Forcing a tri-state checkbox to not move to indeterminate state - wpf

Question

I'm working in WPF and i have an interesting requirement.
I need my checkboxes to be ThreeState, so if only some of the child elements are selected it shows as indeterminate. But when a use clicks it, i want it to select either true or false.
Here is a story to represent my requirements:
item - indeterminate
subItem - checked
subItem - unchecked
subItem - checked
When a user clicks item, the checkbox should alternate between checked and unchecked. The user should never be able to select 'indeterminate' as a state. Is this possible?

Answer 1

XAML:
<CheckBox IsThreeState="True" IsChecked="{x:Null}" Click="CheckBox_Clicked" />
Code-behind:
private void CheckBox_Clicked(object sender, RoutedEventArgs e)
{
var cb = e.Source as CheckBox;
if (!cb.IsChecked.HasValue)
cb.IsChecked = false;
}
If you don't like the code-behind solution, then you could sub-class your own control like in the solution for this question.

Answer 2

It's much easier if you use Binding with your CheckBox.
XAML:
<Window x:Class="WpfApplication39Checkbox.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>
<CheckBox Content="CheckBox" HorizontalAlignment="Left" Margin="128,95,0,0" VerticalAlignment="Top" IsThreeState="False" IsChecked="{Binding CheckState}"/>
<Button Content="Button" HorizontalAlignment="Left" Margin="46,241,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click"/>
<Button Content="Button" HorizontalAlignment="Left" Margin="139,241,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click_1"/>
<Button Content="Button" HorizontalAlignment="Left" Margin="235,241,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click_2"/>
</Grid>
</Window>
Code-behind:
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
namespace WpfApplication39Checkbox
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window, INotifyPropertyChanged
{
public MainWindow()
{
InitializeComponent();
DataContext = this;
}
private bool? checkState;
public bool? CheckState
{
get { return checkState; }
set
{
checkState = value;
OnPropertyChanged("CheckState");
}
}
private void Button_Click(object sender, RoutedEventArgs e)
{
CheckState = false;
}
private void Button_Click_1(object sender, RoutedEventArgs e)
{
CheckState = true;
}
private void Button_Click_2(object sender, RoutedEventArgs e)
{
CheckState = null;
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
You see the point? You set the CheckBox to IsThreeState="False" but set the third state from CodeBehind and the CheckBox behaves as expected.