why can not set Canvas.TopProperty in usercontrol

public static UserControl1 getInstance()
        {
            if (instance == null)
                instance = new UserControl1();
            return instance;
        }


private void Label_MouseEnter(object sender, MouseEventArgs e)
        {
            //UserControl3.getInstance().SetValue(Canvas.LeftProperty, this.Width - 300);
            for (int i = 2; i < 100; i += 2)
            {
                UserControl3.getInstance().SetValue(Canvas.TopProperty, 300+i); // error here
            }
        }

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