void dataGridView1_Paint(object sender, PaintEventArgs e)

        {

            //Get the cell bounds of the header cell in column3

            Rectangle r1 = this.dataGridView1.GetCellDisplayRectangle(2, -1, true);

            //Get the cell bounds of the header cell in column4

            Rectangle r2 = this.dataGridView1.GetCellDisplayRectangle(3, -1, true);

            r1.X += 1;

            r1.Y += 2;

            r1.Width += r2.Width - 2;

            r1.Height -= 6;

 

            //cover the rectangle right over the headers.

            using (SolidBrush br = new SolidBrush(

                this.dataGridView1.ColumnHeadersDefaultCellStyle.BackColor))

            {

                e.Graphics.FillRectangle(br, r1);

            }

            //draw text

            using (SolidBrush br = new SolidBrush(

                this.dataGridView1.ColumnHeadersDefaultCellStyle.ForeColor))

            {

                StringFormat sf = new StringFormat();

                sf.LineAlignment = StringAlignment.Center;

                sf.Alignment = StringAlignment.Center;

                e.Graphics.DrawString("merged header",

                    this.dataGridView1.ColumnHeadersDefaultCellStyle.Font, br, r1, sf);

            }

        }