一、简单键联接

下面的示例创建两个集合,其中包含两种用户定义类型 Person 和 Pet 的对象。 查询使用 C# 中的 join 子句将 Person 对象与 Owner 是该 Person 的 Pet 对象匹配。 C# 中的 select 子句定义结果对象的外观。 在此示例中,结果对象是由所有者名字和宠物姓名组成的匿名类型。

class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
} class Pet
{
public string Name { get; set; }
public Person Owner { get; set; }
} /// <summary>
/// Simple inner join.
/// </summary>
public static void InnerJoinExample()
{
Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };
Person rui = new Person { FirstName = "Rui", LastName = "Raposo" }; 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 bluemoon = new Pet { Name = "Blue Moon", Owner = rui };
Pet daisy = new Pet { Name = "Daisy", Owner = magnus }; // Create two lists.
List<Person> people = new List<Person> { magnus, terry, charlotte, arlene, rui };
List<Pet> pets = new List<Pet> { barley, boots, whiskers, bluemoon, daisy }; // Create a collection of person-pet pairs. Each element in the collection
// is an anonymous type containing both the person's name and their pet's name.
var query = from person in people
join pet in pets on person equals pet.Owner
select new { OwnerName = person.FirstName, PetName = pet.Name }; foreach (var ownerAndPet in query)
{
Console.WriteLine($"\"{ownerAndPet.PetName}\" is owned by {ownerAndPet.OwnerName}");
}
} // This code produces the following output:
//
// "Daisy" is owned by Magnus
// "Barley" is owned by Terry
// "Boots" is owned by Terry
// "Whiskers" is owned by Charlotte
// "Blue Moon" is owned by Rui

请注意,LastName 是“Huff”的 Person 对象未出现在结果集中,因为没有 Pet 对象的 Pet.Owner 等于该 Person

二、组合键联接

可以使用复合键基于多个属性来比较元素,而不是只基于一个属性使元素相关联。 为此,请为每个集合指定键选择器函数,以返回由要比较的属性组成的匿名类型。 如果对属性进行标记,则它们必须在每个键的匿名类型中具有相同标签。 属性还必须按相同顺序出现。

class Employee
{
public string FirstName { get; set; }
public string LastName { get; set; }
public int EmployeeID { get; set; }
} class Student
{
public string FirstName { get; set; }
public string LastName { get; set; }
public int StudentID { get; set; }
} /// <summary>
/// Performs a join operation using a composite key.
/// </summary>
public static void CompositeKeyJoinExample()
{
// Create a list of employees.
List<Employee> employees = new List<Employee> {
new Employee { FirstName = "Terry", LastName = "Adams", EmployeeID = },
new Employee { FirstName = "Charlotte", LastName = "Weiss", EmployeeID = },
new Employee { FirstName = "Magnus", LastName = "Hedland", EmployeeID = },
new Employee { FirstName = "Vernette", LastName = "Price", EmployeeID = } }; // Create a list of students.
List<Student> students = new List<Student> {
new Student { FirstName = "Vernette", LastName = "Price", StudentID = },
new Student { FirstName = "Terry", LastName = "Earls", StudentID = },
new Student { FirstName = "Terry", LastName = "Adams", StudentID = } }; // Join the two data sources based on a composite key consisting of first and last name,
// to determine which employees are also students.
IEnumerable<string> query = from employee in employees
join student in students
on new { employee.FirstName, employee.LastName }
equals new { student.FirstName, student.LastName }
select employee.FirstName + " " + employee.LastName; Console.WriteLine("The following people are both employees and students:");
foreach (string name in query)
Console.WriteLine(name);
} // This code produces the following output:
//
// The following people are both employees and students:
// Terry Adams
// Vernette Price

三、多联接

可以将任意数量的联接操作相互追加,以执行多联接。 C# 中的每个 join 子句会将指定数据源与上一个联接的结果相关联。

class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
} class Pet
{
public string Name { get; set; }
public Person Owner { get; set; }
} class Cat : Pet
{ } class Dog : Pet
{ } public static void MultipleJoinExample()
{
Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };
Person rui = new Person { FirstName = "Rui", LastName = "Raposo" };
Person phyllis = new Person { FirstName = "Phyllis", LastName = "Harris" }; Cat barley = new Cat { Name = "Barley", Owner = terry };
Cat boots = new Cat { Name = "Boots", Owner = terry };
Cat whiskers = new Cat { Name = "Whiskers", Owner = charlotte };
Cat bluemoon = new Cat { Name = "Blue Moon", Owner = rui };
Cat daisy = new Cat { Name = "Daisy", Owner = magnus }; Dog fourwheeldrive = new Dog { Name = "Four Wheel Drive", Owner = phyllis };
Dog duke = new Dog { Name = "Duke", Owner = magnus };
Dog denim = new Dog { Name = "Denim", Owner = terry };
Dog wiley = new Dog { Name = "Wiley", Owner = charlotte };
Dog snoopy = new Dog { Name = "Snoopy", Owner = rui };
Dog snickers = new Dog { Name = "Snickers", Owner = arlene }; // Create three lists.
List<Person> people =
new List<Person> { magnus, terry, charlotte, arlene, rui, phyllis };
List<Cat> cats =
new List<Cat> { barley, boots, whiskers, bluemoon, daisy };
List<Dog> dogs =
new List<Dog> { fourwheeldrive, duke, denim, wiley, snoopy, snickers }; // The first join matches Person and Cat.Owner from the list of people and
// cats, based on a common Person. The second join matches dogs whose names start
// with the same letter as the cats that have the same owner.
var query = from person in people
join cat in cats on person equals cat.Owner
join dog in dogs on
new { Owner = person, Letter = cat.Name.Substring(, ) }
equals new { dog.Owner, Letter = dog.Name.Substring(, ) }
select new { CatName = cat.Name, DogName = dog.Name }; foreach (var obj in query)
{
Console.WriteLine(
$"The cat \"{obj.CatName}\" shares a house, and the first letter of their name,
with \"{obj.DogName}\".");
}
} // This code produces the following output:
//
// The cat "Daisy" shares a house, and the first letter of their name, with "Duke".
// The cat "Whiskers" shares a house, and the first letter of their name, with "Wiley".

四、使用分组联接的内联

下面的示例演示如何使用分组联接实现内部联接。

query1 的结果等效于通过使用 join 子句(不使用 into 子句)执行内部联接来获取的结果集。 query2 变量演示了此等效查询。

class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
} class Pet
{
public string Name { get; set; }
public Person Owner { get; set; }
} /// <summary>
/// Performs an inner join by using GroupJoin().
/// </summary>
public static void InnerGroupJoinExample()
{
Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" }; 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 bluemoon = new Pet { Name = "Blue Moon", Owner = terry };
Pet daisy = new Pet { Name = "Daisy", Owner = magnus }; // Create two lists.
List<Person> people = new List<Person> { magnus, terry, charlotte, arlene };
List<Pet> pets = new List<Pet> { barley, boots, whiskers, bluemoon, daisy }; var query1 = from person in people
join pet in pets on person equals pet.Owner into gj
from subpet in gj
select new { OwnerName = person.FirstName, PetName = subpet.Name }; Console.WriteLine("Inner join using GroupJoin():");
foreach (var v in query1)
{
Console.WriteLine($"{v.OwnerName} - {v.PetName}"));
} var query2 = from person in people
join pet in pets on person equals pet.Owner
select new { OwnerName = person.FirstName, PetName = pet.Name }; Console.WriteLine("\nThe equivalent operation using Join():");
foreach (var v in query2)
Console.WriteLine($"{v.OwnerName} - {v.PetName}"));
} // This code produces the following output:
//
// Inner join using GroupJoin():
// Magnus - Daisy
// Terry - Barley
// Terry - Boots
// Terry - Blue Moon
// Charlotte - Whiskers
//
// The equivalent operation using Join():
// Magnus - Daisy
// Terry - Barley
// Terry - Boots
// Terry - Blue Moon
// Charlotte - Whiskers

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