WPF ValueConverter - Standard return for unconvertible value

Question

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;

  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...
}}

So my question is, is there a 'standard' way to indicate that an input value can't be converted?

Answer 1

According to MSDN - IValueConverter:

The data binding engine does not catch exceptions that are thrown by a user-supplied converter. Any exception that is thrown by the Convert method, or any uncaught exceptions that are thrown by methods that the Convert method calls, are treated as run-time errors. Handle anticipated problems by returning DependencyProperty.UnsetValue.

The key line is Handle anticipated problems by returning DependencyProperty.UnsetValue.

Answer 2

When you look these values up you will find out what they mean. Then pick the right one to return in your converter.

The main issue is that null might be a valid value for the property.

DependencyProperty.UnsetValue to indicate that the property exists, but does not have its value set by the property system

Binding.DoNothing to instruct the binding engine not to transfer a value to the binding target, not to move to the next Binding in a PriorityBinding, or not to use the FallBackValue or default value

EDIT

To indicate that you can't convert the value you should simply return the given value. That is the best you can do because it returns the problem to the author of the binding. If you meddle with the value it becomes very hard to findout what is going on.

Answer 3

After much thinking and digging around, it seems that DependencyProperty.UnsetValue is the appropriate choice. I have moved everything in house over to this pattern with much success. Also, the text in the 'Remarks' section of this page indicates that I this is probably the best choice..

There is also some discussion about returning the input value if a binding can't be converted, but this can easily "break" the binding system. Think of a case where the binding input is a 'string' and the output is a 'brush'. Returning a string is not going to work!

Answer 4

what you return as a default depends on the situation. You don't want to return an int as the default for a converter to a bool, or return a bool for a converter for the visibility enum.