LINQ内加入VS左加入

本文介绍了LINQ内加入VS左加入的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 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();