linq to sql　左联接出错，未将对象引用设置到实例

var result = from a in model
join b in orderDetailModel on a.FoodMenuID equals b.FoodMenuID into g
from b in g.DefaultIfEmpty()
select new Model.WebApiModel.WMFoodMenu
{
BarginPrice = a.BarginPrice,
Credits = a.Credits,
FoodMenuCategoryID = a.FoodMenuCategoryID,
FoodMenuID = a.FoodMenuID,
RestaurantID = a.RestaurantID,
FoodMenuName = a.FoodMenuName,
SaleCount = b != null ? b.FoodMenuSaleCount : 0,
ImgUrl100 = a.ImgUrl100,
ImgUrl150 = a.ImgUrl150,
IsRecommend = a.IsRecommend,
Price = a.Price
};

这里，必须要注意一点是：SaleCount = b != null ? b.FoodMenuSaleCount : 0，这里如果SaleCount字段不允许为空，必须先判定。

如果写成　SaleCount = b.FoodMenuSaleCount，当右边数据为空时，将出现未将对象引用设置对实例的错误