public partial class Form1 : Form
{
  private TextBox textBox1;

  public Form1()
  {
    InitializeComponent();
    this.Load += new System.EventHandler(this.Form1_Load);
  }

  private void Form1_Load(object sender, EventArgs e)
  {
    textBox1 = new TextBox(); textBox1.Width = 150; Controls.Add(textBox1);
    new Thread(new ThreadStart(ThreadMethode)).Start();
  }
  private int counter = 0;
  public void ThreadMethode()
  {
    while (counter < 500)
    {
      textBox1.Invoke(new UpdateTextCallback(this.UpdateText),
          "aus Nicht-UI-Thread:" + (counter++).ToString());
      Thread.Sleep(5);
    }
  }
  public delegate void UpdateTextCallback(string text);
  private void UpdateText(string text)
  {
    textBox1.Text = text;
  }
}

private TextBox textBox1;int counter = 0;
BackgroundWorker bw;

private void Form1_Load(object sender,EventArgs e)
{ textBox1 =new TextBox(); textBox1.Width = 150; 
  Controls.Add(textBox1); bw =new BackgroundWorker();
  bw.WorkerReportsProgress =true;
  bw.DoWork +=new DoWorkEventHandler(bw_DoWork); bw.RunWorkerAsync();
  bw.ProgressChanged +=new ProgressChangedEventHandler(bw_ProgressChanged);
}

void bw_ProgressChanged(object sender,ProgressChangedEventArgs e)
{ textBox1.Text ="aus UI-Thread:" + (counter).ToString();
}

void bw_DoWork(object sender,DoWorkEventArgs e)
{
  while (counter < 500)
  { bw.ReportProgress(counter++ / 500);
    Thread.Sleep(5);//Nicht notwendig, nur künstliche Verlangsamung
  }
}

<Window x:Class="WpfApplication1.Window1"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
   Title ="Window1" Height="300" Width="300" Loaded="Window_Loaded">
  <Canvas Name="cvs">
    <TextBox Name="textBox1">Anfangs-Text</TextBox>
  </Canvas>
</Window>

private void Window_Loaded(object sender, RoutedEventArgs e)
{
  ThreadStart start = delegate()
  {
    Dispatcher.Invoke(DispatcherPriority.Normal,
      new Action<string>(SetzeStatus), "aus anderem Thread");
  };
  new Thread(start).Start();
}

void SetzeStatus(string meldung)
{
  textBox1.Text = meldung;
}