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📜  LINQ-对象

📅  最后修改于: 2020-11-21 07:07:18             🧑  作者: Mango

LINQ to Objects可以使用支持IEnumerable 的任何LINQ查询来访问内存中的数据集合，而无需使用LINQ提供程序(API)，例如LINQ to SQL或LINQ to XML。

LINQ对象介绍

LINQ to Objects中的查询仅返回通常为IEnumerable 类型的变量。简而言之，LINQ to Objects提供了一种较新的集合方法，为从集合中检索数据编写长编码(非常复杂的foreach循环)至关重要，现在已由编写清楚描述所需数据的声明性代码代替需要检索。

与传统的foreach循环相比，LINQ to Objects还具有许多优点，例如，更高的可读性，强大的过滤功能，分组功能，增强的排序功能以及最少的应用程序编码。这样的LINQ查询在本质上也更加紧凑，并且可以在不进行任何修改或稍加修改的情况下移植到任何其他数据源。

以下是一个简单的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(工具)数组用作要使用LINQ to Objects查询的对象的集合。

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

使用LINQ对对象进行内存集合查询

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 departments = new List();
            
         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

编译并执行以上C＃或VB的代码时，将产生以下结果-

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

Press any key to continue.