title: Binding "Text-Property" of a derived textbox to another textbox doesn´t work
body: <p>i have a class 'MyTextBox' that derives from the default TextBox in Silverlight. This class currently contains no additional code. </p>
  
  <p>I set up a binding in xaml to bind the Text-Property of MyTextbox to another Textbox to reflect the input made in the Textbox. The effect is that MyTextBox doesn´t update and not display the text of the other Textbox.</p>
  
  <p>Additional i made an equal binding for a normal Textbox. And this works.</p>
  
  <p>Here´s the XAML for the bindings.</p>
  
  <pre><code>&lt;UserControl x:Class="Silverlight.Sample.Dummy"
  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:my="clr-namespace:Sample"
  mc:Ignorable="d"
  d:DesignHeight="300" d:DesignWidth="400"&gt;
  
  &lt;StackPanel x:Name="LayoutRoot" Background="White"&gt;
      &lt;TextBox Height="23" x:Name="textBox2"  Width="120" /&gt;
      &lt;TextBox Text="{Binding ElementName=textBox2, Path=Text, Mode=TwoWay}" Width="120" /&gt;
      &lt;my:NumberTextBox Width="120" Text="{Binding ElementName=textBox2, Path=Text, Mode=OneWay}" /&gt;
  &lt;/StackPanel&gt;
  </code></pre>
  
  <p></p>
  
  <p>Is there something special to set for binding, when i derive from a control.</p>
  
  <p>PS: I tried a binding to a dummy object with INotifyPropertyChanged and set it as DataContext for the existing Textboxes. This test works as expected and my derived textbox gets updated. </p>