พอมีตัวอย่าง code C# ให้ดูบ้างใหม่คับ ขอบรบกวนท่านเทพทั้งหลาย ช่วยหน่อนนะคับ

  int i = 0;
            double a = 0;
            string tmpProductID;
            for (i = 0; i <= dataGridView1.Rows.Count - 1; i++)
            {
                tmpProductID = dataGridView1.Rows[i].Cells[1].Value.ToString();

                if (tmpProductID ==Items )
                {

                    a += Convert.ToDouble(dataGridView1.Rows[i].Cells[5].Value);
                }
                dataGridView1.Rows[i].Cells[1].Value = a.ToString("#,##0");
            }

 int sum = 0;
           
           if (e.RowIndex == this.dataGridView1.NewRowIndex && e.ColumnIndex > -1)
           {           
               for (int i = 0; i < this.dataGridView1.NewRowIndex; i++)
               {
                    if (this.dataGridView1.Rows[i].Cells[e.ColumnIndex].Value.ToString().Trim() != "")
                   {
                       sum += (int)this.dataGridView1.Rows[i].Cells[e.ColumnIndex].Value;
                    }
               }
               e.PaintBackground(e.CellBounds, false);
               e.Graphics.DrawString(sum.ToString(), 
              this.dataGridView1.Font, Brushes.Black, e.CellBounds.Left + 2,e.CellBounds.Top + 2);
               
               e.Handled = true;
           }