LINQ之Join(from join on )与GroupJoin(from join on into)将两个集合进行关联与分组

[C#] LINQ之Join与GroupJoin - cnxy - 博客园 (cnblogs.com)

[C#]使用Join与GroupJoin将两个集合进行关联与分组 - cnxy - 博客园 (cnblogs.com)

LINQ TO SQL 中的join - min.jiang - 博客园 (cnblogs.com)

Enumerable.Join 方法 (System.Linq) | Microsoft Docs

Enumerable.GroupJoin 方法 (System.Linq) | Microsoft Docs

代码1如下

void Main()
{
    Person[] persons = new Person[]
        {
        new Person{ CityID = 1, Name = "ABC" },
        new Person{ CityID = 1, Name = "EFG" },
        new Person{ CityID = 2, Name = "HIJ" },
        new Person{ CityID = 3, Name = "KLM" },
        new Person{ CityID = 3, Name = "NOP" },
        new Person{ CityID = 4, Name = "QRS" },
        new Person{ CityID = 5, Name = "TUV" }
        };

    City[] cities = new City[]
    {
        new City{ ID = 1,Name = "Guangzhou" },
        new City{ ID = 2,Name = "Shenzhen" },
        new City{ ID = 3,Name = "Beijing" },
        new City{ ID = 4,Name = "Shanghai" }
    };


    Console.WriteLine("********Join第一种用法*************************************");
    //public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, TInner, TResult> resultSelector);
    //官方释义：基于匹配键对两个序列的元素进行关联。使用默认的相等比较器对键进行比较。

    var result1 = persons.Join(cities, p => p.CityID, c => c.ID, (p, c) => new { PersonName = p.Name, CityName = c.Name });

    //    其等价的LINQ语句为：  
    var result = from p in persons
                 join c in cities on p.CityID equals c.ID
                 select new { PersonName = p.Name, CityName = c.Name };

    foreach (var item in result1)
    {
        Console.WriteLine($"{item.PersonName},{item.CityName}");
    }
    Console.WriteLine();


    Console.WriteLine("********Join第二种用法*************************************");
    //public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, TInner, TResult> resultSelector, IEqualityComparer<TKey> comparer);
    //官方释义：基于匹配键对两个序列的元素进行关联。使用指定的IEqualityComparer<TKey> 对键进行比较。



    Console.WriteLine("********GroupJoin第一种方法*************************************");
    //public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector);
    //官方释义： 基于键相等对两个序列的元素进行关联并对结果进行分组。使用默认的相等比较器对键进行比较。

    var result3 = persons.GroupJoin(cities, p => p.CityID, c => c.ID, (p, cs) => new { PersonName = p.Name, Citys = cs });

    //    其等价的LINQ语句为：
    //var result3 = from p in persons
    //              join c in cities on p.CityID equals c.ID into cs
    //              select new { PersonName = p.Name, Citys = cs };

    foreach (var item in result3)
    {
        Console.Write($"{item.PersonName}	");
        foreach (var city in item.Citys)
        {
            Console.Write($"{city.Name}");
        }
        Console.WriteLine();
    }
    Console.WriteLine();


    //使用cities去左关联persons
    var result4 = cities.GroupJoin(persons, p => p.ID, c => c.CityID, (p, cs) => new { PersonName = p.Name, Persons = cs });
    foreach (var item in result4)
    {
        Console.Write($"{item.PersonName}:");
        foreach (var city in item.Persons)
        {
            Console.Write($"{city.Name}	");
        }
        Console.WriteLine();
    }


    Console.WriteLine();

    Console.WriteLine("********GroupJoin第二种方法*************************************");
    //public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector, IEqualityComparer<TKey> comparer);
    //官方释义：基于键相等对两个序列的元素进行关联并对结果进行分组。使用指定的IEqualityComparer<TKey> 对键进行比较。
}

class Person
{
    public int CityID { set; get; }
    public string Name { set; get; }
}
class City
{
    public int ID { set; get; }
    public string Name { set; get; }
}

结果1：

********Join第一种用法*************************************
ABC,Guangzhou
EFG,Guangzhou
HIJ,Shenzhen
KLM,Beijing
NOP,Beijing
QRS,Shanghai

********Join第二种用法*************************************
********GroupJoin第一种方法*************************************
ABC  Guangzhou
EFG  Guangzhou
HIJ  Shenzhen
KLM  Beijing
NOP  Beijing
QRS  Shanghai
TUV  

Guangzhou:ABC  EFG  
Shenzhen:HIJ  
Beijing:KLM  NOP  
Shanghai:QRS  

********GroupJoin第二种方法*************************************

代码2如下

void Main()
{
    List<Person> pList = new List<Person> {
        new Person(){ Name = "ABC", Age = 18 },
        new Person(){ Name = "EFG", Age = 19 },
        new Person(){ Name = "LMN", Age = 20 },
        new Person(){ Name = "XYZ", Age = 21 }
        };

    List<Department> dList = new List<Department> {
        new Department() { Name = "A1", Employee = pList[0] },
        new Department() { Name = "A2", Employee = pList[1] },
        new Department() { Name = "A3", Employee = pList[0] },
        new Department() { Name = "B1", Employee = pList[2] },
        new Department() { Name = "B2", Employee = pList[3] },
        new Department() { Name = "B3", Employee = pList[3] }
        };

    Console.WriteLine("********Join方法*************************************");


    var result = pList.Join(dList,
        person => person,
        department => department.Employee,
        (person, department) => new
        {
            Person = person,
            Department = department
        });

    foreach (var item1 in result)
    {
        Console.WriteLine($"Name:{item1.Person} & Department:{item1.Department} ");

    }
    Console.WriteLine();

    Console.WriteLine("********GroupJoin方法*************************************");
    var result2 = pList.GroupJoin(dList,
        person => person,
        department => department.Employee,
        (person, departments) => new
        {
            Person = person,
            Department = departments.Select(d => d)
        });

    foreach (var item1 in result2)
    {
        Console.WriteLine($"Name:{item1.Person} & ");
        foreach (var item2 in item1.Department)
        {
            if (item1.Department.First() == item2)
                Console.Write($"Department:{item2} ");
            else
                Console.Write($"{item2} ");
        }
    }
}
class Person
{
    public string Name { set; get; }
    public int Age { set; get; }
    public override string ToString()
    {
        return $"{Name},{Age}";
    }
}

class Department
{
    public string Name { set; get; }
    public Person Employee { set; get; }
    public override string ToString()
    {
        return $"{Name}";
    }
}

结果：

********Join方法*************************************
Name:ABC,18 & Department:A1 
Name:ABC,18 & Department:A3 
Name:EFG,19 & Department:A2 
Name:LMN,20 & Department:B1 
Name:XYZ,21 & Department:B2 
Name:XYZ,21 & Department:B3 

********GroupJoin方法*************************************
Name:ABC,18 & 
Department:A1 A3 Name:EFG,19 & 
Department:A2 Name:LMN,20 & 
Department:B1 Name:XYZ,21 & 
Department:B2 B3