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