title: C# WPF Textboxbinding: using exception and want after a empty textbox
body: <p>i wrote a WPF userapplication for some administration taks and finaly im ready ... up to one last problem:</p>
  
  <p>I have 2 TextBox controlls and i dont want to make it possible for the user to fill both at the same time so i did this:</p>
  
  <pre><code>      private string versichertennummerAlt = "";
              public string VersichertennummerAlt
              {
                  get { return versichertennummerAlt; }
                  set
                  {
                      if (Versichertennummer.Length &gt; 0)
                      {
                          versichertennummerAlt = "";
                          OnPropertyChanged("VersichertennummerAlt");
                          throw new ArgumentException("Es kann nur die neue ODER die alte Versichertennummer eingegeben werden.\n Eine von beiden löschen.");                  
                      }
  ........
  }
  }
  </code></pre>
  
  <p>It works how it should, and i get the exception and the string is "" if the user try to fill both at the same time, but somehow the textbox is still showing the last pressed key, even the property is "".</p>
  
  <p>What am i doing wrong?
  Or how can i make sure the TextBox dosnt show anything after the exception? </p>
  
  <p>Thanks for helping :)</p>