print a windows form in c# having textbox

 Values display in grid , but  print preview is blank.what  should I do
  public void grid()
        {
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = con;
         
            cmd.CommandText = "Select Name,RegNo from Booking";
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            da.Fill(dt);
            if (dt.Rows.Count > 0)
            {
                dataGridView1.DataSource = dt;
                dataGridView1.Visible = true;
               
            }
        }



     
       

        private void button6_Click(object sender, EventArgs e)
        {
            grid();
            System.Windows.Forms.Panel panel = new System.Windows.Forms.Panel();
            this.Controls.Add(panel);

            //Create a Bitmap of size same as that of the Form.
            Graphics grp = panel.CreateGraphics();
            Size formSize = this.ClientSize;
            bitmap = new Bitmap(formSize.Width, formSize.Height, grp);
            grp = Graphics.FromImage(bitmap);

            //Copy screen area that that the Panel covers.
            Point panelLocation = PointToScreen(panel.Location);
            grp.CopyFromScreen(panelLocation.X, panelLocation.Y, 0, 0, formSize);

            //Show the Print Preview Dialog.
            printPreviewDialog1.Document = printDocument1;
            printPreviewDialog1.PrintPreviewControl.Zoom = 1;
            printPreviewDialog1.ShowDialog();
        }

        private void PrintPage(object sender, System.Drawing.Printing.PrintPageEventArgs e)
        {
            //Print the contents.
            e.Graphics.DrawImage(bitmap, 0, 0);
        }