ItemsSource更改时，ComboBox不会更新

Question

I hav a ComboBox that is bound to a List<string> . When the List changes, the ComboBox does not, even though PropertyChanged was raised. When debugging, I found out that the List Property is even read.

The error can be reproduced using the following code:

XAML

<Window x:Class="ComboBoxTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="90" Width="400">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <ComboBox ItemsSource="{Binding Source, Mode=OneWay}"/>
        <Button Grid.Column="1" Content="add string" Command="{Binding}" CommandParameter="Add"/>
    </Grid>
</Window>

Code behind

using System.Windows;

namespace ComboBoxTest
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new ViewModel();
        }
    }
}

ViewModel

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Windows.Input;

namespace ComboBoxTest
{
    class ViewModel : INotifyPropertyChanged, ICommand
    {
        public ViewModel()
        {
            Source = new List<string>();
            Source.Add("Test1");
            Source.Add("Test2");
            Source.Add("Test3");
        }

        private List<string> _Source;

        public List<string> Source
        {
            get { return _Source; }
            set
            {
                _Source = value;
                OnPropertyChanged("Source");
            }
        }

        private void OnPropertyChanged(string property)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(property));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        public bool CanExecute(object parameter)
        {
            return true;
        }

        public event System.EventHandler CanExecuteChanged;

        public void Execute(object parameter)
        {
            if ((string)parameter == "Add")
            {
                Source.Add("New string");
                OnPropertyChanged("Source");
            }
        }
    }
}

Why isn't the ComboBox updating?

Answer 1

The ComboBox does not update because it doesn't see any changes when it checks the List. The reference stays the same and the ComboBox is not informed about changes inside the List.

Refactoring the code to use ObservableCollection instead of List will solve the problem, because ObservableCollection implements INotifyCollectionChanged , what is necessary to inform the View about Changes inside an Object.