my code is::
=================
<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="Datacollection.aspx.cs" Inherits="New_NPS.Datacollection" %>
<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
    <div>
     <script type="text/javascript">
      function isNumberKey(evt) {
            var charCode = (evt.which) ? evt.which : event.keyCode
            if (charCode > 31 && (charCode < 48 || charCode > 57))
                return false;
            return true;
        }

   </script>




    <h1> put collection amount</h1>
        <asp:GridView ID="GridView1" runat="server" AllowPaging="True"
            AllowSorting="True" AutoGenerateColumns="False" DataKeyNames="Member_ID"
            DataSourceID="SqlDataSource1" CellPadding="4" ForeColor="#333333"
            GridLines="None" style="margin-right: 54px" Width="303px">

                <AlternatingRowStyle BackColor="White" />

                <Columns>
                <asp:BoundField DataField="Member_ID" HeaderText="Member_ID" ReadOnly="True"
                        SortExpression="Member_ID" />
                    <asp:BoundField DataField="First_Name" HeaderText="First_Name"
                        SortExpression="First_Name" ReadOnly="True" />
                    <asp:BoundField DataField="Last_Name" HeaderText="Last_Name"
                        SortExpression="Last_Name" ReadOnly="True" />


                    <asp:templatefield headertext="Collection Amount">
                    <itemtemplate>
                    <asp:textbox id="TextBox1" onkeypress="return isNumberKey(event)" type="Text" runat="server"></asp:textbox>
                    </itemtemplate>
                    </asp:templatefield>

                        </Columns>
                <EditRowStyle BackColor="#2461BF" />
                <FooterStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
                <HeaderStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
                <PagerStyle BackColor="#2461BF" ForeColor="White" HorizontalAlign="Center" />
                <RowStyle BackColor="#EFF3FB" />
                <SelectedRowStyle BackColor="#D1DDF1" Font-Bold="True" ForeColor="#333333" />
                <SortedAscendingCellStyle BackColor="#F5F7FB" />
                <SortedAscendingHeaderStyle BackColor="#6D95E1" />
                <SortedDescendingCellStyle BackColor="#E9EBEF" />
                <SortedDescendingHeaderStyle BackColor="#4870BE" />
        </asp:GridView>
        <asp:SqlDataSource ID="SqlDataSource1" runat="server"
                ConnectionString="<%$ ConnectionStrings:MemberConnectionString %>"

            SelectCommand="SELECT [Member_Info].[Member_ID], [Member_Info].[First_Name], [Member_Info].[Last_Name] FROM [Member_Info] inner join [Membermaster] on [Member_Info].[Member_ID]=[Membermaster].[Member_ID]"></asp:SqlDataSource>



          <asp:Button ID="btsave" runat="server" onclick="btsave_Click" Text="Save" />



          </div>



</asp:Content>


====================
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;

namespace New_NPS
{
    public partial class Datacollection : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void btsave_Click(object sender, EventArgs e)
        {

            string Member_ID = string.Empty;
            //string Frist_Name = string.Empty;
            //string Last_Name = string.Empty;
            string TextBox1 = string.Empty;



            int GVCount = GridView1.Rows.Count;

            foreach (GridViewRow GVRow in GridView1.Rows)
            {
                Member_ID = GVRow.Cells[0].Text;
               // Frist_Name = GVRow.Cells[1].Text;
                //Last_Name = GVRow.Cells[2].Text;
                TextBox1 = GVRow.Cells[3].Text;
            }

                SqlConnection myConnection = new SqlConnection("user id=sa;" +
                                           "password=prodip@km;server=PRODIP;" +
                                           "Trusted_Connection=yes;" +
                                           "database=Member;");
                myConnection.Open();

                SqlCommand myCommand = new SqlCommand("INSERT INTO [Collectionamt ] (Member_ID,collectionamt,collectiondate)" +
    "Values(@Member_ID,@TextBox1,GETDATE())");

                //int Member_ID = int.Parse(GVRow.Cells[1].Text);


                myCommand.Parameters.Add("@Member_ID", System.Data.SqlDbType.Int).Value = Convert.ToInt32(Member_ID);
                myCommand.Parameters.Add("@TextBox1", System.Data.SqlDbType.Int).Value = Convert.ToInt32(TextBox1);


                myCommand.Connection = myConnection;
                myCommand.ExecuteNonQuery();

                Response.Write("<script language='javascript'>{alert('" + "Data is Save')}</script>");

                myConnection.Close();

        }
    }
}


Problem:
1. when i save it then show ''Input string was not in a correct format.''