Retain TextBox value in DataTable and bind GridView before inserting to Database in ASP.Net

Design:

<div>
    <table>
        <tr>
            <td>
                CustomerId:
            </td>
            <td>
                <asp:TextBox ID="txtid" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                Name:
            </td>
            <td>
                <asp:TextBox ID="txtname" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                Country:
            </td>
            <td>
                <asp:TextBox ID="txtcountry" runat="server" />
            </td>
        </tr>
        <tr>
            <td align="center">
                <asp:Button ID="btnAdd" Text="ADD" runat="server" OnClick="btnAdd_Click" />
            </td>
            <td align="center">
                <asp:Button ID="btnInsert" Text="Insert" runat="server" OnClick="Save" />
            </td>
        </tr>
    </table>
    <asp:GridView runat="server" ID="gvResult" AutoGenerateColumns="false" EmptyDataText="No Record To Display !">
        <Columns>
            <asp:BoundField DataField="CustomerId" HeaderText="Id" />
            <asp:BoundField DataField="Name" HeaderText="Name" />
            <asp:BoundField DataField="Country" HeaderText="Country" />
        </Columns>
    </asp:GridView>
</div>

 SOURCE CODE:

using System;
using System.Data;
using System.Data.SqlClient;

namespace WebApplication2
{
    public partial class _Default : System.Web.UI.Page
    {
        SqlConnection con = new SqlConnection("Data Source=ADMIN-PC\\SQLEXPRESS;Initial Catalog=sanjay;Integrated Security=True");
        SqlCommand cmd;
        DataTable dt;
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                BindGrid();
            }
        }
        private void BindGrid()
        {
            cmd = new SqlCommand("SELECT * FROM Customers", con);
            con.Open();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable(); da.Fill(dt);
            gvResult.DataSource = dt;
            gvResult.DataBind();
            con.Close();
        }
        protected void Save(object sender, EventArgs e)
        {
            con.Open();
            cmd = new SqlCommand("insert into Customers values('" + txtid.Text + "','" + txtname.Text + "','" + txtcountry.Text + "')", con);
            cmd.ExecuteNonQuery();
            con.Close();
            BindGrid();
            Response.Write("Details are inserted");
        }
        protected void btnAdd_Click(object sender, EventArgs e)
        {
        }
    }
}

step:1 onclick evenet add button

protected void btnAdd_Click(object sender, EventArgs e)

{

}   

griddisplay Id    Name      Country

1     muthu          india

SQL SERVER 

ID       NAME      COUNTRY

STEP2: ONCLICK EVENET INSERT BUTTON

grid display

id   Name    country

1     muthu    india

sql server display

ID     NAME      CONTRY

1       MUTHU     INDIA