Eu criei um controle personalizado, substitui o evento paint. Quando tento descartar os gráficos criados, eles simplesmente desaparecem da tela. Não preciso usar o descarte em controles personalizados?

Edição: eu incluí um trecho de código. Por que não posso descartar o objeto gráfico dc, criado a partir do PaintEventArgs? Preciso descartá-lo?

class canvas : Control
    {

        PointF mouseDown;

        float newX;
        float newY;
        float zoomFactor = 1F;

        Graphics _dc;

        public canvas()
        {
            this.DoubleBuffered = true;
            mouseDown = new PointF(0F, 0F);
            this.Paint += new PaintEventHandler(ctrl_Paint);
        }

        private void ctrl_Paint(object sender, PaintEventArgs e)
        {


            Graphics dc = e.Graphics;
            _dc = dc;

            dc.SmoothingMode = SmoothingMode.AntiAlias;

            Color gridColor = Color.FromArgb(230, 230, 230);
            Pen gridPen = new Pen(gridColor, 1);

            float offX = (float)((Math.Sqrt(Math.Pow(newX, 2)) % (30 * zoomFactor)));
            float offY = (float)((Math.Sqrt(Math.Pow(newY, 2)) % (30 * zoomFactor)));

            for (float y = offY; y < this.Height; y = y + 30 * zoomFactor)
            {
                dc.DrawLine(gridPen, 0, y, this.Width, y);
            }
            for (float x = offX; x < this.Width; x = x + 30 * zoomFactor)
            {
                dc.DrawLine(gridPen, x, 0, x, this.Height);
            }

            dc.TranslateTransform(newX, newY);
            dc.ScaleTransform(zoomFactor, zoomFactor, MatrixOrder.Prepend);

            float XPosition = 10;
            float YPosition = 10;
            float CornerRadius = 5;
            float Width = 50;
            float Height = 50;

            Color BoxColor = Color.FromArgb(0, 0, 0);
            Pen BoxPen = new Pen(BoxColor, 2);

            GraphicsPath Path = new GraphicsPath();

            Path.AddLine(XPosition + CornerRadius, YPosition, XPosition + Width - (CornerRadius * 2), YPosition);
            Path.AddArc(XPosition + Width - (CornerRadius * 2), YPosition, CornerRadius * 2, CornerRadius * 2, 270, 90);
            Path.AddLine(XPosition + Width, YPosition + CornerRadius, XPosition + Width, YPosition + Height - (CornerRadius * 2));
            Path.AddArc(XPosition + Width - (CornerRadius * 2), YPosition + Height - (CornerRadius * 2), CornerRadius * 2, CornerRadius * 2, 0, 90);
            Path.AddLine(XPosition + Width - (CornerRadius * 2), YPosition + Height, XPosition + CornerRadius, YPosition + Height);
            Path.AddArc(XPosition, YPosition + Height - (CornerRadius * 2), CornerRadius * 2, CornerRadius * 2, 90, 90);
            Path.AddLine(XPosition, YPosition + Height - (CornerRadius * 2), XPosition, YPosition + CornerRadius);
            Path.AddArc(XPosition, YPosition, CornerRadius * 2, CornerRadius * 2, 180, 90);

            Path.CloseFigure();

            dc.DrawPath(BoxPen, Path);

            LinearGradientBrush lgb = new LinearGradientBrush(new PointF(XPosition + (Width / 2), YPosition), new PointF(XPosition + (Width / 2), YPosition + Height), Color.RosyBrown, Color.Red);

            dc.FillPath(lgb, Path);

        }
}