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Multiple-Selection ComboBox for Silverlight

, 12 Mar 2013
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A tutorial on how to create a Multiple-Selection ComboBox for Silverlight

Introduction

ComboBox is a widely used control. Sometimes, we want to go beyond its default capability, such as multiple-selection. In this article, I will walk through with you how to create a multiple-selection ComboBox for Silverlight. The same approach applies to WPF. I will use Expression Blend for design. It is efficient to use the Blend to create a resource skeleton; and you can use either Blend or Visual Studio to edit the code.

In this updated version, I answered the most asked question: How to display the selected items?

Step by Step

The goal is to create a multiple-selection ComboBox as shown in the left side of following figure. The items in the dropdown should display with CheckBoxes. The right side ListBox demonstrates the same XAML code can be used for a multiple selection ListBox. One stone kills two birds!

Assume you have a ComboBox in place, and want to make it supporting multiple-selection. First, right-click the ComboBox to popup the context menu. In the menu, select Edit Template -> Edit a Copy. This will open the Create Style Resource dialog box. You can rename the default Name (Key) in the dialog. And then click OK to close the dialog.

Now you get the default template of the ComboBox in your .xaml file. You can browse the details in the following window, and figure out how the ComboBox works: when you click the ComboBox, it pops up a Popup control. By default, the Popup control uses an ItemsPresenter to display the SourceItems of the ComboBox. We need to replace it with a ListBox which supports multiple-selection.

Tips

  • The Windows theme setting will effect on the Blend generated template. I suggest you avoid Basic Themes.
  • The Blend generated template could be slightly different with different versions of Silverlight. If you cannot make the ComboBox template generated with Silverlight 3 work with Silverlight 4, regenerate it Silverlight 4.

We also need to the replace the ListBoxItem with CheckBox so that the user can easily identify the selection status. It would be efficient to do this in the XAML window. Let’s create the CheckBoxListBoxItemStyle style as follows:

<Style x:Key="CheckBoxListBoxItemStyle" TargetType="ListBoxItem">
    <Setter Property="Foreground" Value="#FF000000" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ListBoxItem">
                <Grid x:Name="RootElement">
                    <CheckBox ClickMode="Press" Content="{Binding Path=Name}"
                          IsChecked="{Binding Path=IsSelected, Mode=TwoWay}" />
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style> 

Make sure you set ClickMode="Press"; and Mode=TwoWay.

The data I am using here is very simple: a Name property, and an IsSelected property.

 

Set the ItemContainerStyle in the ListBox to the CheckBoxListBoxItemStyle as follows:

<ListBox x:Name="lstBox" SelectionMode="Multiple"
                 ItemsSource="{TemplateBinding ItemsSource}"
                 ItemContainerStyle="{StaticResource CheckBoxListBoxItemStyle}"
                 HorizontalAlignment="Stretch" />

Now you can build and run the project. In the demo page, I included an additional ListBox, which is bound to the same data. The ListBox reuses the same style CheckBoxListBoxItemStyle. With the ListBox, you can validate that the selection is updated on both the ComboBox and the ListBox.

Now it’s time to create a Resource Dictionary file, and move the resource (templates and styles) to the Resource Dictionary.

It seems ambiguous to display the selected items in the collapsed ComboBox, which is designed to display a single item. Here we need the creativity of the User Interface design. For instance, we can reduce the width of the ComboBox, and use a separate ItemsControl to display the selected items. The ItemsControl binds to the same data as ComboBox, but displays selected items only. We can implement a converter class with IValueConverter to convert the IsSelected bool property to Visibility. In this way, the ItemsControl displays selected items only.

<ItemsControl ItemsSource="{Binding Data}" >
<ItemsControl.ItemTemplate>
              <DataTemplate>
                    <TextBlock Text="{Binding Name}"
			Visibility="{Binding IsSelected,
			Converter={StaticResource BoolToVisibilityConverter}}" />
              </DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>

Further, we can use the Tooltip to display the selected items. The following figure shows the Tooltip with an ItemsControl embedded within.

Following is the code snippet for how to embed an ItemsControl in the Tooltip:

<ComboBox ItemsSource="{Binding Data}" Width="26" VerticalAlignment="Center"
	Style="{StaticResource MultiSelComboBoxStyle}" >
    <ToolTipService.ToolTip>
        <ToolTip>
            <ItemsControl ItemsSource="{Binding Data}" Margin="4" >
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <TextBlock Text="{Binding Name}"
			Visibility="{Binding IsSelected,
			Converter={StaticResource BoolToVisibilityConverter}}" />
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </ToolTip>
    </ToolTipService.ToolTip>
</ComboBox>

Conclusion

The ComboBox contains a Popup control for hosting the item selection control. You can replace the default one to customize the behavior, such as multiple selection. Technically, you can embed a UserControl (or almost any control) in the Popup control. This opens the door for creating more sophisticated User Interface. For detail. please read How to Popup Anything? 

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Frank W. Wu
Technical Lead
United States United States
No Biography provided

Comments and Discussions

 
QuestionAdd a "Title" to the closed ComboBox PinmemberNicolas Cordouan12-Mar-13 0:44 
QuestionUnable to populate labels/text/ PinmemberRavichandran M. Kaushika3-Dec-12 12:03 
AnswerRe: Unable to populate labels/text/ PinmemberFrank W. Wu3-Dec-12 14:21 
GeneralRe: Unable to populate labels/text/ PinmemberRavichandran M. Kaushika4-Dec-12 5:28 
GeneralMy vote of 3 PinmemberUday P.Singh11-Jun-12 6:45 
GeneralRe: My vote of 3 Pinmembernare s14-Mar-13 2:55 
QuestionHow to get the selected index changed event fired? PinmemberUday P.Singh8-Jun-12 23:25 
AnswerRe: How to get the selected index changed event fired? PinmemberFrank W. Wu11-Jun-12 3:26 
GeneralRe: How to get the selected index changed event fired? Pinmemberdbernhardt14-Jun-12 5:44 
I anxiously await your post.
 
I tried adding a SelectionChanged event handler to the Combo object. The routine was never called.
 
I tried adding a Click event handler to the CheckBoxListBoxStyle/ControlTemplate in MultiSelComboBoxStyle.xaml --
CheckBox ClickMode=Press .. Click=CheckBox_Click
I get the error below at runtime when I click one of the checkboxes.
Failed to assign to property 'System.Windows.Controls.Primitives.ButtonBase.Click
no matter how I define CheckBox_Click in MainPage.xaml.cs --
public void CheckBox_Click(object sender, ButtonBase e) { }
public void CheckBox_Click(object sender, RoutedEventArgs e) {}
 
I tried creating a custome event handler routine in ViewModel.cs to handle the MyData.PropertyChanged event, but didn't figure out how to wire it to the events.
SuggestionRe: How to get the selected index changed event fired? PinmemberFrank W. Wu10-Jul-12 8:35 
GeneralRe: How to get the selected index changed event fired? Pinmemberdbernhardt18-Jul-12 4:08 
GeneralRe: How to get the selected index changed event fired? [modified] PinmemberFrank W. Wu18-Jul-12 8:12 
QuestionCodeplex project PinmemberAdriaan Davel29-Mar-12 20:27 
GeneralMy vote of 5 PinmemberLR___1-Dec-11 7:36 
QuestionCustomize your great control PinmemberMr. J.4-Aug-11 21:10 
AnswerRe: Customize your great control PinmemberFrank W. Wu7-Aug-11 3:43 
GeneralRe: Customize your great control PinmemberMr. J.6-Sep-11 20:44 
GeneralRe: Customize your great control PinmemberFrank W. Wu7-Sep-11 2:41 
GeneralMy vote of 3 PinmemberJohnny J.2-May-11 3:28 
GeneralRe: My vote of 3 PinmemberShimmy Weitzhandler6-Mar-12 18:17 
GeneralHow to display text on this multiselect combobox?? PinmemberPiyush Nandanwar28-Apr-11 20:33 
GeneralRe: How to display text on this multiselect combobox?? PinmemberFrank W. Wu29-Apr-11 4:28 
GeneralRe: How to display text on this multiselect combobox?? PinmemberJohnny J.2-May-11 3:27 
GeneralRe: How to display text on this multiselect combobox?? PinmemberFrank W. Wu2-May-11 6:47 
QuestionRe: How to display text on this multiselect combobox?? PinmemberShimmy Weitzhandler6-Mar-12 18:16 

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