Ever need to show something in a databound gridview that isn't in your datasource? Don't worry, relax, forget about that datasource. Just create that extra row dynamically.

Lets assume I have a GridView with 2 columns. The first column shows the items on an order,  the second column shows the price of that item and the footer shows the order total.  How would I inject a row that shows the tax?

Something like this in the RowDataBound EventHandler would do the trick:

decimal orderTotal = 0.0m;
decimal tax = 0.0m;
protected void gvOrderDetail_RowDataBound(object sender, GridViewRowEventArgs e)
{
    //TODO: Calculate the orderTotal and the tax when RowType = DataRow
    if (e.Row.RowType == DataControlRowType.Footer)
    {
        //Create the Row
        GridViewRow row = new GridViewRow(-1, -1, DataControlRowType.DataRow, DataControlRowState.Normal);
        //Add the two Columns
        row.Cells.AddRange(CreateCells());
        //get a reference to the table that holds this row
        Table tbl = (e.Row.Parent as Table);
        //Add the row at the end of the list, but before the footer.
        tbl.Rows.AddAt(gvOrderDetail.Rows.Count + 1, row);

        //Don't forget to account for any changes in the footer. Since we added a row to show the tax,
        //that tax must also be accounted for in our footer. Calculating the orderTotal and the tax
        //is an exercise for the reader.
        Label lbl;
        lbl = (Label)e.Row.FindControl("lblTotal");
        lbl.Text = String.Format("{0:C}", (orderTotal + tax));
    }
}

private TableCell[] CreateCells()
{

    TableCell[] cells = new TableCell[2];
    TableCell cell;
    Label lbl;

    //The order item column
    cell = new TableCell();
    lbl = new Label();
    lbl.Text = "Sales Tax";
    cell.Controls.Add(lbl);
    cells[0] = cell;

    //The price column
    cell = new TableCell();
    lbl = new Label();
    lbl.Font.Bold = true;
    lbl.Text = tax.ToString("C");
    cell.HorizontalAlign = HorizontalAlign.Right;
    cell.Controls.Add(lbl);
    cells[1] = cell;

    return cells;
}