How do I make two asymmetrically linked CheckBoxes in XAML?

I have two CheckBoxes in XAML. When the first one is unchecked, I want the second one to be unchecked. But when the first one is checked, I don't want anything to happen with the second one. I also want both CheckBoxes checked by default. This seems like it should be very simple with a DataTrigger, but I can't quite get it. I know this would be trivial to accomplish in code with event handlers, but I wanted to do it in XAML. Here is what I have:

    <CheckBox Name="CheckBox1" Content="CheckBox1" IsChecked="True" />
    <CheckBox Name="CheckBox2" Content="CheckBox2">
        <CheckBox.Style>
            <Style TargetType="{x:Type CheckBox}">
                <Setter Property="CheckBox.IsChecked" Value="True" />
                <Style.Triggers>
                    <DataTrigger Binding="{Binding ElementName=CheckBox1, Path=IsChecked}" Value="False">
                        <Setter Property="CheckBox.IsChecked" Value="False" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </CheckBox.Style>
    </CheckBox>

Both CheckBoxes start off checked, and when I uncheck CheckBox1, CheckBox2 is also unchecked, correctly. But when I check CheckBox1, CheckBox2 is incorrectly checked also. I tried several variations of this with no luck. If I remove the first Setter it works correctly, but then CheckBox2 doesn't start out checked.

Solution

You can try the other way around using event trigger directly from CheckBox1 :

  <CheckBox Name="CheckBox1" Content="CheckBox1" IsChecked="True">
      <CheckBox.Triggers>
          <EventTrigger RoutedEvent="CheckBox.Unchecked">
              <BeginStoryboard>
                  <Storyboard>
                      <BooleanAnimationUsingKeyFrames Storyboard.TargetName="CheckBox2" Storyboard.TargetProperty="(CheckBox.IsChecked)">
                          <DiscreteBooleanKeyFrame KeyTime="0:0:0" Value="False"/>
                      </BooleanAnimationUsingKeyFrames>
                  </Storyboard>
              </BeginStoryboard>
          </EventTrigger>
      </CheckBox.Triggers>
  </CheckBox>
  <CheckBox Name="CheckBox2" Content="CheckBox2" IsChecked="True"/>

Alternatively If You want this too look cleaner, You might explore : Microsoft.Xaml.Behaviors.* packages.

Problem with Your solution was, that the initial <Setter../> that You used to set the initial value of CheckBox, has been repeatedly working alongside the DataTrigger.