Gridview refresh textbox values

See more:

VB

ASP.NET

WHen i enter Product in Gridview and set quantity which i want to sell and it also show sum total in Lable Filed but one i enter second Product It reset Quantity to 0 which i enter before and also reset SUm total Value on Button Click.

What I have tried:

on Page Load

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
   If Not IsPostBack Then
     Dim dt As New DataTable()
            dt.Columns.AddRange(New DataColumn(3) {New DataColumn("ID"), New DataColumn("Item"), New DataColumn("Price"), New DataColumn("txtQuantity")})
            ViewState("Customers") = dt

   End If
End Sub

On Button Click Event

Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
       Dim dt As DataTable = DirectCast(ViewState("Customers"), DataTable)
       dt.Rows.Add(TextBox2.Text.Trim, itemcode.Trim(), 1000)
       ViewState("Customers") = dt
       GridView1.DataSource = dt
       GridView1.DataBind()
   End Sub

and in Design View

<asp:GridView ID="GridView1" runat="server" CssClass="table table-responsive table-striped footable" PageSize="20" Style="max-width: 100%" AutoGenerateColumns="false" AutoGenerateDeleteButton="True" OnRowEditing="OnRowEditing" OnRowCancelingEdit="OnRowCancelingEdit" AutoGenerateEditButton="True">
    <Columns>
         <asp:BoundField DataField="ID" HeaderText="ID" ItemStyle-Width="10%"/>
        <asp:BoundField DataField="Item" HeaderText="Item" ItemStyle-Width="50%"/>
        <asp:BoundField DataField="Price" HeaderText="Price" ItemStyle-CssClass="price" ItemStyle-Width="10%"/>

        <asp:TemplateField HeaderText="Quantity" ItemStyle-Width="10%">
            <ItemTemplate>
                <asp:TextBox ID="txtQuantity" runat="server" ></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Total" ItemStyle-Width="20%">
            <ItemTemplate>
                <asp:Label ID="lblTotal" runat="server" Text="0"></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>