C# Linq 内部连接

Question

我只想选择养宠物的人。

当我执行查询时

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

没有宠物的人也被选中。

我的 list 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

那是因为您正在使用 join ... into 来加入群组。您只需要一个普通的连接:

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

或者，如果你想要有宠物的人和他们的主人，你可以这样做:

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

这将给出一个结果，您可以获得每个主人和他们的宠物，但仅限于拥有宠物的人。

如果您想要其他东西，请告诉我们...

顺便说一句，现在是学习对象和集合初始化器的好时机。下面是初始化 people 列表的更简单方法，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 };

使用组连接可以有效地模拟左连接。听起来你已经深入了解了 C#，我建议你通读第 11 章 :)