以下代码产生两个项目。第一项包含Component对象列表。第二项还包含Component对象列表。在此示例中，第二个列表应该为空，因为该Component列表没有DishID=10。我该如何解决这个问题？
换句话说result[1].components.Count应为0。
namespace ConsoleApplication3
{
class Program
{
static void Main(string[] args)
{
List<Component> Component = new List<Component>();
List<Dish> Dish = new List<Dish>();
Dish.Add(new Dish { DishID = 9, CategoryID = 6, DishName = "Pork" });
Dish.Add(new Dish { DishID = 10, CategoryID = 6, DishName = "Beef" });
Component.Add(new Component { ComponentID = 1, DishID = 9, AmountID = "1", NameID = "1" });
List<Item> result = (from dishes in Dish
join components in Component on dishes.DishID equals components.DishID
into item
from p in item.DefaultIfEmpty()
select new { CategoryID = dishes.CategoryID, DishID = dishes.DishID, ComponentID = p != null ? p.ComponentID : default(int), DishName = dishes.DishName, AmountID = p != null ? p.AmountID : null, NameID = p != null ? p.NameID : null }).ToList().GroupBy(key => key.DishID)
.Select(g => new Item()
{
DishID = g.Key,
components = g.Select(t => new Component { AmountID = t.AmountID, ComponentID = t.ComponentID, DishID = t.DishID, NameID = t.NameID }).ToList()
})
.ToList();
}
}
public class Item
{
public int DishID { get; set; }
public List<Component> components { get; set; }
}
public partial class Component
{
public int ComponentID { get; set; }
public int DishID { get; set; }
public string AmountID { get; set; }
public string NameID { get; set; }
}
public partial class Dish
{
public int DishID { get; set; }
public int CategoryID { get; set; }
public string DishName { get; set; }
}
}
如果ComponentId为空（在这种情况下为0），则不存在匹配项，因此Component在创建结果列表项时过滤空对象，在这种情况下.Where (x => x.ComponentID > 0)，请在最后一个select语句之前使用：
List<Item> result = (from dishes in Dish
join components in Component on dishes.DishID equals components.DishID into item
from p in item.DefaultIfEmpty()
select new
{
CategoryID = dishes.CategoryID,
DishID = dishes.DishID,
ComponentID = p != null ? p.ComponentID : default(int),
DishName = dishes.DishName,
AmountID = p != null ? p.AmountID : null,
NameID = p != null ? p.NameID : null
})
.ToList()
.GroupBy(key => key.DishID)
.Select(g => new Item()
{
DishID = g.Key,
components = g
.Where (x => x.ComponentID > 0)
.Select(t => new Component
{
AmountID = t.AmountID,
ComponentID = t.ComponentID,
DishID = t.DishID,
NameID = t.NameID
})
.ToList()
})
.ToList();
现在，结果包含两个项目，第一个具有DishId=9，第二个列表（用于DishId=10）为空。
如果我正确理解您的要求，我认为代码可以简化：
代码如下：
List<Item> result1 = new List<Item>();
Dish.ForEach(dish =>
{
var item = new Item
{
DishID = dish.DishID,
components = Component.Where (c => c.DishID == dish.DishID).ToList()
};
result1.Add(item);
});
输出与您的代码相同。
由于您正在LEFT JOIN使用LINQ（带有from p in item.DefaultIfEmpty()），因此我假设您还希望从结果中排除空项目。这可以通过在第一个查询中添加where条件（where ((p != null) && (p.ComponentID > 0))）来完成：
List<Item> result = (from dishes in Dish
join components in Component on dishes.DishID equals components.DishID into item
from p in item.DefaultIfEmpty()
where ((p != null) && (p.ComponentID > 0))
select new
{
CategoryID = dishes.CategoryID,
DishID = dishes.DishID,
ComponentID = p != null ? p.ComponentID : default(int),
DishName = dishes.DishName,
AmountID = p != null ? p.AmountID : null,
NameID = p != null ? p.NameID : null
})
.ToList()
.GroupBy(key => key.DishID)
.Select(g => new Item()
{
DishID = g.Key,
components = g.Select(t => new Component
{
AmountID = t.AmountID,
ComponentID = t.ComponentID,
DishID = t.DishID,
NameID = t.NameID
})
.ToList()
})
.ToList();
结果与预期的一样，仅包含的Component项目DishId=9。
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