ComboBox that supports the "empty value" item

Question

I want a combobox where a user can make a selection of a list of items, including a default one (none). I don't want the developer that uses the combobox to add the empty item to the combobox manually.

Therefore, I tried something like this (sorry for the long code). I will first explain my problem further, the the code is at the bottom of this post.

What I actually do is bind the itemssource to a binding or dynamic resource. Then, when the ItemsSource changes, I store the original items source and create my own with the none item. This works great the first time. However, then I loose the connection with the DynamicResource and thus I am not aware of future updates. I have used reflector to see what happens under the hood, but as always, the part I need is internal... Is there a better way to implement my requirements or is there a way I can subscribe to changes of a dynamic resource.

    /// <summary>
    /// Implements an advanced combobox with the following features:
	/// 
	/// * Automatically selects the value in case that there is only one 
	///   value available in the items source.
	/// * Supports the MaxLength property.
    /// </summary>
    public class ComboBoxEx : System.Windows.Controls.ComboBox
    {
        #region Variables
		private ComboBoxItem _emptyItem = null;
		private IEnumerable _originalItemsSource = null;
		private ObservableCollection<object> _customItemsSource = new ObservableCollection<object>();
        #endregion

        #region Constructor & destructor
		/// <summary>
		/// Initializes a new instance of this control.
		/// </summary>
		public ComboBoxEx()
		{
			// Set default values
			_emptyItem = new ComboBoxItem();
		}
        #endregion

        #region Properties
		/// <summary>
		/// Gets or sets whether the control is currently changing the items source.
		/// </summary>
		private bool IsChangingItemsSource { get; set; }

		/// <summary>
		/// Wrapper for the EmptyValueText dependency property.
		/// </summary>
		public string EmptyValueText
		{
			get { return (string)GetValue(EmptyValueTextProperty); }
			set { SetValue(EmptyValueTextProperty, value); }
		}

		// Using a DependencyProperty as the backing store for EmptyValueText.  This enables animation, styling, binding, etc...
		public static readonly DependencyProperty EmptyValueTextProperty = DependencyProperty.Register("EmptyValueText", typeof(string),
			typeof(ComboBoxEx), new UIPropertyMetadata(Properties.Resources.Empty, new PropertyChangedCallback(EmptyValueText_Changed)));
       #endregion

        #region Methods
		/// <summary>
		/// Invoked when the EmptyValueText dependency property has changed.
		/// </summary>
		/// <param name="sender">The object that contains the dependency property.</param>
		/// <param name="e">The event data.</param>
		private static void EmptyValueText_Changed(DependencyObject sender, DependencyPropertyChangedEventArgs e)
		{
			// Get typed sender
			ComboBoxEx typedSender = sender as ComboBoxEx;
			if (typedSender != null)
			{
				// Update content
				typedSender._emptyItem.Content = e.NewValue;
			}
		}

		/// <summary>
		/// Invoked when the original items source has changed.
		/// </summary>
		/// <param name="sender">Sender.</param>
		/// <param name="e">EventArgs.</param>
		private void OriginalItemsSource_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
		{
			// Update items source
			UpdateItemsSource();
		}

        /// <summary>
        /// Invoked when the ItemsSource-property has changed.
        /// </summary>
        /// <param name="oldValue">Old value.</param>
        /// <param name="newValue">New value.</param>
        protected override void OnItemsSourceChanged(System.Collections.IEnumerable oldValue, System.Collections.IEnumerable newValue)
        {
			// Call base
			base.OnItemsSourceChanged(oldValue, newValue);
			
			// Are we already changing?
			if (IsChangingItemsSource) return;

			// We are changing
			IsChangingItemsSource = true;

			// Store original itemssource
			_originalItemsSource = newValue;
			if (_originalItemsSource is INotifyCollectionChanged)
			{
			    // Subscribe to changes
			    ((INotifyCollectionChanged)_originalItemsSource).CollectionChanged += new NotifyCollectionChangedEventHandler(OriginalItemsSource_CollectionChanged);
			}

			// Update items source
			UpdateItemsSource();

			// Not changing any longer
			IsChangingItemsSource = false;
        }

		/// <summary>
		/// Updates the itemssource.
		/// </summary>
		private void UpdateItemsSource()
		{
			// Clear custom items source
			_customItemsSource.Clear();

			// Enable empty value if required
			_customItemsSource.Add(_emptyItem);

			// Fill custom items source
			if (_originalItemsSource != null)
			{
				foreach (object obj in _originalItemsSource)
				{
					_customItemsSource.Add(obj);
				}
			}

			// Update itemssource
			ItemsSource = _customItemsSource;
		}
        #endregion
    }

---

Geert van Horrik - CatenaLogic
Visit my blog: http://blog.catenalogic.com

Looking for a way to deploy your updates to all your clients? Try Updater!

Answer 1

(Please don't copy code directly from VS. Use '</>' icon to insert code snippet.)

You don't need all that. Do you need more than this:

<Window x:Class="MSDNTest.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:MSDNTest"
    Width="500" Height="500" >
    
    <Window.Resources>
        <Style TargetType="{x:Type local:EmptyItemComboBox}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type local:EmptyItemComboBox}">
                        <ComboBox>
                            <ComboBox.Resources>
                                <CollectionViewSource x:Key="list" Source="{Binding RelativeSource={RelativeSource AncestorType={x:Type local:EmptyItemComboBox}}, Path=ItemsSource}"/>
                            </ComboBox.Resources>
                            <ComboBox.ItemsSource>
                                <CompositeCollection>
                                    <ComboBoxItem Content="{TemplateBinding EmptyItemText}"/>
                                    <CollectionContainer Collection="{Binding Source={StaticResource list}}"/>
                                </CompositeCollection>
                            </ComboBox.ItemsSource>
                        </ComboBox>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>

    <StackPanel>
        <local:EmptyItemComboBox x:Name="combo1" EmptyItemText="Choose one"/>
        <local:EmptyItemComboBox x:Name="combo2" EmptyItemText="What ever"/>
    </StackPanel>
</Window><br/>

using System.Collections.Generic;
using System.Windows;
using System.Windows.Controls;

namespace MSDNTest
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
            var list1 = new List<MyClass1>();
            for (var i = 1; i <= 5; i++)
                list1.Add(new MyClass1 { Name = "My Class1 " + i });
            combo1.ItemsSource = list1;

            var list2 = new List<MyClass1>();
            for (var i = 1; i <= 5; i++)
                list2.Add(new MyClass1 { Name = "My Class2 " + i });
            combo2.ItemsSource = list2;
        }
    }

    public class EmptyItemComboBox : ComboBox
    {
        public static readonly DependencyProperty EmptyItemTextProperty = DependencyProperty.Register("EmptyItemText",
                                                                            typeof(string),
                                                                            typeof(EmptyItemComboBox));

        public string EmptyItemText
        {
            get { return (string)GetValue(EmptyItemTextProperty); }
            set { SetValue(EmptyItemTextProperty, value); }
        }
    }

    public class MyClass1
    {
        public string Name { get; set; }
        public override string ToString() { return Name; }
    }

    public class MyClass2
    {
        public string Name { get; set; }
        public override string ToString() { return Name; }
    }
}

---

Bigsby, Lisboa, Portugal
O que for, quando for, é que será o que é...
Wenn ist das Nunstruck git und Slotermeyer? Ja! ... Beiherhund das Oder die Flipperwaldt gersput!
http://bigsby.eu

Answer 2

I did use the "Insert code block" tool button. Apparently it can't handle comments, so sorry for the messed up code.

But, back on topic, thanks for your help :)

---

Geert van Horrik - CatenaLogic
Visit my blog: http://blog.catenalogic.com

Looking for a way to deploy your updates to all your clients? Try Updater!

Answer 3

Hi,

Great code.  I'm having a problem getting the SelectedValue and SelectedItem.  It seems the ItemsSource property is being changed to IEnumerable of objects from a Dictionary and no longer recognises the Key and Value property names that I'm supplying in DisplayMemberPath and SelectedValuePath.

Any ideas?

Thanks,

Colm

Answer 4

Bind inner ComboBox DisplayMemberPath and SelectedValuePath to TemplateBinding. Something like so:

<ControlTemplate ... >
  <ComboBox
    DisplayMemberPath="{TemplateBinding DisplayMemberPath}"
    SelectedValuePath="{TemplateBinding SelectedValuePath}">
  </ComboBox>
</ControlTemplate>

---

Bigsby, Lisboa, Portugal
O que for, quando for, é que será o que é...
Wenn ist das Nunstruck git und Slotermeyer? Ja! ... Beiherhund das Oder die Flipperwaldt gersput!
http://bigsby.eu

Answer 5

The problem is that TemplateBinding is a oneway binding, and that is not sufficient in all cases. I have created a blog post for the control I created.

---

Geert van Horrik - CatenaLogic
Visit my blog: http://blog.catenalogic.com

Looking for a way to deploy your updates to all your clients? Try Updater!

Answer 6

Thanks for the replies.  I'll have a look at that blog - been pulled onto another project so will let you know if I get sorted whenever I get back to this...if ever. :)

Thanks again.

Colm