Nullable ComboBox for Silverlight and WPF

• Download Silverlight version - 50.1 KB
• Download WPF version - 33.1 KB

Introduction

This article describes a simple and easy way to change the current behavior of your Silverlight or WPF ComboBox by using an attached property and a ComboBox style.

Here is a live example of the Silverlight version.

All that you need to do is set the IsNullable property to True on the ComboBox that you want to have this behavior:

<ComboBox ItemsSource="{Binding Source={StaticResource ViewModel}, Path=Actors}"
          library:Combobox.IsNullable="True" />

Using the code

First, you need to change the default ComboBox style so that it will contain the X button. You can find the default style for Silverlight here and for WPF here. You need to make the following changes to the Template property of the ComboBox:

<ContentPresenter x:Name="ContentPresenter" Margin="{TemplateBinding Padding}"  
        HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" 
        VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
    <TextBlock Text=" " />
</ContentPresenter>
<Button x:Name="PART_ClearButton" Visibility="Collapsed" 
    HorizontalAlignment="Right" Margin="0,0,20,0">
    <Path Data="M0,0 L1.6,0 L3,1.56 L4.4,0 L6,0 L3.8,2.5 L6,5 L4.4,5 L3,
                3.49 L1.59,5 L-4.2E-09,5 L2.18,2.5 z" 
      Fill="#CC111111" Height="5" 
      Stretch="Fill" Width="7"/>
</Button>

<TextBox x:Name="PART_EditableTextBox" Style="{x:Null}"
        Template="{StaticResource ComboBoxTextBox}"
        HorizontalAlignment="Left"
        VerticalAlignment="Bottom"
        Margin="3,3,23,3"
        Focusable="True"
        Background="Transparent"
        Visibility="Hidden"
        IsReadOnly="{TemplateBinding IsReadOnly}" />
<Button x:Name="PART_ClearButton" 
        Visibility="Collapsed" 
        HorizontalAlignment="Right" 
        Margin="0,0,22,0">
    <Path Data="M0,0 L1.6,0 L3,1.56 L4.4,0 L6,0 L3.8,2.5 L6,5 L4.4,
                5 L3,3.49 L1.59,5 L-4.2E-09,5 L2.18,2.5 z" 
      Fill="#CC111111" Height="5" 
      Stretch="Fill" Width="7"/>
</Button>

When you have completed these changes, you need to create the IsNullable attached property that will do all the work. When this property is set to True on a ComboBox, it will enable the nullable functionality.

The nullable functionality is obtained by using these two functions:

private static void ApplyIsNullable(ComboBox comboBox)
{
    var isNullable = GetIsNullable(comboBox);
    var clearButton = (Button)GetClearButton(comboBox);
    if (clearButton != null)
    {
        clearButton.Click -= clearButton_Click;
        clearButton.Click += clearButton_Click;

        if (isNullable && comboBox.SelectedIndex != -1)
        {
            clearButton.Visibility = Visibility.Visible;
        }
        else
        {
            clearButton.Visibility = Visibility.Collapsed;
        }
    }
}
 
private static void clearButton_Click(object sender, RoutedEventArgs e)
{
    var clearButton = (Button)sender;
    var parent = VisualTreeHelper.GetParent(clearButton);

    while (!(parent is ComboBox))
    {
        parent = VisualTreeHelper.GetParent(parent);
    }

    var comboBox = (ComboBox)parent;
    //clear the selection
    comboBox.SelectedIndex = -1;
}

The ApplyIsNullable function refreshes the X button state and is called whenever the selection of the ComboBox changes. The clearButton_Click function clears the ComboBox selection and is called when someone clicks the X button.

Before we finish, let's make the X button look better by adding a MouseOver animation by using this custom button style:

<Style x:Key="ClearSelectionButtonStyle" TargetType="Button">
    <Setter Property="Background" Value="#FF3C688D"/>
    <Setter Property="BorderBrush" Value="#FF617584"/>
    <Setter Property="BorderThickness" Value="1"/>
    <Setter Property="Width" Value="15"/>
    <Setter Property="Height" Value="15"/>
    <Setter Property="Padding" Value="0"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="Button">
                <Grid Background="Transparent">
                    <VisualStateManager.VisualStateGroups>
                        <VisualStateGroup x:Name="CommonStates">
                            <VisualState x:Name="Normal"/>
                            <VisualState x:Name="MouseOver">
                                <Storyboard>
                                    <ObjectAnimationUsingKeyFrames Duration="0" 
                                          Storyboard.TargetProperty="(UIElement.Visibility)" 
                                          Storyboard.TargetName="MouseOverElement">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <Visibility>Visible</Visibility>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Pressed">
                                <Storyboard>
                                    <ObjectAnimationUsingKeyFrames Duration="0" 
                                           Storyboard.TargetProperty="(UIElement.Visibility)" 
                                           Storyboard.TargetName="MouseOverElement">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <Visibility>Visible</Visibility>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Disabled"/>
                        </VisualStateGroup>
                        <VisualStateGroup x:Name="FocusStates">
                            <VisualState x:Name="Focused"/>
                            <VisualState x:Name="Unfocused"/>
                        </VisualStateGroup>
                    </VisualStateManager.VisualStateGroups>
                    <Border x:Name="MouseOverElement" 
                        BorderThickness="{TemplateBinding BorderThickness}" 
                        Background="#FFC8E4ED" BorderBrush="#FF3F6A8E" 
                        Visibility="Collapsed"/>
                    <ContentPresenter x:Name="contentPresenter" 
                      ContentTemplate="{TemplateBinding ContentTemplate}" 
                      Content="{TemplateBinding Content}" 
                      HorizontalAlignment="Center" 
                      Margin="{TemplateBinding Padding}" 
                      VerticalAlignment="Center"/>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

That's it! :)

History

• Version 1.