Behaviors and Subclassing in Silverlight4

using System;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;

namespace SilverlightApplication1
{
    public static class TextBoxProperties
    {
        public static readonly DependencyProperty SelectAllProperty =
            DependencyProperty.RegisterAttached("SelectAll", typeof(bool),
            typeof(TextBoxProperties), new PropertyMetadata(false, OnSelectAllChanged));

        public static void SetSelectAll(DependencyObject o, bool value)
        {
            o.SetValue(SelectAllProperty, value);
        }

        public static bool GetSelectAll(DependencyObject o)
        {
            return (bool)o.GetValue(SelectAllProperty);
        }

        private static void OnSelectAllChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if ((bool)e.NewValue == true)
            {
                ((TextBox)d).GotFocus += new RoutedEventHandler(TextBoxProperties_GotFocus);
            }
        }

        private static void TextBoxProperties_GotFocus(object sender, RoutedEventArgs e)
        {
            ((TextBox)sender).SelectAll();
        }
    }
}

using System;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using System.Windows.Interactivity;

namespace SilverlightApplication1
{
    public class SelectAllBehavior : Behavior<TextBox>
    {
        protected override void OnAttached()
        {
            base.OnAttached();
            AssociatedObject.GotFocus += new RoutedEventHandler(AssociatedObject_GotFocus);
        }

        void AssociatedObject_GotFocus(object sender, RoutedEventArgs e)
        {
            ((TextBox)sender).SelectAll();
        }
    }
}

<UserControl x:Class="SilverlightApplication1.MainPage3"
    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"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400" xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk"
            
    xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
    xmlns:behaviors="clr-namespace:SilverlightApplication1;assembly=SilverlightApplication1">
   
    <Grid x:Name="LayoutRoot" Background="White">
        <TextBox Name="textBox1" Width="120" Height="23" Text="computer" HorizontalAlignment="Left">
            <i:Interaction.Behaviors>
                <behaviors:SelectAllBehavior/>
            </i:Interaction.Behaviors>
        </TextBox>

        <TextBox  Name="textBox2" Width="120" Height="23"  Text="中华人民共和国,china" HorizontalAlignment="Center" behaviors:TextBoxProperties.SelectAll="True"/>
    </Grid>
</UserControl>