using (Graphics g = Graphics.FromImage(bmp))
{
    g.InterpolationMode = InterpolationMode.NearestNeighbor;
    g.DrawImage(bmp, new Rectangle(Point.Empty, bmp.Size));
}

protected override void OnPaint(PaintEventArgs pe)
{
   pe.Graphics.InterpolationMode = InterpolationMode.NearestNeighbor;
   base.OnPaint(pe);
}