[Solved] ASP.Net DataTable Sum function Error: Specified cast is not valid

Hi makumbi,

Make sure the DataType is same as in the database when using Sum function.

Please refer below sample.

HTML

<asp:GridView ID="gvProduct" runat="server" AutoGenerateColumns="false" 
    ShowFooter="true" Width="237px">
    <Columns>
        <asp:BoundField DataField="ProductId" HeaderText="ProductId" />
        <asp:BoundField DataField="ProductName" HeaderText="ProductName" />
        <asp:BoundField DataField="UnitsInStock" HeaderText="UnitsInStock" />
        <asp:TemplateField HeaderText="UnitPrice">
            <ItemTemplate>
                <asp:Label ID="lblPrice" runat="server" Text='<%# Eval ("UnitPrice") %>'></asp:Label>
            </ItemTemplate>
            <FooterTemplate>
                <asp:Label ID="lblTotal" runat="server"></asp:Label>
            </FooterTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>

Namespaces

C#

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

VB.Net

Imports System.Data
Imports System.Data.SqlClient
Imports System.Configuration

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindGrid();
    }

}

private void BindGrid()
{
    string query = "SELECT TOP 5 ProductId,ProductName,UnitPrice,UnitsInStock from Products";
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand(query))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                using (DataTable dt = new DataTable())
                {
                    sda.Fill(dt);
                    gvProduct.DataSource = dt;
                    gvProduct.DataBind();
                    if (gvProduct.Rows.Count <= 0)
                    {
                    }
                    else
                    {
                        Decimal total = dt.AsEnumerable().Sum(row => row.Field<Decimal>("UnitPrice"));
                        gvProduct.FooterRow.Cells[2].Text = "GrandTotal";
                        gvProduct.FooterRow.Cells[2].HorizontalAlign = HorizontalAlign.Right;
                        gvProduct.FooterRow.Cells[3].Text = total.ToString("N2");
                    }
                }
            }
        }
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Me.BindGrid()
    End If
End Sub

Private Sub BindGrid()
    Dim query As String = "SELECT TOP 5 ProductId,ProductName,UnitPrice,UnitsInStock from Products"
    Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As SqlConnection = New SqlConnection(constr)
        Using cmd As SqlCommand = New SqlCommand(query)
            Using sda As SqlDataAdapter = New SqlDataAdapter()
                cmd.Connection = con
                sda.SelectCommand = cmd
                Using dt As DataTable = New DataTable()
                    sda.Fill(dt)
                    gvProduct.DataSource = dt
                    gvProduct.DataBind()

                    If gvProduct.Rows.Count <= 0 Then
                    Else
                        Dim total As Decimal = dt.AsEnumerable().Sum(Function(row) row.Field(Of Decimal)("UnitPrice"))
                        gvProduct.FooterRow.Cells(2).Text = "GrandTotal"
                        gvProduct.FooterRow.Cells(2).HorizontalAlign = HorizontalAlign.Right
                        gvProduct.FooterRow.Cells(3).Text = total.ToString("N2")
                    End If
                End Using
            End Using
        End Using
    End Using
End Sub

Screenshot