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-->public  void checkAll_Click(object sender, RoutedEventArgs e)  //checkAll is my button

        {

           var objects = GetChilds(MyTreeview).OfType<CheckBox>();

            foreach (CheckBox d in objects)

            {

                d.IsChecked = true;

            }

        }

Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->  private static IEnumerable<DependencyObject> GetChilds(DependencyObject root)
        {
            List<DependencyObject> lst = new List<DependencyObject>();
            lst.Add(root);
            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(root); i++)
                lst.AddRange(GetChilds(VisualTreeHelper.GetChild(root, i)));
            return lst;
        }