private void dtgworkingdays_DataBindingComplete(object sender, DataGridViewBindingCompleteEventArgs e)
{
    this.FillRecordNo();
}


private void FillRecordNo()
{
    for (int i = 0; i < this.dtworkingdays.Rows.Count; i++)
    {
        this.dtgworkingdays.Rows[i].HeaderCell.Value = (i + 1).ToString();
    }
}

另见 在 DataGridView 的行标题中显示行号.