How to cancel ComboBox change?

Question

Let's analyze the following scenario. Window contains a combo box. When user changes the value, under some circumstances, it may cause data loss, so program asks user, whether he really want to change this value. If he answers "No", value in the
combo box should remain unchanged.
Here's XAML part of the PoC:
<Window x:Class="BindingProblem.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 ItemsSource="{Binding ItemsSource}" SelectedItem="{Binding SelectedItem, Mode=TwoWay}" HorizontalAlignment="Center" VerticalAlignment="Center" />
</Grid>
</Window>
And there's code-behind part (for simplicity, windows is its own DataContext):
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
namespace BindingProblem
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window, INotifyPropertyChanged
private int selectedItem;
private void SetSelectedItem(int value)
if (MessageBox.Show("Are you sure?", "Test", MessageBoxButton.YesNo) == MessageBoxResult.Yes)
selectedItem = value;
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs("SelectedItem"));
else
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs("SelectedItem"));
return;
public MainWindow()
InitializeComponent();
ItemsSource = new List<int>() {1, 2, 3, 4, 5};
DataContext = this;
public List<int> ItemsSource
get;
private set;
public int SelectedItem
get
return selectedItem;
set
SetSelectedItem(value);
public event PropertyChangedEventHandler PropertyChanged;
Try:
Run the program
Choose "1" in combo box
Choose "Yes". Value in the combo box will change to "1"
Choose "2" in combo box
Choose "No". Value in the combo box will change to "2" despite that value in the viewmodel will remain "1".
What's interesting is that property getter is actually being called during execution of the setter. But combo box still displays incorrect value.
How can I fix that?

Answer 1

>>What's interesting is that property getter is actually being called during execution of the setter. But combo box still displays incorrect value.
>>How can I fix that?
Just raise the PropertyChanged using the dispatcher:
private void SetSelectedItem(int value)
if (MessageBox.Show("Are you sure?", "Test", MessageBoxButton.YesNo) == MessageBoxResult.Yes)
selectedItem = value;
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs("SelectedItem"));
else
Application.Current.Dispatcher.BeginInvoke(new Action(() =>
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs("SelectedItem"));
return;
Then the event will be raised after the ComboBox control have sets its internal selection.
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