Linq 简明教程

LINQ - SQL

LINQ to SQL 为管理对象关系数据提供一个基础结构（运行时）。它是 .NET Framework 3.5 版本的一个组件，能够将对象模型的语言集成查询翻译成 SQL。然后将这些查询发送到数据库以供执行。在从数据库获取结果后，LINQ to SQL 会再次将结果转换为对象。

Introduction of LINQ To SQL

对于大多数的 ASP.NET 开发人员而言，LINQ to SQL（也称为 DLINQ）是语言集成查询中令人振奋的部分，因为它可以通过使用通常的 LINQ 表达式来查询 SQL 服务器数据库。它还允许更新、删除和插入数据，但它所遭受的唯一缺点是它仅限于 SQL 服务器数据库。然而，LINQ to SQL 相较于 ADO.NET 具有许多优势，例如复杂度降低、编码行数减少等等。

以下是显示 LINQ to SQL 执行架构的图表。

How to Use LINQ to SQL?

Step 1 - 与数据库服务器建立新的“数据连接”。查看 & Farrar; 服务器资源管理器 & Farrar; 数据连接 & Farrar; 添加连接

Step 2 - 添加 LINQ To SQL 类文件

Step 3 - 从数据库中选择表并将其拖放到新的 LINQ to SQL 类文件中。

Step 4 - 已将表添加到类文件中。

Querying with LINQ to SQL

使用 LINQ to SQL 执行查询的规则类似于标准 LINQ 查询，即延迟或立即执行查询。有各种组件在使用 LINQ to SQL 执行查询中发挥作用，如下所示。

LINQ to SQL API - 代表应用程序请求查询执行，并将其发送到 LINQ to SQL 提供程序。
LINQ to SQL Provider - 将查询转换为事务 SQL（T-SQL），并将新查询发送到 Ado 提供程序以供执行。
ADO Provider - 在执行查询后，将结果以 DataReader 形式发送到 LINQ to SQL 提供程序，而后者又将其转换为用户对象的形式。

需要注意的是，在执行 LINQ to SQL 查询之前，必须通过 DataContext 类连接到数据源。

Insert, Update and Delete using LINQ To SQL

Add OR Insert

using System;
using System.Linq;

namespace LINQtoSQL {
   class LinqToSQLCRUD {
      static void Main(string[] args) {

         string connectString = System.Configuration.ConfigurationManager.ConnectionStrings["LinqToSQLDBConnectionString"].ToString();

         LinqToSQLDataContext db = new LinqToSQLDataContext(connectString);

         //Create new Employee

         Employee newEmployee = new Employee();
         newEmployee.Name = "Michael";
         newEmployee.Email = "yourname@companyname.com";
         newEmployee.ContactNo = "343434343";
         newEmployee.DepartmentId = 3;
         newEmployee.Address = "Michael - USA";

         //Add new Employee to database
         db.Employees.InsertOnSubmit(newEmployee);

         //Save changes to Database.
         db.SubmitChanges();

         //Get new Inserted Employee
         Employee insertedEmployee = db.Employees.FirstOrDefault(e ⇒e.Name.Equals("Michael"));

         Console.WriteLine("Employee Id = {0} , Name = {1}, Email = {2}, ContactNo = {3}, Address = {4}",
                          insertedEmployee.EmployeeId, insertedEmployee.Name, insertedEmployee.Email,
                          insertedEmployee.ContactNo, insertedEmployee.Address);

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

Module Module1

   Sub Main()

      Dim connectString As String = System.Configuration.ConfigurationManager.ConnectionStrings("LinqToSQLDBConnectionString").ToString()

      Dim db As New LinqToSQLDataContext(connectString)

      Dim newEmployee As New Employee()

      newEmployee.Name = "Michael"
      newEmployee.Email = "yourname@companyname.com"
      newEmployee.ContactNo = "343434343"
      newEmployee.DepartmentId = 3
      newEmployee.Address = "Michael - USA"

      db.Employees.InsertOnSubmit(newEmployee)

      db.SubmitChanges()

      Dim insertedEmployee As Employee = db.Employees.FirstOrDefault(Function(e) e.Name.Equals("Michael"))

      Console.WriteLine("Employee Id = {0} , Name = {1}, Email = {2}, ContactNo = {3},
         Address = {4}", insertedEmployee.EmployeeId, insertedEmployee.Name,
         insertedEmployee.Email, insertedEmployee.ContactNo, insertedEmployee.Address)

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

   End Sub

End Module

编译并运行 C# 或 VB 的上述代码后，会产生以下结果 −

Emplyee ID = 4, Name = Michael, Email = yourname@companyname.com, ContactNo =
343434343, Address = Michael - USA

Press any key to continue.

Update

using System;
using System.Linq;

namespace LINQtoSQL {
   class LinqToSQLCRUD {
      static void Main(string[] args) {

         string connectString = System.Configuration.ConfigurationManager.ConnectionStrings["LinqToSQLDBConnectionString"].ToString();

         LinqToSQLDataContext db = new LinqToSQLDataContext(connectString);

         //Get Employee for update
         Employee employee = db.Employees.FirstOrDefault(e =>e.Name.Equals("Michael"));

         employee.Name = "George Michael";
         employee.Email = "yourname@companyname.com";
         employee.ContactNo = "99999999";
         employee.DepartmentId = 2;
         employee.Address = "Michael George - UK";

         //Save changes to Database.
         db.SubmitChanges();

         //Get Updated Employee
         Employee updatedEmployee = db.Employees.FirstOrDefault(e ⇒e.Name.Equals("George Michael"));

         Console.WriteLine("Employee Id = {0} , Name = {1}, Email = {2}, ContactNo = {3}, Address = {4}",
                          updatedEmployee.EmployeeId, updatedEmployee.Name, updatedEmployee.Email,
                          updatedEmployee.ContactNo, updatedEmployee.Address);

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

Module Module1

   Sub Main()

      Dim connectString As String = System.Configuration.ConfigurationManager.ConnectionStrings("LinqToSQLDBConnectionString").ToString()

      Dim db As New LinqToSQLDataContext(connectString)

      Dim employee As Employee = db.Employees.FirstOrDefault(Function(e) e.Name.Equals("Michael"))

      employee.Name = "George Michael"
      employee.Email = "yourname@companyname.com"
      employee.ContactNo = "99999999"
      employee.DepartmentId = 2
      employee.Address = "Michael George - UK"

      db.SubmitChanges()

      Dim updatedEmployee As Employee = db.Employees.FirstOrDefault(Function(e) e.Name.Equals("George Michael"))

      Console.WriteLine("Employee Id = {0} , Name = {1}, Email = {2}, ContactNo = {3},
         Address = {4}", updatedEmployee.EmployeeId, updatedEmployee.Name,
         updatedEmployee.Email, updatedEmployee.ContactNo, updatedEmployee.Address)

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

   End Sub

End Module

编译并运行 C# 或 Vb 的上述代码后，会产生以下结果 −

Emplyee ID = 4, Name = George Michael, Email = yourname@companyname.com, ContactNo =
999999999, Address = Michael George - UK

Press any key to continue.

Delete

using System;
using System.Linq;

namespace LINQtoSQL {
   class LinqToSQLCRUD {
      static void Main(string[] args) {

         string connectString = System.Configuration.ConfigurationManager.ConnectionStrings["LinqToSQLDBConnectionString"].ToString();

         LinqToSQLDataContext db = newLinqToSQLDataContext(connectString);

         //Get Employee to Delete
         Employee deleteEmployee = db.Employees.FirstOrDefault(e ⇒e.Name.Equals("George Michael"));

         //Delete Employee
         db.Employees.DeleteOnSubmit(deleteEmployee);

         //Save changes to Database.
         db.SubmitChanges();

         //Get All Employee from Database
         var employeeList = db.Employees;
         foreach (Employee employee in employeeList) {
            Console.WriteLine("Employee Id = {0} , Name = {1}, Email = {2}, ContactNo = {3}",
               employee.EmployeeId, employee.Name, employee.Email, employee.ContactNo);
         }

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

Module Module1

   Sub Main()

      Dim connectString As String = System.Configuration.ConfigurationManager.ConnectionStrings("LinqToSQLDBConnectionString").ToString()

      Dim db As New LinqToSQLDataContext(connectString)

      Dim deleteEmployee As Employee = db.Employees.FirstOrDefault(Function(e) e.Name.Equals("George Michael"))

      db.Employees.DeleteOnSubmit(deleteEmployee)

      db.SubmitChanges()

      Dim employeeList = db.Employees

      For Each employee As Employee In employeeList
         Console.WriteLine("Employee Id = {0} , Name = {1}, Email = {2}, ContactNo = {3}",
            employee.EmployeeId, employee.Name, employee.Email, employee.ContactNo)
      Next

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

End Module

编译并运行 C# 或 VB 的上述代码后，会产生以下结果 −

Emplyee ID = 1, Name = William, Email = abc@gy.co, ContactNo = 999999999
Emplyee ID = 2, Name = Miley, Email = amp@esds.sds, ContactNo = 999999999
Emplyee ID = 3, Name = Benjamin, Email = asdsad@asdsa.dsd, ContactNo =

Press any key to continue.