Checkable MenuItem With Sub Menus

If you dig into MenuItem's ControlTemplate, you will see that it uses different templates depending on it's Role property.

Reference:

Menu Styles and Templates

<Style x:Key="{x:Type MenuItem}"
       TargetType="{x:Type MenuItem}">
  <Setter Property="OverridesDefaultStyle"
          Value="True" />
  <Style.Triggers>
    <Trigger Property="Role"
             Value="TopLevelHeader">
      <Setter Property="Template"
              Value="{StaticResource {x:Static MenuItem.TopLevelHeaderTemplateKey}}" />
      <Setter Property="Grid.IsSharedSizeScope"
              Value="true" />
    </Trigger>
    <Trigger Property="Role"
             Value="TopLevelItem">
      <Setter Property="Template"
              Value="{StaticResource {x:Static MenuItem.TopLevelItemTemplateKey}}" />
    </Trigger>
    <Trigger Property="Role"
             Value="SubmenuHeader">
      <Setter Property="Template"
              Value="{StaticResource {x:Static MenuItem.SubmenuHeaderTemplateKey}}" />
    </Trigger>
    <Trigger Property="Role"
             Value="SubmenuItem">
      <Setter Property="Template"
              Value="{StaticResource {x:Static MenuItem.SubmenuItemTemplateKey}}" />
    </Trigger>
  </Style.Triggers>
</Style>

Seems like it can either allow checking or subitems by default.

To workaround that, use following code:

XAML:

<ContextMenu>
    <MenuItem Header="Top Level 1" 
              Mouse.PreviewMouseUp="MenuItem_MouseLeftButtonUp">
        <MenuItem Header="Sub Level" />
        <MenuItem Header="Sub Level" />
    </MenuItem>
    <MenuItem Header="Top Level 2">
        <MenuItem Header="Sub Level" />
        <MenuItem Header="Sub Level" />
    </MenuItem>
</ContextMenu>

Code behind:

private void MenuItem_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
    (sender as MenuItem).IsChecked = !(sender as MenuItem).IsChecked;
}

I strongly recommend converting/encapsulating this piece of functionality into an Attached Property or a Behavior.