WPF Binding and Dynamically Assigning StringFormat Property

Question

I have a form that is generated based on several DataTemplate elements. One of the DataTemplate elements creates a TextBox out of a class that looks like this:

Answer 1

One could create an attached behavior that could replace the binding with one that has the FormatString specified. If the FormatString dependency property then the binding would once again be updated. If the binding is updated then the FormatString would be reapplied to that binding.

The only two tricky things that I can think that you would have to deal with. One issue is whether you want to create two attached properties that coordinate with each other for the FormatString and the TargetProperty on which the binding exist that the FormatString should be applied (ex. TextBox.Text) or perhaps you can just assume which property your dealing with depending on the target control type. The other issue may be that it may be non-trivial to copy an existing binding and modifying it slightly given the various types of bindings out there which might also include custom bindings.

It's important to consider though that all of this only achieves formatting in the direction from your data to your control. As far as I can discover using something like a MultiBinding along with a custom MultiValueConverter to consume both the original value and the FormatString and produce the desired output still suffers from the same problem mainly because the ConvertBack method is only given the output string and you would be expected to decipher both the FormatString and the original value from it which at that point is almost always impossible.

The remaining solutions that should work for bidirectional formatting and unformatting would be the following:

• Write a custom control that extends TextBox that has the desired formatting behavior like Jakob Christensen suggested.
• Write a custom value converter that derives from either DependencyObject or FrameworkElement and has a FormatString DependencyProperty on it. If you want to go the DependencyObject route I believe you can push the value into the FormatString property using the OneWayToSource binding with a "virtual branch" technique. The other easier way may to instead inherit from FrameworkElement and place your value converter into the visual tree along with your other controls so that you can just bind to it when needed by ElementName.
• Use an attached behavior similar to the one I mentioned at the top of this post but instead of setting a FormatString instead have two attached properties, one for a custom value converter and one for the parameter that would be passed to the value converter. Then instead of modifying the original binding to add the FormatString you would be adding the converter and the converter parameter to the binding. Personally I think this option would result in the most readable and intuitive result because attached behaviors tend to be more clean yet still flexible enough to use in a variety of situations other than just a TextBox.