title: Does StringFormat in a TextBox make any sense?
body: <p>If I have a TextBox like this:</p>
  
  <pre><code>&lt;TextBox Text="{Binding Voltage, StringFormat={}{0} kV}" /&gt;
  </code></pre>
  
  <p>and the property Voltage is e.g. 50, I get "50 kV" in my TextBox. This is what I intended.</p>
  
  <p>But now, if the user wants to enter a new value say 40 and types "40 kV" he gets a red border, because there is a FormatException converting back the value.</p>
  
  <pre><code>System.Windows.Data Error: 7 : ConvertBack cannot convert value '40 kV' (type 'String'). BindingExpression:Path=Voltage; DataItem='VMO_VoltageDefinition' (HashCode=19837180); target element is 'TextBox' (Name=''); target property is 'Text' (type 'String') FormatException:'System.FormatException: Die Eingabezeichenfolge hat das falsche Format.
  </code></pre>
  
  <p>I don't think that the users of my program will accept this.</p>
  
  <p>So am I doing something wrong or is this feature just not usable in a reasonable way with a TextBox?</p>