LINQ – 对象

LINQ to Objects 提供了使用任何支持 IEnumerable<T> 的 LINQ 查询来访问内存中的数据集合，而无需像 LINQ to SQL 或 LINQ to XML 那样需要 LINQ 提供程序 (API)。

LINQ to Objects 介绍

LINQ to Objects 中的查询通常仅返回 IEnumerable<T> 类型的变量。简而言之，LINQ to Objects 如前所述提供了一种全新的集合方法，编写长代码（非常复杂的 foreach 循环）以从集合中检索数据至关重要，现在已被编写清晰描述所需数据的声明性代码所取代这是检索所需的。

与传统的 foreach 循环相比，LINQ to Objects 还具有许多优点，例如更具可读性、强大的过滤、分组功能、以最少的应用程序编码增强排序。此类 LINQ 查询本质上也更紧凑，无需任何修改或只需稍加修改即可移植到任何其他数据源。

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();
      }
   }
}

在示例中，字符串数组（工具）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;

当上面的代码被编译和执行时，它会产生以下结果 –

Tablesaw
Bandsaw
Planer
Jointer
Drill
Sander

使用 LINQ to Objects 在内存集合中查询

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

编译并执行上述 C# 或 VB 代码时，会产生以下结果 –

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

Press any key to continue.