WPF ValueConverter - Standard return for unconvertible value

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.

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.

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!

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.

Usually if a value can't be converted, I throw an Exception

This is because if I'm trying to convert a value using an invalid converter, I'd like to be alerted of it so I can alter my code.

In some rare cases, a value may be valid even though it can't be converted, and in that case I usually return the same value that was passed to the converter. This is only used if I want the value to be converted if possible, or to stay the same if not.

Another rare case that I've done on occasion is hardcoding a default value. This is usually done when I know the Converter may be used with an invalid parameter, and I want to return a valid value no matter what the result is. My hard-coded default converters almost always return boolean values.

I don't think I have ever returned any of the 3 you specified (null, DependencyProperty.UnsetValue, or Binding.DoNothing) because those values are often unexpected and not easy to notice unless you specifically look for them.