gridview controls in asp.net

<asp:GridView ID="GVAmountReceived" runat="server"DataKeyNames="sno" EmptyDataText="No Records Found"
                       Style="margin-bottom: 6px" BorderColor="#BDBDBD" AllowPaging="true" PageSize="20">

 <columns>
                            <asp:templatefield headertext="S.No." footerstyle-horizontalalign="Center" itemstyle-horizontalalign="Center" xmlns:asp="#unknown">
                                <itemtemplate>
                                    <%# Container.DataItemIndex + 1 %>
                                </itemtemplate>
                            </asp:templatefield>
<asp:boundfield headertext="Party Name" datafield="partyname" xmlns:asp="#unknown">
ItemStyle-HorizontalAlign="Center" ReadOnly="true" />
 <asp:templatefield headertext="Date" itemstyle-horizontalalign="Center">
            <edititemtemplate>
            <asp:textbox runat="server" id="txt_date" width="90px"></asp:textbox>
            <asp:calendarextender id="calrecdate" runat="server" targetcontrolid="txt_date" onclientdateselectionchanged="checkDate" format="dd/MM/yyyy" enabled="True" />
       <asp:requiredfieldvalidator id="RFVrecdate" runat="server" controltovalidate="txt_date">
         Display="None" ErrorMessage="Must Enter Date" ValidationGroup="duereport"></asp:requiredfieldvalidator>
    <asp:validatorcalloutextender id="VCErecdate" runat="Server" targetcontrolid="RFVrecdate" />
        </edititemtemplate>
        </asp:templatefield>
 <asp:commandfield showeditbutton="true" headertext="Edit" validationgroup="duereport" />

</asp:boundfield></columns>

DataTable dtinsert = new DataTable();
      //TextBox txtparty = (TextBox)GVAmountReceived.Rows[e.RowIndex].Cells[1].Controls[1];
      //DropDownList ddlparty = (DropDownList)GVAmountReceived.Rows[e.RowIndex].Cells[1].Controls[1];
      TextBox txtrecdate = (TextBox)GVAmountReceived.Rows[e.RowIndex].Cells[3].Controls[1];
      TextBox txtrecamount = (TextBox)GVAmountReceived.Rows[e.RowIndex].Cells[4].Controls[1];
      TextBox txtchknumber = (TextBox)GVAmountReceived.Rows[e.RowIndex].Cells[5].Controls[1];


      string partyname = Convert.ToString(GVAmountReceived.DataKeys[e.RowIndex].Values[0].ToString());

      IFormatProvider provider = new System.Globalization.CultureInfo("en-CA", true);
      String recdate1 = txtrecdate.Text.Trim().ToString();
      DateTime recdate2 = DateTime.Parse(recdate1, provider, System.Globalization.DateTimeStyles.NoCurrentDateDefault);


      objRetailPL.Amount = Convert.ToSingle(txtrecamount.Text.ToString());
      objRetailPL.PartyName = partyname;
      objRetailPL.Date = recdate2;
      objRetailPL.Chequeno = txtchknumber.Text.ToString();


      objRetailPL.username = Session["username"].ToString();
      dtinsert = objRetailBAL.InsertReceivedAmount(objRetailPL);