linq查询加入

Question

我无法准备我需要的查询。我有代码：

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 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(); 
    } 
} 

并且它不能正常工作。即使在Dog对象中的OwnerID为空的情况下，我如何准备查询组的新实例仍然被创建并添加到组变量中？

Answer 1

像这样

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) 
}; 

见http://smehrozalam.wordpress.com/2009/06/10/c-left-outer-joins-with-linq/

尽管他的例子说明转换为SQL，同样适用于LINQ对象

Answer 2

从你的描述，这听起来像你想在狗身上做左外连接查询。 查看这个来自微软LINQ的example关于如何使用LINQ函数的例子。