public class CornerRadiusSetter
{
    public static CornerRadius GetCornerRadius(DependencyObject obj) => (CornerRadius)obj.GetValue(CornerRadiusProperty);

    public static void SetCornerRadius(DependencyObject obj, CornerRadius value) => obj.SetValue(CornerRadiusProperty, value);

    public static readonly DependencyProperty CornerRadiusProperty =
        DependencyProperty.RegisterAttached(nameof(Border.CornerRadius), typeof(CornerRadius),
            typeof(CornerRadiusSetter), new UIPropertyMetadata(new CornerRadius(), CornerRadiusChangedCallback));

    public static void CornerRadiusChangedCallback(object sender, DependencyPropertyChangedEventArgs e)
    {
        Control control = sender as Control;

        if (control == null) return;

        control.Loaded += new RoutedEventHandler(Control_Loaded);
    }

    private static void Control_Loaded(object sender, RoutedEventArgs e)
    {
        Control control = sender as Control;

        if (control == null || control.Template == null) return;

        control.ApplyTemplate();

        Border border = control.Template.FindName("border", control) as Border;

        if (border == null) return;

        border.CornerRadius = GetCornerRadius(control);
    }
}

<TextBox local:CornerRadiusSetter.CornerRadius="5" />

<Button local:CornerRadiusSetter.CornerRadius="15">Button</Button>

Control toggleButton = control.Template.FindName("toggleButton", control) as Control;

if (toggleButton != null)
    SetCornerRadius(toggleButton, GetCornerRadius(control));

Border dropDownBorder = control.Template.FindName("DropDownBorder", control) as Border;

if (dropDownBorder != null)
    dropDownBorder.CornerRadius = GetCornerRadius(control);