สอบถามในกรณีที่ต้องการเขียนโค๊ดคำนวณตัวเลขที่ได้จากการกรอกข้อมูลใน textbox ต่าง ๆ แล้วนำมาคำนวณตามสุตร...

protected void calculate_Click(object sender, EventArgs e)
{
    for (int i = 0; i < GridView1.Rows.Count; i++)
    {
        TextBox txtqty = (TextBox)GridView1.Rows[i].Cells[3].FindControl("txtqty");
        CheckBox chk1 = (CheckBox)GridView1.Rows[i].Cells[5].FindControl("chk1");
        String pid = Convert.ToString(GridView1.Rows[i].Cells[0].Text);
        DataRow[] drdel = DS.Tables["cart"].Select("pid='" + pid.Trim() + "'");
        if (drdel.Length != 0)
        {
            if (chk1.Checked) drdel[0].Delete();
            else
            {
                drdel[0]["qty"] = txtqty.Text.Trim();
                drdel[0]["sumprice"] = Convert.ToInt64(drdel[0]["unitprice"]) * Convert.ToInt64(txtqty.Text.Trim());
            }
        }
        DS.Tables["cart"].AcceptChanges();
    }
    GridView1.DataSource = DS.Tables["cart"];
    DataBind();
}
int QtyTotal = 0;
double SumTotal = 0;
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        e.Row.Cells[2].HorizontalAlign = HorizontalAlign.Right;
        e.Row.Cells[4].HorizontalAlign = HorizontalAlign.Right;
        double uprice = Convert.ToDouble(DataBinder.Eval(e.Row.DataItem, "Unitprice"));
        e.Row.Cells[2].Text = uprice.ToString("#,##0.00");
        if (uprice >= 50) e.Row.BackColor = Color.DeepSkyBlue;
        else e.Row.BackColor = Color.LightPink;
        double sprice = Convert.ToDouble(DataBinder.Eval(e.Row.DataItem, "sumprice"));
        e.Row.Cells[4].Text = sprice.ToString("#,##0.00");
        QtyTotal += Convert.ToInt32(DataBinder.Eval(e.Row.DataItem, "qty"));
        SumTotal += Convert.ToDouble(DataBinder.Eval(e.Row.DataItem, "sumprice"));
    }
    else if (e.Row.RowType == DataControlRowType.Footer)
    {
        e.Row.Cells[0].Text = "Totals:";
        e.Row.Cells[3].Text = QtyTotal.ToString("d");
        e.Row.Cells[4].Text = SumTotal.ToString("#,##0.00");
        e.Row.Cells[3].HorizontalAlign = HorizontalAlign.Center;
        e.Row.Cells[4].HorizontalAlign = HorizontalAlign.Right;
        e.Row.Font.Bold = true;
    }
}