silverlight2中的定时器，以及如何动态改变控件的坐标

<UserControl
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="SilverlightApplication2.Page"
    Width="300" Height="300" xmlns:vsm="clr-namespace:System.Windows;assembly=System.Windows" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d"> 

 <Canvas x:Name="LayoutRoot" Background="White">
 
 <TextBlock x:Name="txtTime" Height="10" Width="10" Text="A" Canvas.Left="10" Canvas.Top="10" FontSize="12" Foreground="Red" />
 <Ellipse Height="200" Width="200" Canvas.Left="10" Canvas.Top="10" Fill="{x:Null}" Stroke="#22000000" Canvas.ZIndex="-1"/>
 
 
 </Canvas>
</UserControl>

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

namespace SilverlightApplication2
{
 public partial class Page : UserControl
    {
 public Page()
        {
 
            InitializeComponent();
            DispatcherTimer dt = new DispatcherTimer();
            dt.Interval = new TimeSpan(0, 0, 0, 0, 100); // 50 Milliseconds
            dt.Tick += new EventHandler(dt_Tick);
            dt.Start();   
        }

 int angle = 0;
 int radius = 100;        

 void dt_Tick(object sender, EventArgs e)
        {
 
            angle = angle + 1;
            txtTime.Text = new System.Random().Next(1, 10).ToString();
 //Margin.Left = 半径*sin(角度),Margin.Top = 半径*cos(角度)－－注意：角度要换成弧度，同时加上平移量
            txtTime.Margin = new System.Windows.Thickness(radius * Math.Cos(angle * Math.PI / 180) + radius, radius * Math.Sin(angle * Math.PI / 180) + radius, 0, 0);
 if (angle > 360) { angle = 0; }
        }


    }
}