How to do something when a checkbox is unchecked in c#

I want a select all checkbox which when checked, checks all other checkboxes and when unchecked, unchecks the others. I have written this code:

private void all_CheckedChanged(object sender, RoutedEventArgs e)
    {
        if (all.IsChecked == true)        //this is working
        {
           issue.IsChecked = true;
           summary.IsChecked = true;
           type.IsChecked = true;
           status.IsChecked = true;
           label.IsChecked = true;
           components.IsChecked = true;
           empty.IsChecked = true;
           reporter.IsChecked = true;
           requester.IsChecked = true;
           team.IsChecked = true;
           assignee.IsChecked = true;
        }
        else if (all.IsChecked == false)       //this is not doing anything
        {
            issue.IsChecked = false;
            summary.IsChecked = false;
            type.IsChecked = false;
            //same for other checkboxes
        }

    }

when I debug it I see that the event takes places when I check the checkbox but nothing happens when I uncheck it. Someone, please suggest me how to do this in case I am doing something wrong. I am quite new to c# so might have missed something obvious.