WPF/XAML - compare the "SelectedIndex" of two comboboxes (DataTrigger?)

Question

hello,

i've got two comboboxes with the same content. the user should not be allowed to choose the same item twice. therefore the comboboxes' contents (= selectedindex?) should never be equal.

my first attempt was to comapare the selectedindex with a datatrigger to show/hide a button:

<DataTrigger Binding="{Binding ElementName=comboBox1, Path=SelectedIndex}" Value="{Binding ElementName=comboBox2, Path=SelectedIndex}">
     <Setter Property="Visibility" Value="Hidden" />
</DataTrigger>

it seems that it is not possible to use Value={Binding}. is there any other way (if possible without using a converter)? thanks in advance!

Answer 1

Option 1

You could use ValidationRules -- it can also be done in XAML, and will work for a one-off situation. It would be extremely localized and not something I would recommend since the rule would not be reusable. Maybe somebody else can come up with a generic rule to encompass different inputs. Try this out.

<ComboBox>
    <ComboBox.SelectedValue>
        <Binding Path="Whatever" UpdateSourceTrigger="PropertyChanged">
            <Binding.ValidationRules>
                <local:ComparisonValidationRule />
            </Binding.ValidationRules>
        </Binding>
    </ComboBox.SelectedValue>
</ComboBox>

And maybe the ComparisonRule looks like this, and it would have to be in the code-behind for that rule to see the controls in the visual tree.

public class ComparisonValidationRule : ValidationRule
    {
        public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
        {
            if (this.firstComboBox.SelectedIndex == this.secondComboBox.SelectedIndex)
                return new ValidationResult(false, "These two comboboxes must supply different values.");
            else return new ValidationResult(true, null);
        }
    }

OR you could definitely do it with a trigger if you wanted to set some interesting things outside of an error template.

Option 2

Use a trigger & converter.. It's really not too difficult. Here's how I would do it.

    <Style x:Key="{x:Type ComboBox}" TargetType="{x:Type ComboBox}">
        <Style.Triggers>
            <DataTrigger Value="True">
                <DataTrigger.Binding>
                    <MultiBinding Converter="{StaticResource EqualsConverter}">
                        <Binding ElementName="cbOne" Path="SelectedIndex"/>
                        <Binding ElementName="cbTwo" Path="SelectedIndex"/>
                    </MultiBinding>
                </DataTrigger.Binding>
                <Setter Property="Background" Value="Yellow"/>
            </DataTrigger>
        </Style.Triggers>
    </Style>

and the converter in code-behind

public class EqualsConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType,
                          object parameter, CultureInfo culture)
    {
        if (values[0] is int && values[1] is int && values[0] == values[1])
            return true;
        return false;
    }

    public object[] ConvertBack(object value, Type[] targetTypes,

                                object parameter, CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}

Answer 2

If the ComboBoxes share the same itemsource, set a flag on the underlying data object when an item is selected via the first ComboBox.

In the datatemplate for the data object displayed in the second combobox write a datatrigger that binds to that property and does something appropriate.

Ensure your binding is TwoWay for the first ComboBox.