ComboBox ItemsSource changed => SelectedItem is ruined

Question

Ok, this has been bugging me for a while now. And I wonder how others handle the following case:

Answer 1

Unfortunately when setting ItemsSource on a Selector object it immediately sets SelectedValue or SelectedItem to null even if corresponding item is in new ItemsSource.

No matter if you implement Equals.. functions or you use a implicitly comparable type for your SelectedValue.

Well, you can save SelectedItem/Value prior to setting ItemsSource and than restore. But what if there's a binding on SelectedItem/Value which will be called twice: set to null restore original.

That's additional overhead and even it can cause some undesired behavior.

Here's a solution which I made. Will work for any Selector object. Just clear SelectedValue binding prior to setting ItemsSource.

UPD: Added try/finally to protect from exceptions in handlers, also added null check for binding.

public static class ComboBoxItemsSourceDecorator
{
    public static readonly DependencyProperty ItemsSourceProperty = DependencyProperty.RegisterAttached(
        "ItemsSource", typeof(IEnumerable), typeof(ComboBoxItemsSourceDecorator), new PropertyMetadata(null, ItemsSourcePropertyChanged)
    );

    public static void SetItemsSource(UIElement element, IEnumerable value)
    {
        element.SetValue(ItemsSourceProperty, value);
    }

    public static IEnumerable GetItemsSource(UIElement element)
    {
        return (IEnumerable)element.GetValue(ItemsSourceProperty);
    }

    static void ItemsSourcePropertyChanged(DependencyObject element, 
                    DependencyPropertyChangedEventArgs e)
    {
        var target = element as Selector;
        if (element == null)
            return;

        // Save original binding 
        var originalBinding = BindingOperations.GetBinding(target, Selector.SelectedValueProperty);

        BindingOperations.ClearBinding(target, Selector.SelectedValueProperty);
        try
        {
            target.ItemsSource = e.NewValue as IEnumerable;
        }
        finally
        {
            if (originalBinding != null)
                BindingOperations.SetBinding(target, Selector.SelectedValueProperty, originalBinding);
        }
    }
}

Here's a XAML example:

                <telerik:RadComboBox Grid.Column="1" x:Name="cmbDevCamera" DataContext="{Binding Settings}" SelectedValue="{Binding SelectedCaptureDevice}" 
                                     SelectedValuePath="guid" e:ComboBoxItemsSourceDecorator.ItemsSource="{Binding CaptureDeviceList}" >
                </telerik:RadComboBox>

Unit Test

Here is a unit test case proving that it works. Just comment out the #define USE_DECORATOR to see the test fail when using the standard bindings.

#define USE_DECORATOR

using System.Collections;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Security.Permissions;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Data;
using System.Windows.Threading;
using FluentAssertions;
using ReactiveUI;
using ReactiveUI.Ext;
using ReactiveUI.Fody.Helpers;
using Xunit;

namespace Weingartner.Controls.Spec
{
    public class ComboxBoxItemsSourceDecoratorSpec
    {
        [WpfFact]
        public async Task ControlSpec ()
        {
            var comboBox = new ComboBox();
            try
            {

                var numbers1 = new[] {new {Number = 10, i = 0}, new {Number = 20, i = 1}, new {Number = 30, i = 2}};
                var numbers2 = new[] {new {Number = 11, i = 3}, new {Number = 20, i = 4}, new {Number = 31, i = 5}};
                var numbers3 = new[] {new {Number = 12, i = 6}, new {Number = 20, i = 7}, new {Number = 32, i = 8}};

                comboBox.SelectedValuePath = "Number";
                comboBox.DisplayMemberPath = "Number";


                var binding = new Binding("Numbers");
                binding.Mode = BindingMode.OneWay;
                binding.UpdateSourceTrigger=UpdateSourceTrigger.PropertyChanged;
                binding.ValidatesOnDataErrors = true;

#if USE_DECORATOR
                BindingOperations.SetBinding(comboBox, ComboBoxItemsSourceDecorator.ItemsSourceProperty, binding );
#else
                BindingOperations.SetBinding(comboBox, ItemsControl.ItemsSourceProperty, binding );
#endif

                DoEvents();

                var selectedValueBinding = new Binding("SelectedValue");
                BindingOperations.SetBinding(comboBox, Selector.SelectedValueProperty, selectedValueBinding);

                var viewModel = ViewModel.Create(numbers1, 20);
                comboBox.DataContext = viewModel;

                // Check the values after the data context is initially set
                comboBox.SelectedIndex.Should().Be(1);
                comboBox.SelectedItem.Should().BeSameAs(numbers1[1]);
                viewModel.SelectedValue.Should().Be(20);

                // Change the list of of numbers and check the values
                viewModel.Numbers = numbers2;
                DoEvents();

                comboBox.SelectedIndex.Should().Be(1);
                comboBox.SelectedItem.Should().BeSameAs(numbers2[1]);
                viewModel.SelectedValue.Should().Be(20);

                // Set the list of numbers to null and verify that SelectedValue is preserved
                viewModel.Numbers = null;
                DoEvents();

                comboBox.SelectedIndex.Should().Be(-1);
                comboBox.SelectedValue.Should().Be(20); // Notice that we have preserved the SelectedValue
                viewModel.SelectedValue.Should().Be(20);


                // Set the list of numbers again after being set to null and see that
                // SelectedItem is now correctly mapped to what SelectedValue was.
                viewModel.Numbers = numbers3;
                DoEvents();

                comboBox.SelectedIndex.Should().Be(1);
                comboBox.SelectedItem.Should().BeSameAs(numbers3[1]);
                viewModel.SelectedValue.Should().Be(20);


            }
            finally
            {
                Dispatcher.CurrentDispatcher.InvokeShutdown();
            }
        }

        public class ViewModel<T> : ReactiveObject
        {
            [Reactive] public int SelectedValue { get; set;}
            [Reactive] public IList<T> Numbers { get; set; }

            public ViewModel(IList<T> numbers, int selectedValue)
            {
                Numbers = numbers;
                SelectedValue = selectedValue;
            }
        }

        public static class ViewModel
        {
            public static ViewModel<T> Create<T>(IList<T> numbers, int selectedValue)=>new ViewModel<T>(numbers, selectedValue);
        }

        /// <summary>
        /// From http://stackoverflow.com/a/23823256/158285
        /// </summary>
        public static class ComboBoxItemsSourceDecorator
        {
            private static ConcurrentDictionary<DependencyObject, Binding> _Cache = new ConcurrentDictionary<DependencyObject, Binding>();

            public static readonly DependencyProperty ItemsSourceProperty = DependencyProperty.RegisterAttached(
                "ItemsSource", typeof(IEnumerable), typeof(ComboBoxItemsSourceDecorator), new PropertyMetadata(null, ItemsSourcePropertyChanged)
            );

            public static void SetItemsSource(UIElement element, IEnumerable value)
            {
                element.SetValue(ItemsSourceProperty, value);
            }

            public static IEnumerable GetItemsSource(UIElement element)
            {
                return (IEnumerable)element.GetValue(ItemsSourceProperty);
            }

            static void ItemsSourcePropertyChanged(DependencyObject element,
                            DependencyPropertyChangedEventArgs e)
            {
                var target = element as Selector;
                if (target == null)
                    return;

                // Save original binding 
                var originalBinding = BindingOperations.GetBinding(target, Selector.SelectedValueProperty);
                BindingOperations.ClearBinding(target, Selector.SelectedValueProperty);
                try
                {
                    target.ItemsSource = e.NewValue as IEnumerable;
                }
                finally
                {
                    if (originalBinding != null )
                        BindingOperations.SetBinding(target, Selector.SelectedValueProperty, originalBinding);
                }
            }
        }

        [SecurityPermission(SecurityAction.Demand, Flags = SecurityPermissionFlag.UnmanagedCode)]
        public static void DoEvents()
        {
            DispatcherFrame frame = new DispatcherFrame();
            Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Background, new DispatcherOperationCallback(ExitFrame), frame);
            Dispatcher.PushFrame(frame);
        }

        private static object ExitFrame(object frame)
        {
            ((DispatcherFrame)frame).Continue = false;
            return null;
        }


    }
}

Answer 2

This is the top google result for "wpf itemssource equals" right now, so to anyone trying the same approach as in the question, it does work as long as you fully implement equality functions. Here is a complete MyItem implementation:

public class MyItem : IEquatable<MyItem>
{
    public int Id { get; set; }

    public bool Equals(MyItem other)
    {
        if (Object.ReferenceEquals(other, null)) return false;
        if (Object.ReferenceEquals(other, this)) return true;
        return this.Id == other.Id;
    }

    public sealed override bool Equals(object obj)
    {
        var otherMyItem = obj as MyItem;
        if (Object.ReferenceEquals(otherMyItem, null)) return false;
        return otherMyItem.Equals(this);
    }

    public override int GetHashCode()
    {
        return this.Id.GetHashCode();
    }

    public static bool operator ==(MyItem myItem1, MyItem myItem2)
    {
        return Object.Equals(myItem1, myItem2);
    }

    public static bool operator !=(MyItem myItem1, MyItem myItem2)
    {
        return !(myItem1 == myItem2);
    }
}

I successfully tested this with a multiple selection ListBox, where listbox.SelectedItems.Add(item) was failing to select the matching item, but worked after I implemented the above on item.

Answer 3

    public MyItem SelectedItem { get; set; }
    private MyItem selectedItem ;
    // <summary>
    ///////
    // </summary>
    public MyItem SelectedItem 
    {
        get { return selectedItem ; }
        set
        {
            if (value != null && selectedItem != value)
            {
                selectedItem = value;
                if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs("SelectedItem ")); }
            }
        }
    }

Answer 4

I just implemented a very simple override and it seems to be working visually, however this cuts off bunch of internal logic, so I'm not sure it's safe solution:

public class MyComboBox : ComboBox 
{
    protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
    {
        return;
    }
}

So if you use this control then changing Items/ItemsSource won't affect SelectedValue and Text - they will remains untouched.

Please let me know if you find problems it causes.

Answer 5

You can consider using a valueconverter to select the correct SlectedItem from your collection

Answer 6

The standard ComboBox doesn't have that logic. And as you mentioned SelectedItem becomes null already after you call Clear, so the ComboBox has no idea about you intention to add the same item later and therefore it does nothing to select it. That being said, you will have to memorize the previously selected item manually and after you've updated you collection restore the selection also manually. Usually it is done something like this:

public void RefreshMyItems()
{
    var previouslySelectedItem = SelectedItem;

    MyItems.Clear();
    foreach(var myItem in LoadItems()) MyItems.Add(myItem);

    SelectedItem = MyItems.SingleOrDefault(i => i.Id == previouslySelectedItem.Id);

}

If you want to apply the same behavior to all ComboBoxes (or perhaps all Selector controls), you can consider creating a Behavior(an attached property or blend behavior). This behavior will subscribe to the SelectionChanged and CollectionChanged events and will save/restore the selected item when appropriate.