LINQ 中的连接操作符

联接指的是对彼此具有难以直接跟踪的关系的数据源进行定位的一项操作。

联接示例 - 查询表达式

C#

using System;
using System.Collections.Generic;
using System.Linq;

namespace Operators {
   class JoinTables {
      class DepartmentClass {
         public int DepartmentId { get; set; }
         public string Name { get; set; }
      }

      class EmployeeClass {
         public int EmployeeId { get; set; }
         public string EmployeeName { get; set; }
         public int DepartmentId { get; set; }
      }

      static void Main(string[] args) {
      
         List <DepartmentClass> departments = new List <DepartmentClass>();
         departments.Add(new DepartmentClass { DepartmentId = 1, Name = "Account" });
         departments.Add(new DepartmentClass { DepartmentId = 2, Name = "Sales" });
         departments.Add(new DepartmentClass { DepartmentId = 3, Name = "Marketing" });
       
         List <EmployeeClass> employees = new List <EmployeeClass>();
         employees.Add(new EmployeeClass { DepartmentId = 1, EmployeeId = 1, EmployeeName = "William" });
         employees.Add(new EmployeeClass { DepartmentId = 2, EmployeeId = 2, EmployeeName = "Miley" });
         employees.Add(new EmployeeClass { DepartmentId = 1, EmployeeId = 3, EmployeeName = "Benjamin" });


         var list = (from e in employees join d in departments on e.DepartmentId equals d.DepartmentId select new {
            EmployeeName = e.EmployeeName,
            DepartmentName = d.Name
         });
            
         foreach (var e in list) {
            Console.WriteLine("Employee Name = {0} , Department Name = {1}", e.EmployeeName, e.DepartmentName);
         }

         Console.WriteLine("\nPress any key to continue.");
         Console.ReadKey(); 
      }
   }
}

VB

Module Module1

   Sub Main()
   
      Dim account As New Department With {.Name = "Account", .DepartmentId = 1}
      Dim sales As New Department With {.Name = "Sales", .DepartmentId = 2}
      Dim marketing As New Department With {.Name = "Marketing", .DepartmentId = 3}

      Dim departments As New System.Collections.Generic.List(Of Department)(New Department() {account, sales, marketing})

      Dim william As New Employee With {.EmployeeName = "William", .EmployeeId = 1, .DepartmentId = 1}
      Dim miley As New Employee With {.EmployeeName = "Miley", .EmployeeId = 2, .DepartmentId = 2}
      Dim benjamin As New Employee With {.EmployeeName = "Benjamin", .EmployeeId = 3, .DepartmentId = 1}

      Dim employees As New System.Collections.Generic.List(Of Employee)(New Employee() {william, miley, benjamin})

      Dim list = (From e In employees
                  Join d In departments On e.DepartmentId Equals d.DepartmentId
                  Select New Person With {.EmployeeName = e.EmployeeName, .DepartmentName = d.Name})

      For Each e In list
         Console.WriteLine("Employee Name = {0} , Department Name = {1}", e.EmployeeName, e.DepartmentName)
      Next

      Console.WriteLine(vbLf &"Press any key to continue.")
      Console.ReadKey()
   End Sub

   Class Employee
      Public Property EmployeeId As Integer
      Public Property EmployeeName As String
      Public Property DepartmentId As Integer
   End Class

   Class Department
      Public Property Name As String
      Public Property DepartmentId As Integer
   End Class

   Class Person
      Public Property EmployeeName As String
      Public Property DepartmentName As String
   End Class
   
End Module

当编译并执行上述 C# 或 VB 代码时，会产生以下结果 −

Emplyee Name = William, Department Name = Account
Emplyee Name = Miley, Department Name = Sales
Emplyee Name = Benjamin, Department Name = Account

Press any key to continue.