Windows Presentation Foundation binds UI controls directly to model properties, and custom type conversions go through the IValueConverter interface with its famously ambiguous Convert and ConvertBack methods. Microsoft's own sample code converts a Color into a SolidColorBrush and simply returns null from ConvertBack, because the reverse operation is not meaningful.

A submission by a reader named Fredrika shows where this leads in practice. Binding a text box to a double? field mostly works out of the box, except WPF's built-in converter refuses to treat an empty string as null. A co-worker therefore wrote a StringToDoubleConverter: its Convert merely casts the value to double? and returns it unchanged, while ConvertBack takes the string from the text box, checks whether it equals string.Empty, and returns null in that case — otherwise it returns the raw string, leaning on the framework's built-in conversion machinery to do the actual parsing.

Author Remy Porter struggles to pin the WTF on a single party: he dislikes WPF's converter API on principle, finds this particular implementation unhelpful, and notes that naming both variables 'parsed' clarifies nothing. Treating empty text boxes as null may be the correct behavior here, he concedes, but he suspects it will cause problems down the road. His closing summary has no neat bow: he simply dislikes all of it.