title: WPF ValueConverter - Standard return for unconvertible value
body: <p>Over the course of the last year or so I have seen many different value converters for many different purposes, from many different authors.  One thing that sticks out in my mind is the wide variance of the 'default' values that are returned by them.  For example;</p>
  
  <pre><code>  public class MyConverter: IValueConverter
    {
      public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
      {
        // OK, we test for some undesirable, unconvertable situation, typically null...
        if (value == null)
        {
          // And here are a variety of 'defaults' that I have seen, these begin the most typical.
          return null;
          return DependencyProperty.UnsetValue;
          return Binding.DoNothing;
        }
          //...... other code.. whatever...
  }}
  </code></pre>
  
  <p>So my question is, is there a 'standard' way to indicate that an input value can't be converted?</p>