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C# Linq的Join()方法 – 将两个序列中的元素联接在一起

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下面是关于 C# Linq 的 Join() 的详细攻略及两个样例示例。

Join() 方法概述

Join() 是 C# Linq 中用于将两个集合中符合条件的元素进行连接的方法。它需要两个集合参数和两个 Key Selector 函数，其中一个 Key Selector 用于指定第一个集合中的连接键，另一个 Key Selector 用于指定第二个集合中的连接键。Join() 方法的返回值是一个新集合，其中的元素包含了两个集合中连接键相等的元素对。

Join() 方法的语法如下：

Join<TOuter, TInner, TKey, TResult>(
    IEnumerable<TOuter> outer,
    IEnumerable<TInner> inner,
    Func<TOuter, TKey> outerKeySelector,
    Func<TInner, TKey> innerKeySelector,
    Func<TOuter, TInner, TResult> resultSelector
)

参数说明：

  • outer ：第一个集合。
  • inner ：第二个集合。
  • outerKeySelector ：指定第一个集合中的连接键。
  • innerKeySelector ：指定第二个集合中的连接键。
  • resultSelector ：组合两个集合中连接键相等的元素对得到的新元素对象。

示例 1

下面是一个简单的示例，用于演示如何使用 Join() 方法连接两个集合：

// 创建“外部的”集合
List<Department> departments = new List<Department>()
{
    new Department { Id = 1, Name = "HR" },
    new Department { Id = 2, Name = "Finance" },
    new Department { Id = 3, Name = "IT" }
};

// 创建“内部的”集合
List<Employee> employees = new List<Employee>()
{
    new Employee { Id = 1, Name = "Alice", DepartmentId = 1 },
    new Employee { Id = 2, Name = "Bob", DepartmentId = 1 },
    new Employee { Id = 3, Name = "Charlie", DepartmentId = 3 },
    new Employee { Id = 4, Name = "David", DepartmentId = 1 },
    new Employee { Id = 5, Name = "Eve", DepartmentId = 2 },
};

// 使用 Join() 方法连接两个集合
var query = departments.Join(
    employees,
    department => department.Id,
    employee => employee.DepartmentId,
    (department, employee) => new { Department = department.Name, Employee = employee.Name }
);

// 输出连接的结果
foreach (var item in query)
{
    Console.WriteLine($"Department: {item.Department}, Employee: {item.Employee}");
}

运行以上代码后，输出结果如下：

Department: HR, Employee: Alice
Department: HR, Employee: Bob
Department: HR, Employee: David
Department: IT, Employee: Charlie
Department: Finance, Employee: Eve

以上代码创建了两个集合：departments 和 employees，其中 Department 对象包含 Id 和 Name 属性，Employee 对象包含 Id、Name 和 DepartmentId 属性。然后使用 Join() 方法连接这两个集合，按照 Department 对象的 Id 属性与 Employee 对象的 DepartmentId 属性进行匹配，返回结果包含了每个部门和该部门的所有员工的名称。

示例 2

下面是一个稍微复杂一些的示例，用于演示如何使用 Join() 方法连接三个集合，并且使用一个匿名类型把多个属性放在同一个对象内：

// 创建“外部的”集合
List<Department> departments = new List<Department>()
{
    new Department { Id = 1, Name = "HR" },
    new Department { Id = 2, Name = "Finance" },
    new Department { Id = 3, Name = "IT" }
};

// 创建“中间的”集合
List<Position> positions = new List<Position>()
{
    new Position { Id = 1, Name = "Manager", DepartmentId = 1 },
    new Position { Id = 2, Name = "Specialist", DepartmentId = 2 },
    new Position { Id = 3, Name = "Engineer", DepartmentId = 3 },
};

// 创建“内部的”集合
List<Employee> employees = new List<Employee>()
{
    new Employee { Id = 1, Name = "Alice", PositionId = 1 },
    new Employee { Id = 2, Name = "Bob", PositionId = 1 },
    new Employee { Id = 3, Name = "Charlie", PositionId = 2 },
    new Employee { Id = 4, Name = "David", PositionId = 3 },
    new Employee { Id = 5, Name = "Eve", PositionId = 3 },
};

// 使用 Join() 方法连接三个集合，组合成一个匿名对象
var query = departments.Join(
    positions,
    department => department.Id,
    position => position.DepartmentId,
    (department, position) => new { Department = department.Name, Position = position.Name, PositionId = position.Id })
    .Join(
        employees,
        x => x.PositionId,
        employee => employee.PositionId,
        (x, employee) => new { x.Department, x.Position, Employee = employee.Name }
    );

// 输出连接的结果
foreach (var item in query)
{
    Console.WriteLine($"Department: {item.Department}, Position: {item.Position}, Employee: {item.Employee}");
}

运行以上代码后，输出结果如下：

Department: HR, Position: Manager, Employee: Alice
Department: HR, Position: Manager, Employee: Bob
Department: Finance, Position: Specialist, Employee: Charlie
Department: IT, Position: Engineer, Employee: David
Department: IT, Position: Engineer, Employee: Eve

以上代码创建了三个集合：departments、positions 和 employees，其中 Department 对象包含 Id 和 Name 属性，Position 对象包含 Id、Name 和 DepartmentId 属性，Employee 对象包含 Id、Name 和 PositionId 属性。然后使用 Join() 方法连接这三个集合，按照 Department 对象的 Id 属性与 Position 对象的 DepartmentId 属性进行匹配，并且按照 Position 对象的 Id 属性与 Employee 对象的 PositionId 属性进行匹配，返回结果包含了每个部门、职位和该职位的员工的名称。

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