WPF 附加属性

附加属性本质也是依赖属性，依赖属性的宿主位置不一样。

XAML：

<Window x:Class="WpfApp1.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:WpfApp1"
        mc:Ignorable="d" WindowStartupLocation="CenterScreen"
        Title="MainWindow" Height="250" Width="350">
    <Grid>
        <Button Content="Grade" Width="100" Height="50" Click="Button_Click"/>
    </Grid>
</Window>

C# Behind：

namespace WpfApp1
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Human human = new Human();
            School.SetGrade(human, 6);
            int grade = School.GetGrade(human);
            MessageBox.Show(grade.ToString());

            Student stu = new Student();
            School.SetGrade(stu, 6);
            int g = School.GetGrade(stu);
            MessageBox.Show(g.ToString());
        }
    }

    public class School:DependencyObject
    {
        public static int GetGrade(DependencyObject obj)
        {
            return (int)obj.GetValue(GradeProperty);
        }

        public static void SetGrade(DependencyObject obj, int value)
        {
            obj.SetValue(GradeProperty, value);
        }

        // Using a DependencyProperty as the backing store for Grade.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty GradeProperty =
            DependencyProperty.RegisterAttached("Grade", typeof(int), typeof(School), new PropertyMetadata(0));
    }

    public class Human:DependencyObject
    {
    }

    public class Student:DependencyObject
    {
    }
}

运行结果：

▲ 运行结果, 两个弹窗都显示 6 没有问题

以上可见，附加属性和依赖属性保存值过程是一样的。只是用来检索值的依赖属性(附加属性)并不是以Human为宿主，而是寄宿在School里面，而这是没有什么关系的。因为，CLR 属性名和宿主类型只是用来生成 hash code 和 GlobalIndex 的，保证键值对唯一。

另外，既然附加属性和依赖属性本质是一样的，那么附加属性也是可以用来 Binding 的。

XAML：

<Window x:Class="WpfApp1.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:WpfApp1"
        mc:Ignorable="d" WindowStartupLocation="CenterScreen"
        Title="MainWindow" Height="250" Width="350">
    <Canvas>
        <Slider x:Name="sliderX" Canvas.Top="10" Canvas.Left="10" Width="320" Minimum="50" Maximum="260"/>
        <Slider x:Name="sliderY" Canvas.Top="50" Canvas.Left="10" Width="320" Minimum="50" Maximum="260"/>
        <Rectangle x:Name="rect" Fill="Blue" Width="30" Height="30" Canvas.Left="{Binding ElementName=sliderX, Path=Value}" 
                   Canvas.Top="{Binding Path=Value, ElementName=sliderY}" />
    </Canvas>
</Window>