Cant able to make mdi child as transparent

protected override CreateParams CreateParams
            {
                  get
                  {
                        CreateParams cp = base.CreateParams;
                        cp.ExStyle |= 0x00000020;//WS_EX_TRANSPARENT
                        return cp;
                  }

private int opacity1;
        public int Opacity1
        {
            get { return opacity1; }
            set
            {
            opacity1 = value;
            Rectangle rc = new Rectangle(this.Location, this.Size);
            Parent.Invalidate(rc, true);
            }
        }

protected override void OnPaintBackground(PaintEventArgs e)
        {
            Color bk = Color.FromArgb(Opacity1, this.BackColor);
            e.Graphics.FillRectangle(new SolidBrush(bk), e.ClipRectangle);
        }