ModernWPF.Internal.LegacyBorderManager.SetManager C# (CSharp) Method

        public static void SetManager(DependencyObject obj, LegacyBorderManager value)
        {
            obj.SetValue(ManagerProperty, value);
        }

 void _contentWindow_Closed(object sender, EventArgs e)
 {
     LegacyBorderManager.SetManager(_contentWindow, null);
 }