如何在c# LINQ中执行左外连接到对象而不使用join-on-equal -into子句?有办法用where子句来实现吗?
正确的问题:
内连接很简单，我有一个这样的解决方案
List<JoinPair> innerFinal = (from l in lefts from r in rights where l.Key == r.Key
select new JoinPair { LeftId = l.Id, RightId = r.Id})
但是对于左外连接，我需要一个解决方案。我的是这样的，但它不工作
List< JoinPair> leftFinal = (from l in lefts from r in rights
select new JoinPair {
LeftId = l.Id,
RightId = ((l.Key==r.Key) ? r.Id : 0
})
其中JoinPair是一个类:
public class JoinPair { long leftId; long rightId; }
看看这个例子
class Person
{
public int ID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public string Phone { get; set; }
}
class Pet
{
public string Name { get; set; }
public Person Owner { get; set; }
}
public static void LeftOuterJoinExample()
{
Person magnus = new Person {ID = 1, FirstName = "Magnus", LastName = "Hedlund"};
Person terry = new Person {ID = 2, FirstName = "Terry", LastName = "Adams"};
Person charlotte = new Person {ID = 3, FirstName = "Charlotte", LastName = "Weiss"};
Person arlene = new Person {ID = 4, FirstName = "Arlene", LastName = "Huff"};
Pet barley = new Pet {Name = "Barley", Owner = terry};
Pet boots = new Pet {Name = "Boots", Owner = terry};
Pet whiskers = new Pet {Name = "Whiskers", Owner = charlotte};
Pet bluemoon = new Pet {Name = "Blue Moon", Owner = terry};
Pet daisy = new Pet {Name = "Daisy", Owner = magnus};
// Create two lists.
List<Person> people = new List<Person> {magnus, terry, charlotte, arlene};
List<Pet> pets = new List<Pet> {barley, boots, whiskers, bluemoon, daisy};
var query = from person in people
where person.ID == 4
join pet in pets on person equals pet.Owner into personpets
from petOrNull in personpets.DefaultIfEmpty()
select new { Person=person, Pet = petOrNull};
foreach (var v in query )
{
Console.WriteLine("{0,-15}{1}", v.Person.FirstName + ":", (v.Pet == null ? "Does not Exist" : v.Pet.Name));
}
}
// This code produces the following output:
//
// Magnus: Daisy
// Terry: Barley
// Terry: Boots
// Terry: Blue Moon
// Charlotte: Whiskers
// Arlene:
现在你可以从左边包含元素，即使那个元素在右边没有匹配，在我们的例子中，我们检索了Arlene，即使他在右边没有匹配
这是参考资料
如何:执行左外连接(c#编程指南)
使用lambda表达式
db.Categories
.GroupJoin(db.Products,
Category => Category.CategoryId,
Product => Product.CategoryId,
(x, y) => new { Category = x, Products = y })
.SelectMany(
xy => xy.Products.DefaultIfEmpty(),
(x, y) => new { Category = x.Category, Product = y })
.Select(s => new
{
CategoryName = s.Category.Name,
ProductName = s.Product.Name
});
下面是一个版本的扩展方法解决方案，使用IQueryable代替IEnumerable
public class OuterJoinResult<TLeft, TRight>
{
public TLeft LeftValue { get; set; }
public TRight RightValue { get; set; }
}
public static IQueryable<TResult> LeftOuterJoin<TLeft, TRight, TKey, TResult>(this IQueryable<TLeft> left, IQueryable<TRight> right, Expression<Func<TLeft, TKey>> leftKey, Expression<Func<TRight, TKey>> rightKey, Expression<Func<OuterJoinResult<TLeft, TRight>, TResult>> result)
{
return left.GroupJoin(right, leftKey, rightKey, (l, r) => new { l, r })
.SelectMany(o => o.r.DefaultIfEmpty(), (l, r) => new OuterJoinResult<TLeft, TRight> { LeftValue = l.l, RightValue = r })
.Select(result);
}
(from a in db.Assignments
join b in db.Deliveryboys on a.AssignTo equals b.EmployeeId
//from d in eGroup.DefaultIfEmpty()
join c in db.Deliveryboys on a.DeliverTo equals c.EmployeeId into eGroup2
from e in eGroup2.DefaultIfEmpty()
where (a.Collected == false)
select new
{
OrderId = a.OrderId,
DeliveryBoyID = a.AssignTo,
AssignedBoyName = b.Name,
Assigndate = a.Assigndate,
Collected = a.Collected,
CollectedDate = a.CollectedDate,
CollectionBagNo = a.CollectionBagNo,
DeliverTo = e == null ? "Null" : e.Name,
DeliverDate = a.DeliverDate,
DeliverBagNo = a.DeliverBagNo,
Delivered = a.Delivered
});
