Obtener el valor total de una column a de gridview

Descripción del problema

él
Estoy usando ASP. Net/VB. Net y SQL - Server - 2012.
Tengo una column a gridview que contiene tres campos y un campo de plantilla como sigue:
<asp:GridView ID="grdItems" runat="server" AutoGenerateColumns="False" CellPadding="4" DataSourceID="SqlDataSource3" Font-Names="Tahoma" ForeColor="#333333" GridLines="None">
    <AlternatingRowStyle BackColor="White" ForeColor="#284775" />
    <Columns>
        <asp:BoundField DataField="item_name" HeaderText="Item" SortExpression="item_name" />
        <asp:BoundField DataField="item_cost" HeaderText="Cost (inc. VAT)" SortExpression="item_cost" />
        <asp:BoundField DataField="item_quantity" HeaderText="Quantity" SortExpression="item_quantity" />
        <asp:TemplateField HeaderText="Sub-Total (inc. VAT)">
            <ItemTemplate>
                <asp:Label ID="TextBox3" runat="server" Text='<%# Convert.ToInt32(Eval("item_quantity")) * Convert.ToDouble(Eval("item_cost"))%>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField>
            <FooterTemplate>
                <asp:Label ID="lblTotalPrice" runat="server" />
            </FooterTemplate>                  
        </asp:TemplateField>
    </Columns>
    <EditRowStyle BackColor="#999999" />
    <FooterStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
    <HeaderStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" HorizontalAlign="Center" VerticalAlign="Middle" />
    <PagerStyle BackColor="#284775" ForeColor="White" HorizontalAlign="Center" />
    <RowStyle BackColor="#F7F6F3" ForeColor="#333333" HorizontalAlign="Center" VerticalAlign="Middle" />
    <SelectedRowStyle BackColor="#E2DED6" Font-Bold="True" ForeColor="#333333" />
    <SortedAscendingCellStyle BackColor="#E9E7E2" />
    <SortedAscendingHeaderStyle BackColor="#506C8C" />
    <SortedDescendingCellStyle BackColor="#FFFDF8" />
    <SortedDescendingHeaderStyle BackColor="#6F8DAE" />
</asp:GridView>
Código oculto:
Imports System.Data.SqlClient
Imports System.Data

Partial Class ProjectReport
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load

        Dim ProjectID = Session("project_id")
        Session("ProjectID") = ProjectID

    End Sub
    Protected Sub grdItems_RowDataBound(sender As Object, e As GridViewRowEventArgs)

        Dim totalPrice As Decimal = 0

        If e.Row.RowType = DataControlRowType.DataRow Then


            Dim lblPrice As Label = DirectCast(e.Row.FindControl("lblTotalPrice"), Label)

            Dim price As Decimal = [Decimal].Parse(lblPrice.Text)


            totalPrice += price

        End If

        If e.Row.RowType = DataControlRowType.Footer Then
            Dim lblTotalPrice As Label = DirectCast(e.Row.FindControl("lblTotalPrice"), Label)

            lblTotalPrice.Text = totalPrice.ToString()


        End If
    End Sub


End Class
Enlace de datos ()
   Private Sub BindData()

        Dim conn As New SqlConnection("Data Source=BRIAN-PC\SQLEXPRESS;Initial Catalog=master_db;Integrated Security=True")
        Dim query As New SqlCommand("SELECT Items.item_name, Items.item_cost, project_items.item_quantity FROM Items INNER JOIN project_items ON items.item_id = project_items.item_id WHERE project_items.project_id = @parameter", conn)

        query.Parameters.AddWithValue("@UserID", Session("ProjectID"))

        Dim da As New SqlDataAdapter(query, conn)

        da.SelectCommand = query

        Dim table As New DataTable()




        da.Fill(table)

        grdItems.DataSource = table
        grdItems.DataBind()
    End Sub
La última columna (campo plantilla) multiplica el campo cantidad por el campo costo.
¿Cómo calcular todos los valores en un campo de plantilla (A ñadiendo)?

Solución simple

debe sumar valores usando eventos de Unión de datos. Consulte this example y ajuste según sus necesidades:
private Decimal OrderTotal;

protected void GridView1_DataBinding(object sender, EventArgs e)
{ 
    OrderTotal = 0.0M;
}

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        //Keep adding the subtotal here
        OrderTotal += Subtotal;               
    }
}

protected void GridView1_DataBound(object sender, EventArgs e)
{      
    //Set a control with the total sum
    LabelOrderTotal.Text = OrderTotal.ToString("C");
}
Básicamente, sigue a ñadiendo valores a los eventos RowDataBound, y en los eventos DataBound, establece una etiqueta con la suma. Alternativamente, puede iterar sobre la cuadrícula en el evento DataBound y añadir todo.