<UserControl x:Class="DPStudy.MyControl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    >
    <Border CornerRadius="5" BorderThickness="3" BorderBrush="#FFEF410D" Width="300" Height="40" Margin="5"  >        
        <TextBlock x:Name="txt" Text="" VerticalAlignment="Center" HorizontalAlignment="Center"></TextBlock>
    </Border>
</UserControl>

using System.Windows;
using System.Windows.Controls;

namespace DPStudy
{
    public partial class MyControl : UserControl
    {
            
        
        public static readonly DependencyProperty MessageProperty = DependencyProperty.Register("Message", typeof(string), typeof(MyControl), new PropertyMetadata("Message的默认值", new PropertyChangedCallback(OnMessagePropertyChanged)));

public string Message
        {
            get { return (string)GetValue(MessageProperty); }
            set { SetValue(MessageProperty, value); }
        }

/// <summary>
        /// Message变化时的通知处理
        /// </summary>
        /// <param name="d"></param>
        /// <param name="e"></param>
        private static void OnMessagePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            MyControl ctl = d as MyControl;
            ctl.txt.Text = d.GetValue(MyControl.MessageProperty).ToString();
        }

public MyControl()
        {
            InitializeComponent();

this.Loaded += new RoutedEventHandler(MyControl_Loaded);
        }

void MyControl_Loaded(object sender, RoutedEventArgs e)
        {
            this.txt.Text = Message;//初始加载时，显示Message的初始值
        }
    }
}

<UserControl x:Class="DPStudy.MainPage"
    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:l="clr-namespace:DPStudy"
    mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480">
  <StackPanel x:Name="LayoutRoot">        
        <l:MyControl x:Name="myctl"></l:MyControl>
        <Button Click="Button_Click" Content="改变Message属性值" Width="130" ></Button>
    </StackPanel>
</UserControl>

using System.Windows;
using System.Windows.Controls;

namespace DPStudy
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();            
        }

private void Button_Click(object sender, RoutedEventArgs e)
        {
            myctl.SetValue(MyControl.MessageProperty, "新值");
        }
    }
}