <Setter Property="StrokeThickness" Value="2"/>
                <Style.Triggers>
                    <EventTrigger RoutedEvent="MouseEnter">
                        <BeginStoryboard>
                            <Storyboard>
                                <ColorAnimation To="LawnGreen" Duration="0:0:0.5" Storyboard.TargetProperty="Stroke.Color"/>
                            </Storyboard>
                        </BeginStoryboard>
                    </EventTrigger>
                    <EventTrigger RoutedEvent="MouseLeave">
                        <BeginStoryboard>
                            <Storyboard>
                                <ColorAnimation Duration="0:0:1" Storyboard.TargetProperty="Stroke.Color"/>
                            </Storyboard>
                        </BeginStoryboard>
                    </EventTrigger>
                </Style.Triggers>
            </Style>
        </ResourceDictionary>
    </Window.Resources>
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <Rectangle Grid.Column="1" Grid.Row="1"/>
        <Rectangle Grid.Column="1" Grid.Row="2"/>
        <Rectangle Grid.Column="2" Grid.Row="1"/>
        <Rectangle Grid.Column="2" Grid.Row="2"/>
        <Rectangle Grid.Column="3" Grid.Row="1"/>
        <Rectangle Grid.Column="3" Grid.Row="2"/>
    </Grid>
</Window>

代码效果如下：

代码效果