How Do I Multiply Values Of Two Columns And Display The Result In Third Column In Gridview?

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VB

ASP.NET

Hey, My question is How to multiply values of two columns and display the result in third column in gridview? Of which one column consist it textbox, in which the value is changed by the user at run time. And as soon as the value changes the result should be displayed in third column.
Here's my code below. In this if i changed the value manually from designer code then it display the result but not at the run time. How to dynamically change the value at run time?
Designer code:

<asp:GridView ID="GridView1" runat="server" DataSourceID="SqlDataSource1" 
        AutoGenerateColumns="False">
        <Columns>
            <asp:BoundField DataField="Productid" HeaderText="Product Id" 
                InsertVisible="False" ReadOnly="True" SortExpression="Productid" HeaderStyle-Width="90px" ItemStyle-HorizontalAlign="Center"/>
            <asp:BoundField DataField="Pname" HeaderText="Product Name" SortExpression="Pname" HeaderStyle-Width="300px" ItemStyle-HorizontalAlign="Center"/>
            <asp:BoundField DataField="Price" HeaderText="Price" SortExpression="Price" HeaderStyle-Width="100px" ItemStyle-HorizontalAlign="Center"/>
            <asp:TemplateField HeaderText="Quantity" ItemStyle-HorizontalAlign="Center" HeaderStyle-Width="80px"><ItemTemplate>
                <asp:TextBox ID="TextBox1" runat="server" Text="1" Width="20px" AutoPostBack="true"></asp:TextBox></ItemTemplate></asp:TemplateField>
            <asp:TemplateField HeaderText="Total Price" ItemStyle-HorizontalAlign="Center" HeaderStyle-Width="100px">
            <ItemTemplate><asp:Label ID="Label1" runat="server"></asp:Label></ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView><br />
    <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
            ConnectionString="<%$ ConnectionStrings:DBCS %>">
    </asp:SqlDataSource>

VB code:

Public Partial Class WebForm5
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If Session("usertype") = "User" Then
            SqlDataSource1.SelectCommand = "SELECT Productid, Pname, Price FROM Stock WHERE Productid IN (SELECT Productid FROM Cart WHERE Userid='" & Session("userid").ToString & "' AND Datetoday='" & Date.Today & "' AND Payment='No')"
        Else
            Response.Redirect("Account.aspx")
        End If
    End Sub

    Protected Sub GridView1_RowDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles GridView1.RowDataBound
        If e.Row.RowType = DataControlRowType.DataRow Then
            Dim cellquantity As TableCell = e.Row.Cells(3)
            Dim tb As TextBox = CType(cellquantity.FindControl("TextBox1"), TextBox)
            Dim celltotalprice As TableCell = e.Row.Cells(4)
            Dim lbl As Label = CType(celltotalprice.FindControl("Label1"), Label)
            lbl.Text = Convert.ToString(Convert.ToInt32(e.Row.Cells(2).Text) * Convert.ToInt32(tb.Text))
        End If
    End Sub
End Class

Please Help.
Thankyou in Advance.