WPF and MVVM: Changing data binding converter at runtime

6
votes

I am using WPF and the MVVM pattern in my user interface. In my ViewModel I have a List containing distances in millimetres, which I display in a ListView by binding ListView.ItemsSource to the List. However, I would like the values displayed to use a more natural unit - either metres or feet depending on the state of a "metric" checkbox.

I have written a couple of simple classes, MillimetresToMetresConverter and MillimetresToFeetConverter, both of which implement IValueConverter. Although I can set the Converter property on my data binding to one or the other, I am unsure how to change between these converters when the state of the checkbox changes.

My plan was to have a field "IValueConverter lengthConverter" on my ViewModel which I could set to one converter or the other, then in my XAML do ...="{Binding Converter={Binding Path=lengthConverter}}" - unfortunately this does not work since Converter is not a dependency property.

How can change the converter used by the data binding at runtime?

2
wpfdata-bindingmvvmivalueconverter
You should be able to use DependencyObject.GetBinding(DependencyProperty), e.g. myTextBlock.GetBinding(TextBlock.TextProperty) to get (and modify) the Binding object. – Heinzi
Unfortunately, following the MVVM pattern means that code would need to live in the ViewModel - which has no reference to the view and thus cannot touch "myTextBlock". – user200783

2 Answers

5
votes

Most of the time when using the MVVM methodology, you can do the formatting task in the VM classes. In your case, you could add a Format property to the VM class, and based on the value of the Format property, return a well formated string.

See this discussion for more information.

0
votes

If I may suggest a simple alternative solution: Create a small FormatMillimetresConverter in your ViewModel, whose UseMetric property is bound to the "metric" checkbox.