CODEWITHSUNDEEP
wpfdata-bindingivalueconverterimultivalueconverter
NtscCobalt
NtscCobalt

Reputation: 1689

Two way Multiconverter or Converter with property

I need to convert a two part value into a string and back again for example:

{Value = 12.0, Units = DimensionUnits.Inches}

Converts to

"12 in"

This is pretty simple using a multivalue converter to convert from source but becomes impossible to convert back if the user doesn't provide the unit type in the string so a Multivalue converter doesn't look like the solution I need.

A direct converter parameter won't work because the unit type needs to be bound so I researched how to create a bindable parameter. Creating a bindable parameter is actually pretty easy - deriving from DependencyObject - but then you have the problem of your converter not living in the visual tree - and thus not being able to bind to anything - to which there are 2 solutions:

http://tomlev2.wordpress.com/2011/03/21/wpf-how-to-bind-to-data-when-the-datacontext-is-not-inherited/ http://shujaatsiddiqi.blogspot.com/2011/02/wpf-binding-converter-parameter.html

The first method derives your Converter from Freezable instead of DependencyObject to allow DependencyProperties. This works and allows you to bind within the Whatever.Resources section but it has extremely odd behavior like only listening to the binding the first time it is used in your entire application.

The second method doesn't seem to work at all. The dependency property is never updated when the source changes. 

<pf:BindingReflector Target="{Binding Source={StaticResource DistanceConverter}, Path=Units, Mode=TwoWay}"
    Source="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=g:TestWindow, AncestorLevel=1}, Path=Units, Mode=TwoWay}"/>

Does anyone know of a solution to this problem or is this a big limitation of WPF?

Upvotes: 1

Views: 1037

Answers (1)

Reed Copsey
Reed Copsey

Reputation: 564741

Personally, I would actually suggest rethinking this a bit, and doing either:

  1. Keep the final string one-way using a IMultiValueConverter, and have this be input as two separate items. This seems like a reasonable approach, as the units appear to be an enum with a specific set of options. A combo box for units and textbox for amount seems appropriate, and the total display can be done with a one-way converter.

  2. Handle this conversion explicitly in the ViewModel. This has the advantage of allowing much better validation handling, which is probably going to be required as entering two separate values (amount + units) in one control is likely to not validate correctly. By moving the logic directly into your ViewModel instead of binding to the properties, you can correctly handle errors in a clean way.

Upvotes: 3

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