C＃Linq Inner Join

Question

我想选择只有宠物的人。

执行查询时

var query = from p in people
                        join
                        pts in pets
                        on p equals pts.Owner into grp
                        select new {grp=grp,PersonName=p.FirstName};

没有宠物的人也会被选中。

我的名单是

Person[] prn = new Person[3];
prn[0] = new Person();
prn[0].FirstName = "Jon";
prn[0].LastName = "Skeet";

prn[1] = new Person();
prn[1].FirstName = "Marc";
prn[1].LastName = "Gravell";

prn[2] = new Person();
prn[2].FirstName = "Alex";
prn[2].LastName = "Grover";

List<Person> people = new List<Person>();

 foreach (Person p in prn)
 {
     people.Add(p);
 }

 Pet[] pt = new Pet[3];

 pt[0] = new Pet();
 pt[0].Name = "Zonny";
 pt[0].Owner = people[0];

pt[1] = new Pet();
pt[1].Name = "Duggie";
pt[1].Owner = people[0];

pt[2] = new Pet();
pt[2].Name = "Zoggie";
pt[2].Owner = people[1];

List<Pet> pets=new List<Pet>();
 foreach(Pet p in pt)
 {
    pets.Add(p);
 }

Answer 1

那是因为你正在使用join ... into进行群组加入。你只想要一个普通的连接：

var query = (from p in people
             join pts in pets on p equals pts.Owner
             select p).Distinct();

或者，如果您想要有宠物的人及其主人，您可以做以下事情：

var query = pets.GroupBy(pet => pet.Owner)
                .Select(x => new { Owner = x.Key, Pets = x.ToList() });

这会产生一个结果，你可以得到每个主人和他们的宠物，但仅限于拥有宠物的人。

如果你想要别的东西，请告诉我们......

顺便说一句，现在是了解对象和集合初始化程序的好时机。这是一种初始化people列表的简单方法，例如：

List<Person> people = new List<Person>
{
    new Person { FirstName = "Jon", LastName = "Skeet" },
    new Person { FirstName = "Marc", LastName = "Gravell" },
    new Person { FirstName = "Alex", LastName = "Grover" },
};

更加紧凑：）

编辑：交叉连接很简单：

var query = from person in people
            from pet in pets
            select new { person, pet };

使用组连接有效地模拟左连接。听起来你有深度C＃，我建议你仔细阅读第11章：）

Answer 2

这是另一种方法，只添加一行：

var query = from p in people
            join pts in pets
            on p equals pts.Owner into grp
            where grp.Any()             // <--- added this
            select new {grp=grp,PersonName=p.FirstName};

在这里，我像你一样选择组，但我添加了一行，只选择包含至少一个元素的组，并忽略其余的组。

Answer 3

这也可以在一行代码中使用lambda表达式来完成......

IEnumerable<Person> peopleWithPets = people.Where(x => pets.Any(y => y.Owner == x));