Nullable ComboBox for Silverlight and WPF
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This post 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.
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.