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; }
在linqc#中执行左外连接
//执行左外连接
class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
class Child
{
public string Name { get; set; }
public Person Owner { get; set; }
}
public class JoinTest
{
public static void LeftOuterJoinExample()
{
Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };
Child barley = new Child { Name = "Barley", Owner = terry };
Child boots = new Child { Name = "Boots", Owner = terry };
Child whiskers = new Child { Name = "Whiskers", Owner = charlotte };
Child bluemoon = new Child { Name = "Blue Moon", Owner = terry };
Child daisy = new Child { Name = "Daisy", Owner = magnus };
// Create two lists.
List<Person> people = new List<Person> { magnus, terry, charlotte, arlene };
List<Child> childs = new List<Child> { barley, boots, whiskers, bluemoon, daisy };
var query = from person in people
join child in childs
on person equals child.Owner into gj
from subpet in gj.DefaultIfEmpty()
select new
{
person.FirstName,
ChildName = subpet!=null? subpet.Name:"No Child"
};
// PetName = subpet?.Name ?? String.Empty };
foreach (var v in query)
{
Console.WriteLine($"{v.FirstName + ":",-25}{v.ChildName}");
}
}
// This code produces the following output:
//
// Magnus: Daisy
// Terry: Barley
// Terry: Boots
// Terry: Blue Moon
// Charlotte: Whiskers
// Arlene: No Child
https://dotnetwithhamid.blogspot.in/
class Program
{
List<Employee> listOfEmp = new List<Employee>();
List<Department> listOfDepart = new List<Department>();
public Program()
{
listOfDepart = new List<Department>(){
new Department { Id = 1, DeptName = "DEV" },
new Department { Id = 2, DeptName = "QA" },
new Department { Id = 3, DeptName = "BUILD" },
new Department { Id = 4, DeptName = "SIT" }
};
listOfEmp = new List<Employee>(){
new Employee { Empid = 1, Name = "Manikandan",DepartmentId=1 },
new Employee { Empid = 2, Name = "Manoj" ,DepartmentId=1},
new Employee { Empid = 3, Name = "Yokesh" ,DepartmentId=0},
new Employee { Empid = 3, Name = "Purusotham",DepartmentId=0}
};
}
static void Main(string[] args)
{
Program ob = new Program();
ob.LeftJoin();
Console.ReadLine();
}
private void LeftJoin()
{
listOfEmp.GroupJoin(listOfDepart.DefaultIfEmpty(), x => x.DepartmentId, y => y.Id, (x, y) => new { EmpId = x.Empid, EmpName = x.Name, Dpt = y.FirstOrDefault() != null ? y.FirstOrDefault().DeptName : null }).ToList().ForEach
(z =>
{
Console.WriteLine("Empid:{0} EmpName:{1} Dept:{2}", z.EmpId, z.EmpName, z.Dpt);
});
}
}
class Employee
{
public int Empid { get; set; }
public string Name { get; set; }
public int DepartmentId { get; set; }
}
class Department
{
public int Id { get; set; }
public string DeptName { get; set; }
}
输出
根据我对类似问题的回答，在这里:
Linq到SQL左外连接使用Lambda语法和连接2列(复合连接键)
获得代码在这里，或克隆我的github回购，并发挥!
查询:
var petOwners =
from person in People
join pet in Pets
on new
{
person.Id,
person.Age,
}
equals new
{
pet.Id,
Age = pet.Age * 2, // owner is twice age of pet
}
into pets
from pet in pets.DefaultIfEmpty()
select new PetOwner
{
Person = person,
Pet = pet,
};
λ:
var petOwners = People.GroupJoin(
Pets,
person => new { person.Id, person.Age },
pet => new { pet.Id, Age = pet.Age * 2 },
(person, pet) => new
{
Person = person,
Pets = pet,
}).SelectMany(
pet => pet.Pets.DefaultIfEmpty(),
(people, pet) => new
{
people.Person,
Pet = pet,
});
