如何使用ComboBox的Text属性禁用ItemsSource同步

Question

I use the ComboBox for binding to string property of view model. I choose the ComboBox instead of TextBox, because i want to have an option to choose from the list (as a suggestion), but I don't want to change the selected text if the ItemsSource changes.

I tried to set the IsSynchronizedWithCurrentItem property to false, but when the list of suggestions change (at the position of the selected text), the Text changes to empty. It seems that the ComboBox has remembered that the entered text was also in the list and when this item disappears, the Text property is also cleared.

So my question is: Is that a bug, or am I doing something wrong? If it is a bug, could you suggest some work around?

I created a sample project which preproduces this:

in XAML:

<Window x:Class="TestProject1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <ComboBox IsSynchronizedWithCurrentItem="False" ItemsSource="{Binding Items}"
                  IsEditable="True" Text="{Binding SelectedText, UpdateSourceTrigger=PropertyChanged}"
                  HorizontalAlignment="Left" Margin="10,39,0,0" VerticalAlignment="Top" Width="120"/>
        <Button Click="Button_Click" Content="Update list" 
                HorizontalAlignment="Left" Margin="10,82,0,0" VerticalAlignment="Top" Width="75"/>
    </Grid>
</Window>

In code behind:

public partial class MainWindow : Window, INotifyPropertyChanged
    {
        public MainWindow() {
            InitializeComponent();

            this.DataContext = this;
            Items = new List<string>() { "0", "1", "2" };
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void RaisePropertyChanged(string propertyName) {
            if (PropertyChanged != null) {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        private List<string> _items;
        public List<string> Items {// I use IEnumerable<string> with LINQ, but the effect is the same
            get { return _items; }
            set {
                if (_items != value) {
                    _items = value;
                    RaisePropertyChanged("Items");
                }
            }
        }

        private string _selectedText;
        public string SelectedText {
            get { return _selectedText; }
            set {
                if (_selectedText != value) {
                    _selectedText = value;
                    RaisePropertyChanged("SelectedText");
                }
            }
        }

        private void Button_Click(object sender, RoutedEventArgs e) {
            var changed = Items.ToList();//clone
            int index = changed.IndexOf(SelectedText);
            if (index >= 0) {
                changed[index] += "a";//just change the currently selected value
            }
            Items = changed;//update with new list
        }

    }

Answer 1

This is my fix for that issue:

public class ComboBox : System.Windows.Controls.ComboBox
{
    private bool ignore = false;
    protected override void OnSelectionChanged(SelectionChangedEventArgs e)
    {
        if (!ignore)
        {
            base.OnSelectionChanged(e);
        }
    }

    protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
    {
        ignore = true;
        try
        {
            base.OnItemsChanged(e);
        }
        finally
        {
            ignore = false;
        }
    }
}

Answer 2

After your ItemsSource has changed, raise a property changed on your selected text to refresh the UI.

So in your Items collection setter, make the change:

RaisePropertyChanged("Items");
RaisePropertyChanged("SelectedText");

EDIT: in your example you aren't just changing the ItemSource, you are changing the text of the item that is the currently selected one but having a text binding on the old text. What are you expecting to see/happen? Are you wanting the selected item to stay the same, even if its text changes?

Answer 3

Modify Button_Click like this (commented lines are new):

private void Button_Click(object sender, RoutedEventArgs e)
{
    string tempCopy = SelectedText; // Create a copy of the current value

    var changed = Items.ToList();
    int index = changed.IndexOf(SelectedText);
    if (index >= 0)
    {
        changed[index] += "a";
    }
    Items = changed;

    SelectedText = tempCopy; // Replace the selected text with the copy we made
}

All this does is makes a copy of SelectedText before Items changes, and then replaces it once the change has been made.

1. Copy SelectedText
2. Modify items source
3. Replace SelectedText with the copy