Cell Calculation in Gridview in Asp.Net C#

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  protected void txt_DebtPL_TextChanged(object sender, EventArgs e)
        {
            calculate_col(sender);
           // DebitPL_Sum();
        }

     

        private void calculate_col(object sender)
        {
            TextBox txtCredit = sender as TextBox;
            GridViewRow gvr = txtCredit.NamingContainer as GridViewRow;
            TextBox txt1 = gvr.FindControl("txt_DebtPL") as TextBox;
            TextBox txt2 = gvr.FindControl("txt_AC_Bal_Sheet") as TextBox;
            TextBox txt3 = gvr.FindControl("txt_Reimbursement") as TextBox;
            TextBox txt4 = gvr.FindControl("txt_AnyOtherhead") as TextBox;
            TextBox txt5 = gvr.FindControl("txt_TotalPaymentMode") as TextBox;

            double a1=0, a2=0, a3=0, a4=0, a5=0;

            a1 = Convert.ToDouble(txt1.Text.Trim());
            a2 = Convert.ToDouble(txt2.Text.Trim());
            a3 = Convert.ToDouble(txt3.Text.Trim());
            a4 = Convert.ToDouble(txt4.Text.Trim());

            a5 = a1 + a2 + a3 + a4;
            txt5.Text = a5.ToString();


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