gridview sting format string from double

In the gridview rowdatabound event,

Protected Sub gvwRow_Databound(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
        If e.Row.RowType = DataControlRowType.DataRow Then

            Dim idx As Integer = e.Row.RowIndex
            e.Row.Cells(yourcolindex).text   = formatnumber(cdbl( e.Row.Cells(yourcolindex).text),2)
    End Sub

Oops,  I am using C#
thanks

Also the field is string but contain double no.

Protected void gvwRow_Databound(Object sender, GridViewRowEventArgs e)
{
        If e.Row.RowType = DataControlRowType.DataRow
        {

            int idx = e.Row.RowIndex;
            e.Row.Cells(yourcolindex).text   = formatnumber(Convert.ToDouble(e.Row.Cells(yourcolindex).text),2)
        }
}

here is the correct format with c# but formatnumber is not known.
if (e.Row.RowType == DataControlRowType.DataRow )
{
int idx = e.Row.RowIndex;
e.Row.Cells[5].Text = formatnumber(Convert.ToDouble(e.Row.Cells[5].Text),2)
}