ToggleButton group: Ensuring one item is always selected in a ListBox

yeah, i would prefer the radiobutton too for this case, but if you want to use togglebutton then maybe you can bind the isenabled property to ischecked so it cannot be cliked when it's checked

create custom control from ToggleButton, in *.xaml.cs file, declare and define control

    public class ToggleButton2 : ToggleButton
{
    public bool IsNotCheckable
    {
        get { return (bool)GetValue(IsNotCheckableProperty); }
        set { SetValue(IsNotCheckableProperty, value); }
    }

    // Using a DependencyProperty as the backing store for IsNotCheckable.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty IsNotCheckableProperty =
        DependencyProperty.Register("IsNotCheckable", typeof(bool), typeof(ToggleButton2), new FrameworkPropertyMetadata((object)false));

    protected override void OnToggle()
    {
        if(!IsNotCheckable)
        {
            base.OnToggle();
        }
    }
}

in *.xaml, replace ToggleButton with my:ToggleButton2, then you can bind IsNotCheckable to IsChecked, just like below,

              <my:ToggleButton2 IsChecked="{Binding RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type ListBoxItem}},Path=IsSelected}"  IsNotCheckable="{Binding RelativeSource={RelativeSource Self}, Path=IsChecked, Mode=OneWay}">