dynamic gridview rebinding problem

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C#

ASP.NET

C#4.0

Hi,
I am doing the loan calculator and in that I am facing some gridview related problems. so please help me out!!!!!!!!!!
I have taken the template field in grid view like........

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" EnableViewState="true">
                <Columns>
                    <asp:BoundField HeaderText="No." DataField="No" />
                    <asp:BoundField HeaderText="Payment Date" DataField="PaymentDate" />
                    <asp:BoundField HeaderText="Beginning Balance" DataField="BeginningBalance" />
                    <asp:BoundField HeaderText="Scheduled Payment" DataField="ScheduledPayment" />
                    <%-- <asp:BoundField HeaderText="Extra Payment" DataField="ExtraPayment" /> --%>
                    <asp:TemplateField HeaderText="Extra Payment">
                        <ItemTemplate>
                            <asp:TextBox ID="txtExtraPayment" Text="0" EnableViewState="true" AutoPostBack="true"
                                runat="server" OnTextChanged="txtExtraPayment_TextChanged" DataField="ExtraPayment"
                                Width="75"></asp:TextBox>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:BoundField HeaderText="Total Payment" DataField="TotalPayment" />
                    <asp:BoundField HeaderText="Principal" DataField="principal" />
                    <asp:BoundField HeaderText="Interest" DataField="interest" />
                    <asp:BoundField HeaderText="Ending Balance" DataField="EndingBalance" />
                </Columns>
            </asp:GridView>

now i have created one create grid function in which i have creted first row dynamically and then another loop for second to further rows based on the calculation of the first row. so i have to do the dynamic gridview is must.
i have not used any database for this puppose.
the code is as ................

protected void createGrid()
    {
        loanYears = Convert.ToDecimal(txtLoanYears_BoundControl.Text.Trim());
        scheduledNumberOfPayment = loanYears * 12;
        txtScheduledNumberOfPayments.Text = scheduledNumberOfPayment.ToString();

        // Grid Column Calculations

        // 1) Payment Date


        // 2) Begining Balance
        beginingBal = Convert.ToDecimal(txtLoanAmount.Text.Trim());



        // 3) Scheduled Monthly Payment
        annInterest = Convert.ToDecimal(txtAnnInterest_BoundControl.Text.Trim());

        Double roi = fn_InterestRate(Convert.ToDouble(annInterest));

        Double LY = Convert.ToDouble(loanYears);
        Double BBal = Convert.ToDouble(beginingBal);

        Double emi = fn_ScheduledMonthlyPayment(roi, LY, BBal);

        scheduledMonthlyPayment = Convert.ToDecimal(emi);
        txtScheduledMonthlyPayment.Text = scheduledMonthlyPayment.ToString("C");

        // 4) Optional extra payment
        optionalPayment = Convert.ToDecimal(0.00d);

        // 5) Total Payment
        totalPayment = fn_TotalPayment(scheduledMonthlyPayment, Convert.ToDecimal(txtOptionalPayment.Text.Trim()));

        // 6) Interest
        currentInterest = fn_CurrentInterest(beginingBal, annInterest);

        // 7) Calculating principal amount
        principal = fn_Principal(totalPayment, currentInterest);

        // 8) Ending balance
        endingBal = fn_EndingBalance(beginingBal, principal);

        indexNo = indexNo + 1;
        if (dt.Rows.Count < 1)
        {
            dt.Columns.Add("No", typeof(int));
            dt.Columns.Add("PaymentDate", typeof(string));
            dt.Columns.Add("BeginningBalance", typeof(string));
            dt.Columns.Add("ScheduledPayment", typeof(string));
            dt.Columns.Add("ExtraPayment", typeof(string));
            dt.Columns.Add("TotalPayment", typeof(string));
            dt.Columns.Add("principal", typeof(string));
            dt.Columns.Add("interest", typeof(string));
            dt.Columns.Add("EndingBalance", typeof(string));

            dtRow = dt.NewRow();

            dtRow["No"] = "" + indexNo;
            dtRow["PaymentDate"] = "" + txtStartDateOfLoan.Text;
            dtRow["BeginningBalance"] = beginingBal.ToString("C");
            dtRow["ScheduledPayment"] = "" + scheduledMonthlyPayment.ToString("C");
            dtRow["ExtraPayment"] = "" + optionalPayment.ToString("C");
            dtRow["TotalPayment"] = "" + totalPayment.ToString("C");
            dtRow["principal"] = "" + principal.ToString("C");
            dtRow["interest"] = "" + currentInterest.ToString("C");
            dtRow["EndingBalance"] = "" + endingBal.ToString("C");

            dt.Rows.Add(dtRow);
        }

        if (dt.Rows.Count >= 1)
        {
            for (int i = 1; i < 360; i++)
            {

                // 1) Increment date by a month
                rowCount = dt.Rows.Count - 1;
                o = dt.Rows[rowCount]["PaymentDate"];
                date = Convert.ToDateTime(o).AddMonths(1);
                string n = date.ToShortDateString();

                if (date.Month == 12)
                {
                    // GridView1.Rows[rowCount+1].Cells[4].CssClass = "txtBG";
                }
                //date = DateTime.ParseExact(Convert.ToString(o), "d", "");



                // 2) Begining balance from last ending balance
                beginingBal = Convert.ToDecimal(remoFirstChar(dt.Rows[rowCount]["EndingBalance"].ToString()));


                // 3) Scheduled payment remains constant

                // 4) Extra payment will set to its default value 0

                // 5) Total payment
                schPay = remoFirstChar(dt.Rows[rowCount]["ScheduledPayment"].ToString());
                extPay = remoFirstChar(dt.Rows[rowCount]["ExtraPayment"].ToString());

                totalPayment = fn_TotalPayment(Convert.ToDecimal(schPay), Convert.ToDecimal(extPay));


                // 6) Interest count
                currentInterest = fn_CurrentInterest(beginingBal, annInterest);


                // 7) Principal
                principal = fn_Principal(totalPayment, currentInterest);


                // 8) Ending Balance
                endingBal = fn_EndingBalance(beginingBal, principal);


                indexNo = indexNo + 1;

                dtRow = dt.NewRow();

                dtRow["No"] = "" + indexNo;
                dtRow["PaymentDate"] = "" + n;
                dtRow["BeginningBalance"] = "" + beginingBal.ToString("C");
                dtRow["ScheduledPayment"] = "" + scheduledMonthlyPayment.ToString("C");
                dtRow["ExtraPayment"] = "" + optionalPayment.ToString("C");
                dtRow["TotalPayment"] = "" + totalPayment.ToString("C");
                dtRow["principal"] = "" + principal.ToString("C");
                dtRow["interest"] = "" + currentInterest.ToString("C");
                dtRow["EndingBalance"] = "" + endingBal.ToString("C");

                dt.Rows.Add(dtRow);
            }
        }
        GridView1.DataSource = dt;
        Session["tempDt"] = dt;
        GridView1.DataBind();
    }

Then i am using the gridview texbox's checkedchange event to calculate the further loan amount if extra payment payed. so I create and bind the rows again here....
code..........

protected void txtExtraPayment_TextChanged(object sender, EventArgs e)
    {
        TextBox txtsample = (TextBox)sender;
        GridViewRow gvr = (GridViewRow)txtsample.Parent.Parent;
        int index = gvr.RowIndex;

        dt = (DataTable)Session["tempDt"];

        

        loanYears = Convert.ToDecimal(txtLoanYears_BoundControl.Text.Trim());
        scheduledNumberOfPayment = loanYears * 12;
        txtScheduledNumberOfPayments.Text = scheduledNumberOfPayment.ToString();

        // Grid Column Calculations

        // 1) Payment Date                

        //*******************************************************************
        // 2) Begining Balance
        beginingBal = Convert.ToDecimal(txtLoanAmount.Text.Trim());
        //*******************************************************************


        // 3) Scheduled Monthly Payment
        annInterest = Convert.ToDecimal(txtAnnInterest_BoundControl.Text.Trim());

        Double roi = fn_InterestRate(Convert.ToDouble(annInterest));
        Double LY = Convert.ToDouble(loanYears);
        Double BBal = Convert.ToDouble(beginingBal);
        Double emi = fn_ScheduledMonthlyPayment(roi, LY, BBal);

        scheduledMonthlyPayment = Convert.ToDecimal(emi);
        txtScheduledMonthlyPayment.Text = scheduledMonthlyPayment.ToString("C");

        // 4) Optional extra payment
        optionalPayment = Convert.ToDecimal(txtOptionalPayment.Text.ToString());

        // 5) Total Payment
        totalPayment = fn_TotalPayment(scheduledMonthlyPayment, Convert.ToDecimal(txtOptionalPayment.Text.Trim()));

        // 6) Interest
        currentInterest = fn_CurrentInterest(beginingBal, annInterest);

        // 7) Calculating principal amount
        principal = fn_Principal(totalPayment, currentInterest);

        // 8) Ending balance
        endingBal = fn_EndingBalance(beginingBal, principal);

        indexNo = indexNo + 1;

        if (dt.Rows.Count < 1)
        {
            dt.Columns.Add("No", typeof(int));
            dt.Columns.Add("PaymentDate", typeof(string));
            dt.Columns.Add("BeginningBalance", typeof(string));
            dt.Columns.Add("ScheduledPayment", typeof(string));
            dt.Columns.Add("ExtraPayment", typeof(string));
            dt.Columns.Add("TotalPayment", typeof(string));
            dt.Columns.Add("principal", typeof(string));
            dt.Columns.Add("interest", typeof(string));
            dt.Columns.Add("EndingBalance", typeof(string));

            dtRow = dt.NewRow();

            dtRow["No"] = "" + indexNo;
            dtRow["PaymentDate"] = "" + txtStartDateOfLoan.Text;
            dtRow["BeginningBalance"] = beginingBal.ToString("C");
            dtRow["ScheduledPayment"] = "" + scheduledMonthlyPayment.ToString("C");
            dtRow["ExtraPayment"] = "" + optionalPayment.ToString("C");
            dtRow["TotalPayment"] = "" + totalPayment.ToString("C");
            dtRow["principal"] = "" + principal.ToString("C");
            dtRow["interest"] = "" + currentInterest.ToString("C");
            dtRow["EndingBalance"] = "" + endingBal.ToString("C");

            dt.Rows.Add(dtRow);
        }

        if (dt.Rows.Count >= 1)
        {
            for (int i = 1; i < 360; i++)
            {
                // 1) Increment date by a month 
                rowCount = dt.Rows.Count - 1;
                o = dt.Rows[rowCount]["PaymentDate"];
                date = Convert.ToDateTime(o).AddMonths(1);
                string n = date.ToShortDateString();
                //date = DateTime.ParseExact(Convert.ToString(o), "d", "");                    

                //2) Begining balance from last ending balance 
                beginingBal = Convert.ToDecimal(remoFirstChar(dt.Rows[rowCount]["EndingBalance"].ToString()));


                //3) Scheduled payment remains constant 

                //4) Extra payment will set to its default value 0 


                // 5) Total payment ***
                int ind = dt.Rows.Count;

                schPay = remoFirstChar(dt.Rows[rowCount]["ScheduledPayment"].ToString());
                if (ind == index)
                {
                    extPay = txtsample.Text;
                    extPay = Convert.ToDecimal(extPay);
                }
                else
                {
                    extPay = 0.00;
                    extPay = Convert.ToDecimal(extPay);
                }
                totalPayment = fn_TotalPayment(Convert.ToDecimal(schPay), Convert.ToDecimal(extPay));

                // 6) Interest count 
                //annInterest = Convert.ToDecimal(txtAnnInterest_BoundControl.Text.Trim());
                currentInterest = fn_CurrentInterest(beginingBal, annInterest);

                // 7) Principal                     
                principal = fn_Principal(totalPayment, currentInterest);

                // 8) Ending Balance 
                endingBal = fn_EndingBalance(beginingBal, principal);

                indexNo = indexNo + 1;

                dtRow = dt.NewRow();

                dtRow["No"] = "" + indexNo;
                dtRow["PaymentDate"] = "" + n;
                dtRow["BeginningBalance"] = "" + beginingBal.ToString("C");// dt.Rows[rowCount]["EndingBalance"];
                dtRow["ScheduledPayment"] = "" + scheduledMonthlyPayment.ToString("C");
                
                
                dtRow["ExtraPayment"] = "$" + extPay.ToString();
                dtRow["TotalPayment"] = "" + totalPayment.ToString("C");                
                dtRow["principal"] = "" + principal.ToString("C");
                dtRow["interest"] = "" + currentInterest.ToString("C");
                dtRow["EndingBalance"] = "" + endingBal.ToString("C");
                
                dt.Rows.Add(dtRow);
            }
            GridView1.DataSource = dt;
            //Session["temptbl"] = dt;
            GridView1.DataBind();
            //dt.AcceptChanges();
            


                TextBox src = ((TextBox)sender);
            src.Text = "2000";
            
            ////GridViewRow row = (GridViewRow)src.Parent.Parent;
            //foreach(GridViewRow row in GridView1.Rows)
            //{
            //    ((TextBox)row.FindControl("txtExtraPayment")).Text = "df";
            //}

        }
        // actualNoOfPay = fn_ActualPayments();

        //while (Convert.ToDecimal(remoFirstChar(dt.Rows[count]["ExtraPayment"].ToString())) > 0)
        //{
        //    sumOptionalPayment = sumOptionalPayment + Convert.ToDecimal(remoFirstChar(dt.Rows[count]["ExtraPayment"].ToString())); 
        //    totalInterest = totalInterest + Convert.ToDecimal(remoFirstChar(dt.Rows[count]["interest"].ToString())); 
        //}

        //if (scheduledNumberOfPayment != actualNoOfPay)
        //{

        //    NoOfPaySaved = fn_PaymentsSaved(Convert.ToInt32(scheduledNumberOfPayment) , Convert.ToInt32(txtActualNumberOfPayments.Text)); 
        //    txtNumberOfPaymentsSaved.Text = NoOfPaySaved.ToString("C");

        //    dollarAmountOfPaySaved = fn_DollarAmountOfPaySaved();  
        //    txtDollarAmountOfPaymentsSaved.Text = dollarAmountOfPaySaved.ToString("C");

        //   // totalEarlyPayments = fn_TotalOfEarlyPay();
        //   // NoOfYearsSaved = Convert.ToInt32(dollarAmountOfPaySaved / totalEarlyPayments);
        //   // txtNumberOfYearsSaved.Text = NoOfYearsSaved.ToString();

        //    totalInterestSaved = dollarAmountOfPaySaved - totalEarlyPayments;
        //    txtTotalInterest.Text = totalInterestSaved.ToString("C");
        //}

        //DotNetNuke.Data.DataProvider.Instance().ExecuteNonQuery("sp_insert_LoanCalculator", txtLoanAmount.Text.Trim(), annInterest, loanYears, txtStartDateOfLoan.Text.Trim(), optionalPayment, scheduledMonthlyPayment, scheduledNumberOfPayment,,,,,,,,totalPayment);

    }

Now I am facing the problem is on cheked change event
1) all calculations are performing well at first time event raise but the texbox I inserted value and fire the event get cleared after the postback. since I am not able to calculate it again at second time when I raise the same event for another textbox. So please tell me how to preserve the values of gridview textbox during all the postback caused by checkedchange event.