Linq 简明教程

LINQ - Objects

LINQ to Objects 提供对任何支持 IEnumerable<T> 的 LINQ 查询的使用，用于访问内存中数据集合，无需 LINQ 提供程序 (API)，如 LINQ to SQL 或 LINQ to XML。

Introduction of LINQ to Objects

LINQ to Objects 中的查询通常仅返回类型为 IEnumerable<T> 的变量。简而言之，LINQ to Objects 提供了一种新的集合方法，如前所述，以前必须编写长编码（非常复杂的 foreach 循环）才能从集合中检索数据，现在已替换为编写声明性代码，其中明确描述了需要检索的目标数据。

LINQ to Objects 还比传统的 foreach 循环具有更多的优势，例如，可读性更好，过滤功能更强大，分组功能更强大，增强排序功能，同时应用程序编码更少。此类 LINQ 查询本质上也更紧凑，并且可以移植到任何其他数据源，而无需进行任何修改或只需进行少量修改。

下面是一个简单的 LINQ to Objects 示例：

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

namespace LINQtoObjects {
   class Program {
      static void Main(string[] args) {

         string[] tools = { "Tablesaw", "Bandsaw", "Planer", "Jointer", "Drill", "Sander" };
         var list = from t in tools select t;

         StringBuilder sb = new StringBuilder();

         foreach (string s in list) {
            sb.Append(s + Environment.NewLine);
         }

         Console.WriteLine(sb.ToString(), "Tools");
         Console.ReadLine();
      }
   }
}

在该示例中，字符串数组（工具）用作使用 LINQ to Objects 查询的对象集合。

Objects query is:
var list = from t in tools select t;

编译并执行上述代码后，将产生以下结果 −

Tablesaw
Bandsaw
Planer
Jointer
Drill
Sander

Querying in Memory Collections Using LINQ to Objects

C

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

namespace LINQtoObjects {
   class Department {
      public int DepartmentId { get; set; }
      public string Name { get; set; }
   }

   class LinqToObjects {
      static void Main(string[] args) {

         List<Department> departments = new List<Department>();

         departments.Add(new Department { DepartmentId = 1, Name = "Account" });
         departments.Add(new Department { DepartmentId = 2, Name = "Sales" });
         departments.Add(new Department { DepartmentId = 3, Name = "Marketing" });

         var departmentList = from d in departments
                              select d;

         foreach (var dept in departmentList) {
            Console.WriteLine("Department Id = {0} , Department Name = {1}",
               dept.DepartmentId, dept.Name);
         }

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

VB

Imports System.Collections.Generic
Imports System.Linq

Module Module1

   Sub Main(ByVal args As String())

      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 departmentList = From d In departments

      For Each dept In departmentList
         Console.WriteLine("Department Id = {0} , Department Name = {1}", dept.DepartmentId, dept.Name)
      Next

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

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

End Module

当以上 C# 或 VB 代码被编译并执行时，它会生成以下结果 -

Department Id = 1, Department Name = Account
Department Id = 2, Department Name = Sales
Department Id = 3, Department Name = Marketing

Press any key to continue.