WPF Custom Control, Combine multiple ComboBox values into single DependencyProperty

Question

I have a custom control with two ComboBoxes with hard coded ComboBoxItems in them and I would like to be able to bind to the values via a single DependencyProperty but can't seem to get the bindings working. Maybe my approach is way off, but how should I do to get it working?

Generic.xaml:

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:MyApp.Components">
    <Style TargetType="{x:Type local:ContractLimitSelector}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:ContractLimitSelector}">
                    <ControlTemplate.Resources>
                        <BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
                    </ControlTemplate.Resources>
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="auto" />
                            <ColumnDefinition Width="auto" />
                        </Grid.ColumnDefinitions>
                        <ComboBox x:Name="PART_ComboAA" Grid.Column="0" SelectedValue="{TemplateBinding SelectedAA}" />
                        <ComboBox x:Name="PART_ComboFA" Grid.Column="1" SelectedValue="{TemplateBinding SelectedFA}" />
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

Custom Control:

public class SelectorItems
{
    public string AA { get; set; }
    public string FA { get; set; }
}

public class ContractLimitSelector : Control
{
    private static List<string> ComboBoxSource = new List<string>()
    {
        "-",
        "BE",
        "EE",
    };

    static ContractLimitSelector()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(ContractLimitSelector), new FrameworkPropertyMetadata(typeof(ContractLimitSelector)));
    }

    public override void OnApplyTemplate()
    {
        var comboAA = Template.FindName("PART_ComboAA", this) as ComboBox;
        var comboFA = Template.FindName("PART_ComboFA", this) as ComboBox;

        comboAA.ItemsSource = ComboBoxSource;
        comboFA.ItemsSource = ComboBoxSource;

        base.OnApplyTemplate();
    }

    private SelectorItems GetAllValues()
    {
        var AA = (string)GetValue(SelectedAAProperty);
        var FA = (string)GetValue(SelectedFAProperty);

        return new SelectorItems() { AA = AA, FA = FA };
    }

    public static readonly DependencyProperty VisibleHeaderProperty =
        DependencyProperty.Register("VisibleHeader", typeof(bool), typeof(ContractLimitSelector),
            new PropertyMetadata(false));

    public bool VisibleHeader
    {
        get { return (bool)GetValue(VisibleHeaderProperty); }
        set { SetValue(VisibleHeaderProperty, value); }
    }

    public static readonly DependencyProperty SelectedAAProperty =
        DependencyProperty.Register("SelectedAA", typeof(string), typeof(ContractLimitSelector),
            new PropertyMetadata(string.Empty));

    public string SelectedAA
    {
        get { return (string)GetValue(SelectedAAProperty); }
        set
        {
            var all = GetAllValues();
            SetValue(SelectedValuesProperty, new SelectorItems() { AA = value, FA = all.FA, });
            SetValue(SelectedAAProperty, value);
        }
    }

    public static readonly DependencyProperty SelectedFAProperty =
        DependencyProperty.Register("SelectedFA", typeof(string), typeof(ContractLimitSelector),
            new PropertyMetadata(string.Empty));

    public string SelectedFA
    {
        get { return (string)GetValue(SelectedFAProperty); }
        set
        {
            var all = GetAllValues();
            SetValue(SelectedValuesProperty, new SelectorItems() { AA = all.AA, FA = value, });
            SetValue(SelectedFAProperty, value);
        }
    }

    public SelectorItems SelectedValues
    {
        get
        {
            return new SelectorItems()
            {
                AA = (string)GetValue(SelectedAAProperty),
                FA = (string)GetValue(SelectedFAProperty),
            };
        }
        set
        {
            SetValue(SelectedAAProperty, value.AA);
            SetValue(SelectedFAProperty, value.FA);
        }
    }

    public static readonly DependencyProperty SelectedValuesProperty =
        DependencyProperty.Register("SelectedValues", typeof(SelectorItems), typeof(ContractLimitSelector),
            new PropertyMetadata(new SelectorItems()));
}

I use it like this: XAML:

<custom:ContractLimitSelector SelectedValues="{Binding PrepareLockHouseValue, Mode=TwoWay}" />

ViewModel:

private SelectorItems _contractValues;
public SelectorItems ContractValues
{
    get
    {
        return _contractValues;
    }
    set
    {
        _contractValues = value;
        OnPropertyChanged(nameof(ContractValues));
    }
}

I thought using SetValue whenever any of the ComboBoxes change would trigger the "shared" SelectedValues DependecyProperty, but nothing ever gets triggered...

What am I missing?

Answer 1

The SelectedAA and SelectedFA property setters are not called when you call

SetValue(SelectedAAProperty, value.AA);
SetValue(SelectedFAProperty, value.FA);

You would have to write

SelectedAA = value.AA;
SelectedFA = value.FA;

Better move all code except

SetValue(SelectedAAProperty, value);

and

SetValue(SelectedFAProperty, value);

out of the setters and add PropertyChangedCallbacks to execute the other code.

The properties would then also properly work with data bindings and other possible value sources.

---

In general, the get and set methods of the CLR wrapper of a dependency property must not contain anything else than the GetValue and SetValue calls, because get/set may be bypassed by the framework.

See XAML Loading and Dependency Properties for details.