Inverted TwoWay-MultiBinding

Question

I'm trying to express an enumeration property in my view-model as a set of radio buttons in my view. So far, so good; I can express that with a two-way MultiBinding:

(rb1.IsChecked, rb2.IsChecked, rb3.IsChecked) <-> vm.Value

The multi-binding used here would feature a multi-converter that converts between (bool, bool, bool) <-> MyValue; obviously, one of the (three) allowable values of the MyValue type is chosen based on which bool is true, and vice-versa.

This is already a bit inconvenient, though: I cannot define that binding in my view's Xaml, as multi-bindings have to be defined from the side of the single value. Hence, I have to define the multi-binding in code-behind and use SetBinding set it on my view model's Value property.

Now, the issue that I'm stuck at is that I'm not just binding one set of radio buttons to that value, but two. Hence, my bindings would have to look like this:

(rbA1.IsChecked, rbA2.IsChecked, rbA3.IsChecked) <-> vm.Value <-> (rbB1.IsChecked, rbB2.IsChecked, rbB3.IsChecked)

The problem is that I cannot use SetBinding to connect several bindings to vm.Value at a time.

Solutions that I have tried so far are:

• Use one big multi-binding, binding to all radio buttons at a time. This would mean a binding of the form (rbA1.IsChecked, rbA2.IsChecked, rbA3.IsChecked, rbB1.IsChecked, rbB2.IsChecked, rbB3.IsChecked) <-> vm.Value. The problem with this solution is that if one of the radio buttons (say, rbB2) is checked, I have no way of telling whether rbA2 (unchecked) or rbB2 (checked) has the "new, correct" value.
• Abstract the radio button groups first by defining a radio group control that exposes only one SelectedIndex property. This property can then be conveniently bound to my vm.Value property from all instances of my radio group control. While feasible, it requires writing a new control class and I wonder whether this is the only way in WPF.
• Bind one set of radio buttons to another one: By two-way-binding rbB1 to rbA1, rbB2 to rbA2, and so on, and using a multi-binding between my vm.Value and the first set of radio buttons only, I would achieve the desired effect, but I don't like the notion of having a "master radio group". It would be abusing GUI elements for data transfer.
• Do everything in code-behind and manually update radio buttons and view-model value. Of course this is a viable fallback solution, but this feels like it should be feasible in Xaml/with bindings.

Answer 1

Using complex multibinding with converters and codebehind will not only make your code harder to debug but even harder to test. In my opinion it's better to express each set of radio buttons (flags) as a view model. Evaluate your value when any of the radio buttons is checked/unchecked.

RadioButtonGroup

public class RadioButtonGroup : ViewModel {

    public RadioButtonGroup(string groupName, int count, Action<bool[]> whenAnyChanaged = null) {

        RadioButtons = Enumerable.Range(0, count).Select(_ => {
            var button = new RadioButton { GroupName = groupName };
            button.PropertyChanged += (s, e) => {
                if (e.PropertyName == "IsChecked")
                    whenAnyChanaged(Flags);
            };
            return button;
        }).ToList();           
    }

    public List<RadioButton> RadioButtons { get; private set; }

    public bool[] Flags { get { return RadioButtons.Select(rb => rb.IsChecked).ToArray(); } }
}

RadioButton

 public class RadioButton : ViewModel {

    private bool isChecked;

    public bool IsChecked {
        get { return isChecked; }
        set { SetProperty(ref this.isChecked, value); }
    }

    public string GroupName { get; set; }
}

MainViewModel

public class MainViewModel : ViewModel {
    public MainViewModel() {
        GroupA = new RadioButtonGroup("A", 10, flags => GroupToggle(flags, GroupB.Flags));
        GroupB = new RadioButtonGroup("B", 10, flags => GroupToggle(GroupA.Flags, flags));
    }

    public RadioButtonGroup GroupA { get; private set; }

    public RadioButtonGroup GroupB { get; private set; }

    void GroupToggle(bool[] groupA, bool[] groupB) {
        MyValue = Evaluate(groupA, groupB);
    }
}

View

<Window x:Class="WpfLab.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="{Binding Title}" Height="350" Width="525">
<Window.Resources>
    <DataTemplate x:Key="RadioButton">
        <RadioButton IsChecked="{Binding IsChecked, Mode=OneWayToSource}" GroupName="{Binding GroupName}"/>
    </DataTemplate>
</Window.Resources>
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="30"/>
        <RowDefinition Height="30"/>
    </Grid.RowDefinitions>

    <ListBox Grid.Row="0" ItemsSource="{Binding GroupA.RadioButtons}" ItemTemplate="{StaticResource ResourceKey=RadioButton}">
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel Orientation="Horizontal"/>
            </ItemsPanelTemplate>
        </ListBox.ItemsPanel>
    </ListBox>

    <ListBox Grid.Row="1" ItemsSource="{Binding GroupB.RadioButtons}" ItemTemplate="{StaticResource ResourceKey=RadioButton}">
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel Orientation="Horizontal"/>
            </ItemsPanelTemplate>
        </ListBox.ItemsPanel>
    </ListBox>
</Grid>

Answer 2

Bind VMEnum to each RadioButton seperately(use single two-way binding). Each binding should have CommandParameter it's enum. Like:

<RadioButton IsChecked="{Binding VMEnum, Converter={StaticResource EnumConverter}, ConverterParameter={Enums:VMEnums.FirstRadioButtonGroupA}}" />

In the converter,

• Convert should return correct value(true/false) depending of the VMEnum and COmmandParameter. Essentially the logic is VMEnum == (YourEnum)CommandParameter.
• ConvertBack should return correct Enum based on IsChecked. If IsChecked is true, return the correct enum. Otherwise return Binding.DoNothing which will abort the binding for that specific case.