Question

I’m trying to write a user control that has an ItemsControl, the ItemsTemplate of which contains a TextBox that will allow for TwoWay binding. However, I must be making a mistake somewhere in my code, because the binding only appears to work as if Mode=OneWay. This is a pretty simplified excerpt from my project, but it still contains the problem:

<UserControl x:Class="ItemsControlTest.UserControl1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="300" Width="300">
    <Grid>
        <StackPanel>
            <ItemsControl ItemsSource="{Binding Path=.}"
                          x:Name="myItemsControl">
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <TextBox Text="{Binding Mode=TwoWay,
                                                UpdateSourceTrigger=LostFocus,
                                                Path=.}" />
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
            <Button Click="Button_Click"
                    Content="Click Here To Change Focus From ItemsControl" />
        </StackPanel>
    </Grid>
</UserControl>

Here’s the code behind for the above control:

using System;
using System.Windows;
using System.Windows.Controls;
using System.Collections.ObjectModel;

namespace ItemsControlTest
{
    /// <summary>
    /// Interaction logic for UserControl1.xaml
    /// </summary>
    public partial class UserControl1 : UserControl
    {
        public ObservableCollection<string> MyCollection
        {
            get { return (ObservableCollection<string>)GetValue(MyCollectionProperty); }
            set { SetValue(MyCollectionProperty, value); }
        }

        // Using a DependencyProperty as the backing store for MyCollection.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty MyCollectionProperty =
            DependencyProperty.Register("MyCollection",
                                        typeof(ObservableCollection<string>),
                                        typeof(UserControl1),
                                        new UIPropertyMetadata(new ObservableCollection<string>()));

        public UserControl1()
        {
            for (int i = 0; i < 6; i++)
                MyCollection.Add("String " + i.ToString());

            InitializeComponent();

            myItemsControl.DataContext = this.MyCollection;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            // Insert a string after the third element of MyCollection
            MyCollection.Insert(3, "Inserted Item");

            // Display contents of MyCollection in a MessageBox
            string str = "";
            foreach (string s in MyCollection)
                str += s + Environment.NewLine;
            MessageBox.Show(str);
        }
    }
}

And finally, here’s the xaml for the main window:

<Window x:Class="ItemsControlTest.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:src="clr-namespace:ItemsControlTest"
        Title="Window1" Height="300" Width="300">
    <Grid>
        <src:UserControl1 />
    </Grid>
</Window>

Well, that’s everything. I’m not sure why editing the TextBox.Text properties in the window does not seem to update the source property for the binding in the code behind, namely MyCollection. Clicking on the button pretty much causes the problem to stare me in the face;) Please help me understand where I’m going wrong.

Thanx!

Andrew

Answer 1

Ok I believe what is causing this problem is that you are binding directly to a String . Strings are immutable in C# and thus when you change the text, it cannot change the underlying string in the ObservableCollection. What you can do to get around this problem is simply create a model class to hold the string data, and then bind the TextBox.Text to a property inside that class. Here is an example:

public partial class BindingToString : Window
{
    public BindingToString()
    {
        MyCollection = new ObservableCollection<TestItem>();

        for (int i = 0; i < 6; i++)
            MyCollection.Add(new TestItem("String " + i.ToString()));

        InitializeComponent();

        myItemsControl.DataContext = this.MyCollection;
    }

    public ObservableCollection<TestItem> MyCollection
    {
        get;
        set;
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        // Display contents of MyCollection in a MessageBox
        string str = "";
        foreach (TestItem s in MyCollection)
            str += s.Name + Environment.NewLine;
        MessageBox.Show(str);
    }
}

public class TestItem
{
    public string Name
    {
        get;
        set;
    }

    public TestItem(string name)
    {
        Name = name;
    }
}

Notice that I changed your dependency property to a standard property- there is no reason to make the collection a dependency property. Besides that the only difference is the inclusion of the wrapper class TestItem to hold the string data.

<Window x:Class="TestWpfApplication.BindingToString"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="BindingToString " Height="300" Width="300">
<Grid>
    <StackPanel>
        <ItemsControl ItemsSource="{Binding}"
                      x:Name="myItemsControl">
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <TextBox Text="{Binding Path=Name, Mode=TwoWay, UpdateSourceTrigger=LostFocus}"/>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
        <Button Click="Button_Click"
                Content="Click Here To Change Focus From ItemsControl" />
    </StackPanel>
</Grid>

Now the TextBox is bound to the Name path on TestItem, and this binding works and modifies the collection as expected.