Question

I have a list on my WPF xaml which contains two items. Below is the Style template for each item. Now on UI this shows like a group of radio buttons(No. of radio buttons depends on no. of items in my list).

<Style x:Key="RadioButtonListBoxItemStyle" TargetType="{x:Type ListBoxItem}" >
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate>
                    <RadioButton FlowDirection="LeftToRight"
                                     Margin="10 15"
                                     Content="{Binding Value}"                                      
                                     GroupName="{Binding DisplayGroupName}"
                                     IsChecked="{Binding IsSelected, Mode=TwoWay}" />
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

Now I bind a list(having 2 items) using the above style template to get two radio buttons. What happens is everything works pretty fine i.e when I change the selection of radio button on UI the IsSelected property is getting updated properly to true or false depending on whether my radio is checked/un-checked. But if I try to set the list in the code manually, then from that point my binding of the radio button’s with my list is lost and nothing happen’s.

Any help on this would be great and based on my needs I have to set the list in the code manually. So is there any solution in a way that binding will not be lost even though I set the list in my code manually. Thanks.

-Ady.

Answer 1

This is a common problem with radio buttons in WPF, and it has to do an unusual aspect of binding, one that is marginally more feature than bug.

The design of binding assumes that the only two things that change the value of a binding’s target property are a) actions in the UI and b) changes to the source property. If you set the target property of a binding in code – like, you explicitly set the Background of a Border, even though it has a binding – the binding decides that you know what you’re doing, and that it should just get out of the way. So it turns itself off.

This is a pretty sensible design decision, for the most part. It’s better than throwing an exception, for instance. Most of the time, you’re not going to ever set IsEnabled in code anyway; you’ll let the binding do it. Especially if you’re using MVVM.

Okay, so what happens if you have radio buttons in a group?

When you check one button in the group, the WPF code that manages radio button groups unchecks all the other buttons in the group, by setting IsChecked to false in code. The binding disables itself. Oops.

Here’s the solution: If you’re using radio buttons and binding, don’t use groups. Handle the mutual exclusion logic in your view model code. In your case, code your view models so that only one object in a collection can have IsSelected true at any given time. (Yes, this is a pain.)

The radio buttons will still work as expected, but since the only properties being set by code are the source properties, binding won’t break.