LINQ 对象

LINQ to Objects 允许使用任何支持 IEnumerable<T> 的 LINQ 查询，以访问内存中的数据集合，而无需使用 LINQ 供应商 (API)，就像 LINQ to SQL 或 LINQ to XML 中的情况。

LINQ to Objects 简介

LINQ to Objects 中的查询通常只返回类型为 IEnumerable<T> 的变量。简而言之，LINQ to Objects 为集合提供了一种新的方法，因为以前，必须编写冗长的编码（高复杂度的 foreach 循环）才能从集合中检索数据，而现在则改用编写描述该所需检索数据的声明性代码。

LINQ to Objects 相较于传统的 foreach 循环有许多优势，如更强的可读性、强大的筛选、分组能力、最少的应用程序编码即可增强排序。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 (tools) 用作要使用 LINQ to Objects 查询的对象集合。

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

当编译并执行上述代码时，会生成以下结果 −

Tablesaw
Bandsaw
Planer
Jointer
Drill
Sander

使用 LINQ to Objects 查询内存中的集合

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

当编译并执行上述 C# 或 VB 代码时，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.