why Rectangle is blinking ?

private List<Point> GeneralCourses = new List<Point>();

private void dataGridView1_DragDrop(object sender, DragEventArgs e)
        {
            Point p = this.dataGridView1.PointToClient(new Point(e.X, e.Y));
            DataGridView.HitTestInfo info = this.dataGridView1.HitTest(p.X, p.Y);
            if (info.RowIndex != -1 && info.ColumnIndex != -1)
            {
                Object value = (Object)e.Data.GetData(typeof(string));

                this.dataGridView1.Rows[info.RowIndex].Cells[info.ColumnIndex].Value = value;
                GeneralCourses.Add(new Point(info.RowIndex, info.ColumnIndex));
             }
        }

protected void dataGridView1_CellPainting(object sender, DataGridViewCellPaintingEventArgs e)
{
if (GeneralCourses.Contains(new Point(e.RowIndex, e.ColumnIndex)))
            {
                e.Handled = true;
                dataGridView1.Invalidate();

                Rectangle rectDest = Rectangle.Empty;
                StringFormat sf = new StringFormat();
                sf.Alignment = StringAlignment.Center;
                sf.LineAlignment = StringAlignment.Center;
                sf.Trimming = StringTrimming.EllipsisCharacter;
                string str;

                Pen p = new Pen(Color.Blue);
                str = "string example";

 rectDest = new Rectangle(e.CellBounds.X, e.CellBounds.Y, e.CellBounds.Size.Width * 3, e.CellBounds.Height);
                e.Graphics.DrawRectangle(p, rectDest);
                e.Graphics.FillRectangle(Brushes.LightBlue, rectDest);
                e.Graphics.DrawString(str, new Font("Arial", 10, FontStyle.Regular), Brushes.Black, rectDest, sf);
                p.Dispose();
                sf.Dispose();
            }
}