'Silverlight.Canvas' does not contain a defenition for SetZIndex

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XAML

the error occurs like 'Silverlight.Canvas' does not contain a defenition for SetZIndex

my code is given below:

private void rectFirst_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            Canvas.SetZIndex(rectFirst, 2);
            Canvas.SetZIndex(rectSecond, 1);
        }

Thanks in Advance