LINQ内加入VS左加入 [英] LINQ Inner-Join vs Left-Join

问题描述

使用扩展语法我试图创建一个使用LINQ在两个列表中，我有一个左联接。以下是从微软的帮助，但我已经修改了它表明，宠物列表中没有的元素。什么我结束了同是0元素的列表。我认为，这是因为内部联接正在发生。我想结束了是3元（3 Person对象）与填充了缺少的元素空数据的列表。即左加入。这可能吗？

 人马格努斯=新的Person {名称=Hedlund的，马格努斯};
人特里=新的Person {名称=亚当斯，特里};
夏洛特人=新的Person {名称=韦斯，夏洛特};

//宠物大麦=新宠物{名称=大麦，业主=特里};
//宠物靴=新宠物{名称=靴子，业主=特里};
//宠物晶须=新宠物{名称=威威，业主=夏洛特};
//宠物菊花=新宠物{名称=雏菊，业主=马格努斯};

名单＆LT;人＆GT;人=新的名单，其中，人物＆GT; {马格努斯，特里，夏洛特};
//名单，其中，宠物＆GT;宠物=新的名单，其中，宠物＆GT; {大麦，靴子，胡须，菊花};
名单＆LT;宠物＆GT;宠物=新的名单，其中，宠物＆GT;（）;

//创建人物，宠物对，其中一个列表
//每一个元素是一个匿名类型包含
//宠物的名字，并且拥有宠物的人的名字。
VAR的查询=
    people.Join（宠物，
                人=＆GT;人，
                宠物=＆GT; pet.Owner，
                （人，宠物）=＆GT;
                    新{OWNERNAME = person.Name，宠物= pet.Name}）了ToList（）。
 

解决方案

我想，如果你想使用扩展方法，你需要使用的群组加入

  VAR的查询=
    people.GroupJoin（宠物，
                     人=＆GT;人，
                     宠物=＆GT; pet.Owner，
                     （人，petCollection）=＆GT;
                        新{OWNERNAME = person.Name，
                              宠物= PetCollection.Select（P =＆GT; p.Name）
                                                 .DefaultIfEmpty（）}
                    ）.ToList（）;
 

您可能不得不玩的选择EX pression。我不知道它会给你想你想的情况下，你有一个1对多的关系。

我认为这是一个更容易一些与LINQ查询语法

  VAR的查询=（从人context.People
             宠物一起在context.Pets的人等于pet.Owner
             进入tempPets
             从tempPets.DefaultIfEmpty宠物（）
             选择新{OWNERNAME = person.Name，宠物= pets.Name}）
            .ToList（）;
 

Using extension syntax I'm trying to create a left-join using LINQ on two lists that I have. The following is from the Microsoft help but I've modified it to show that the pets list has no elements. What I'm ending up with is a list of 0 elements. I assume that this is because an inner-join is taking place. What I want to end up with is a list of 3 elements (the 3 Person objects) with null data filled in for the missing elements. i.e. a Left-Join. Is this possible?

Person magnus = new Person { Name = "Hedlund, Magnus" };
Person terry = new Person { Name = "Adams, Terry" };
Person charlotte = new Person { Name = "Weiss, Charlotte" };

//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 daisy = new Pet { Name = "Daisy", Owner = magnus };

List<Person> people = new List<Person> { magnus, terry, charlotte };
//List<Pet> pets = new List<Pet> { barley, boots, whiskers, daisy };
List<Pet> pets = new List<Pet>();

// Create a list of Person-Pet pairs where 
// each element is an anonymous type that contains a
// Pet's name and the name of the Person that owns the Pet.
var query =
    people.Join(pets,
                person => person,
                pet => pet.Owner,
                (person, pet) =>
                    new { OwnerName = person.Name, Pet = pet.Name }).ToList();

解决方案

I think if you want to use extension methods you need to use the GroupJoin

var query =
    people.GroupJoin(pets,
                     person => person,
                     pet => pet.Owner,
                     (person, petCollection) =>
                        new { OwnerName = person.Name,
                              Pet = PetCollection.Select( p => p.Name )
                                                 .DefaultIfEmpty() }
                    ).ToList();

You may have to play around with the selection expression. I'm not sure it would give you want you want in the case where you have a 1-to-many relationship.

I think it's a little easier with the LINQ Query syntax

var query = (from person in context.People
             join pet in context.Pets on person equals pet.Owner
             into tempPets
             from pets in tempPets.DefaultIfEmpty()
             select new { OwnerName = person.Name, Pet = pets.Name })
            .ToList();

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