linq to sql 左联接出错，未将对象引用设置到实例

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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