C# 带连接的linq查询
我无法准备我需要的查询。我有密码:C# 带连接的linq查询,c#,.net,linq,collections,C#,.net,Linq,Collections,我无法准备我需要的查询。我有密码: public class Dog { public int id; public int? OwnerID; public string name; } public class Person { public int id; public string Name; } public class Group { public Dog dog; public Person owner; } class
public class Dog
{
public int id;
public int? OwnerID;
public string name;
}
public class Person
{
public int id;
public string Name;
}
public class Group
{
public Dog dog;
public Person owner;
}
class Program
{
static void Main(string[] args)
{
IEnumerable<Dog> dogs = new[] {
new Dog { id = 1, OwnerID = null, name = "Burke" },
new Dog { id = 2, OwnerID = 2, name = "Barkley" }
};
IEnumerable<Person> owners = new[] {
new Person { id = 1, Name = "Jack" },
new Person { id = 2, Name = "Philip" }
};
var groups = from dog in dogs
join owner in owners on dog.OwnerID equals owner.id
select new Group
{
dog = dogs.First(d => d.id == dog.id),
owner = owners.First(o => o.id == owner.id)
};
foreach (var g in groups)
{
var text = g.dog.name + " belongs to " + (g.owner == null ? "no one" : g.owner.Name);
Console.WriteLine(text);
}
Console.ReadLine();
}
}
公共级狗
{
公共int id;
公共内部所有者ID;
公共字符串名称;
}
公共阶层人士
{
公共int id;
公共字符串名称;
}
公共课组
{
公犬;
公众人物所有者;
}
班级计划
{
静态void Main(字符串[]参数)
{
IEnumerable dogs=new[]{
新狗{id=1,OwnerID=null,name=“Burke”},
新狗{id=2,OwnerID=2,name=“Barkley”}
};
IEnumerable owners=new[]{
新人{id=1,Name=“Jack”},
新人{id=2,Name=“Philip”}
};
var组=来自狗中的狗
在dog.OwnerID等于owner.id上加入所有者
选择新组
{
dog=dogs.First(d=>d.id==dog.id),
owner=owner.First(o=>o.id==owner.id)
};
foreach(组中的变量g)
{
var text=g.dog.name+“属于”+(g.owner==null?“没有人”:g.owner.name);
控制台写入线(文本);
}
Console.ReadLine();
}
}
但它并没有像我预期的那样起作用。如何准备查询,即使Dog对象中的OwnerID为null,仍然会创建组的新实例并将其添加到groups变量中?类似于此
var groups = from dog in dogs
join owner in owners on dog.OwnerID equals owner.id
from own in owner.DefaultIfEmpty()
select new Group
{
dog = dogs.First(d => d.id == dog.id),
owner = own.First(o => o.id == owner.id)
};
看
尽管他的示例显示了到SQL的转换,但同样的情况也适用于linq到对象
var groups = from dog in dogs
join owner in owners on dog.OwnerID equals owner.id
from own in owner.DefaultIfEmpty()
select new Group
{
dog = dogs.First(d => d.id == dog.id),
owner = own.First(o => o.id == owner.id)
};
看
尽管他的示例显示了到SQL的转换,但从您所描述的内容来看,这同样适用于linq to objects,这听起来像是要对Dogs执行左外连接查询。
查看Microsoft的LINQ示例,了解如何使用
DefaultIfEmpty()
LINQ函数。根据您的描述,听起来您想对狗执行左外连接查询。
查看Microsoft的LINQ示例,了解如何使用
DefaultIfEmpty()
LINQ函数。非常感谢这一点;)非常感谢,这正是我想要的;)