Question

Before any item in a ComboBox is selected, its SelectedItemis null and the ComboBox itself is visually blank. Once something is selected, there doesn’t seem to be any way for the user to select “the absence of a selection” (though it can be done by setting SelectedItem to null in code).

My ComboBoxes are bound to ObservableCollections of my objects. I don’t want to add a “special” first null-like object to the front of every ObservableCollection. So I’m taking this opportunity to learn a bit about writing a UserControl.

The problem is SelectedItem doesn’t work the way it normally does. That is, the ComboBox is nicely bound to a backing ObservableCollection, but picking something from the ComboBox doesn’t update the SelectedItem it’s supposed to be bound to.

I feel like I need to be passing along some information from the ComboBox in the UserControl to…somewhere. Am I on the right track? What should I be googling for?

C#:

public partial class ClearableComboBox : UserControl
{
    public ClearableComboBox()
    {
        InitializeComponent();
    }

    public IEnumerable ItemsSource
    {
        get { return (IEnumerable)base.GetValue(ItemsSourceProperty); }
        set { base.SetValue(ItemsSourceProperty, value); }
    }

    public static readonly DependencyProperty ItemsSourceProperty =
        DependencyProperty.Register("ItemsSource",
            typeof(IEnumerable),
            typeof(ClearableComboBox));

    public object SelectedItem
    {
        get { return (object)base.GetValue(SelectedItemProperty); }
        set { base.SetValue(SelectedItemProperty, value); }
    }

    public static readonly DependencyProperty SelectedItemProperty =
        DependencyProperty.Register("SelectedItem",
            typeof(object),
            typeof(ClearableComboBox));

    public string DisplayMemberPath
    {
        get { return (string)base.GetValue(DisplayMemberPathProperty); }
        set { base.SetValue(DisplayMemberPathProperty, value); }
    }

    public static readonly DependencyProperty DisplayMemberPathProperty =
        DependencyProperty.Register("DisplayMemberPath",
            typeof(string),
            typeof(ClearableComboBox));

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        comboBox.SelectedItem = null;
    }
}

XAML:

<UserControl x:Class="MyProj.ClearableComboBox"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             x:Name="root">
    <DockPanel>
        <Button DockPanel.Dock="Left" Click="Button_Click" ToolTip="Clear">
            <Image Source="pack://application:,,,/img/icons/silk/cross.png" Stretch="None" />
        </Button>
        <ComboBox
            Name="comboBox"
            ItemsSource="{Binding ElementName=root, Path=ItemsSource}"
            SelectedItem="{Binding ElementName=root, Path=SelectedItem}"
            DisplayMemberPath="{Binding ElementName=root, Path=DisplayMemberPath}" />
    </DockPanel>
</UserControl>

Usage:

<wpfControl:ClearableComboBox ItemsSource="{Binding Path=Things}"
                              DisplayMemberPath="SomeProperty"
                              SelectedItem="{Binding Path=SelectedThing}" />

// Picking a Thing doesn't update SelectedThing :(

Answer 1

Since combobox derives from Selector class which in turn derives from ItemsControl. So, by deriving from UserControl you are devoiding your combobox with properties of Selector class which might internally handle the Selection thing for you. so, i would suggest instead of deriving it from UserControl, you should derive it from Combobox like this –

public partial class ClearableComboBox : ComboBox

So, that ways you won’t have to override the ItemsSource, DisplayMemberPath etc. in your class since it s already present in the ComboBox class. You can always extend your class further to provide addidtional features which is in your case setting the SelectedItem to null on some button click. Hope this is what you want..

EDIT (Custom Control)

Creating a Custom Control is your answer here, to get started if you are not aware of it, look at this for start – http://www.wpftutorial.net/HowToCreateACustomControl.html

When you create a Custom Control say CustomControl1, replace the template for CustomControl1 in your Generic.xaml file with this one –

<ControlTemplate TargetType="{x:Type local:CustomControl1}">
    <Border Background="{TemplateBinding Background}"
            BorderBrush="{TemplateBinding BorderBrush}"
            BorderThickness="{TemplateBinding BorderThickness}">
              <DockPanel>
                 <Button Name="btn" DockPanel.Dock="Left" ToolTip="Clear" Width="20">
                    <Image Source="pack://application:,,,/img/icons/silk/cross.png" Stretch="None" />
                </Button>
                <ComboBox Name="comboBox"
                          ItemsSource="{TemplateBinding ItemsSource}"
                          SelectedItem="{TemplateBinding SelectedItem}"
                          DisplayMemberPath="{TemplateBinding DisplayMemberPath}" />
              </DockPanel>
     </Border>
</ControlTemplate>

By default your CustomControl1 class will be derived from Control. Replace it to derive from class ComboBox so that you don’t have declare DP’s yet over again like this and copy paste this code there –

public class CustomControl1 : ComboBox
{
        private Button clearButton;
        private ComboBox comboBox;

        static CustomControl1()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(CustomControl1), new FrameworkPropertyMetadata(typeof(CustomControl1)));
        }

        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            clearButton = GetTemplateChild("btn") as Button;
            comboBox = GetTemplateChild("comboBox") as ComboBox;
            clearButton.Click += new RoutedEventHandler(clearButton_Click);
        }

        private void clearButton_Click(object sender, RoutedEventArgs e)
        {
            comboBox.SelectedItem = null;
        }
}

Now, your CustomControl1 class is ready for use in your other xaml files like this –

<local:CustomControl1 ItemsSource="{Binding YourSource}"
                      SelectedItem="{Binding YourSelectedItem}"
                      Height="50" Width="200"/>