GetStockSelectListItemsWithOptionGroup.cs

public List<SelectListItem> GetStockSelectListItemsWithOptionGroup(Company[] companies, Stock[] stocks)
{
	var optionGroups = companies.GroupBy(c => c.CompanyId, c => c.CompanyName).Select(c => new SelectListGroup { Name = c.First() });

	return stocks.Select(s => new SelectListItem
	{
	    Text = s.StockName,
	    Value = s.StockId.ToString(),
	    Group = optionGroups.Single(o => o.Name.Trim() == s.CompanyName.Trim())
	}).ToList();
}