gridview

In Vb.net

Private orderTotal As Decimal = 0.0D
Private tax As Decimal = 0.0D
Protected Sub gvOrderDetail_RowDataBound(sender As Object, e As GridViewRowEventArgs)
    'TODO: Calculate the orderTotal and the tax when RowType = DataRow
    If e.Row.RowType = DataControlRowType.Footer Then
        'Create the Row
        Dim row As 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
        Dim tbl As Table = (TryCast(e.Row.Parent, 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.
        Dim lbl As Label
        lbl = DirectCast(e.Row.FindControl("lblTotal"), Label)
        lbl.Text = [String].Format("{0:C}", (orderTotal + tax))
    End If
End Sub

Private Function CreateCells() As TableCell()

    Dim cells As TableCell() = New TableCell(2) {}
    Dim cell As TableCell
    Dim lbl As Label

    '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
End Function

more sample in C#

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try this code sample

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;
}

for reference

more info

http://www.c-sharpcorner.com/UploadFile/Art%20Scott/GridViewInsertANewRow09012006165053PM/GridViewInsertANewRow.aspx

Adding Dynamic Rows in GridView with TextBoxes