LINQ

LINQ 全名 Language-Integrated Query, 於 C# 3.0 引入, 讓程式設計師能使用一致性的語法來查詢不同的資料來源, 如查詢陣列、集合、DataSet、XML、SQL Server 資料庫等等
語法

var 變數 = from 範圍變數 in 集合 orderby 欄位名稱 where 條件 select 範圍變數[.欄位]

一些例子

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

string sentence = "I love cats";
string[] catNames = {"Lucky", "Bella", "Luna", "Oreo", "Simba", "Toby", "Loki", "Oscar"};
int[] numbers = {5, 6, 3, 2, 1, 7, 8, 234, 54, 14, 653, 3, 4, 5, 6, 7};
var getTheNumbers = from number in numbers 
                    where number < 5 
                    orderby number descending
                    select number;
Console.WriteLine(string.Join(",", getTheNumbers));

List<int> newNumbers = new List<int>();

foreach (var number in numbers)
{
  if (number < 5)
  {
    newNumbers.Add(number);
  }
}

Console.WriteLine(string.Join(",", newNumbers));

var catsWithA = from cat in catNames
                where cat.Contains("a") && (cat.Length < 5)
                select cat;
Console.WriteLine(string.Join(",", catsWithA));

internal class Person
{
  private string name;
  private int height;
  private int weight;
  
  private Gender gender;
  
  public string Name {get; set;}
  public int Height {get; set;}
  public int Weight {get; set;}
  public Gender Gender {get; set;}
  
  public Person(string name, int height, int weight, Gender gender)
  {
    this.Name = name;
    this.Height = height;
    this.Weight = weight;
    this.Gender = gender;
  }
}

internal enum Gender
{
  Male,
  Female
}

List<Person> people = new List<Person>()
{
  new Person("Tod", 180, 70, Gender.Female),
  new Person("Kyle", 164, 77, Gender.Male),
  new Person("Anna", 164, 77, Gender.Male)
};

var fourCharPeople = from p in people
                     where (p.Name.Length == 4)
                     orderby p.Weight
                     select p;

foreach (var item in fourCharPeople)
{
  Console.WriteLine($"Name: {item.Name}, Weight: {item.Weight}");
}

var

從 c# 3.0 開始引入, 在方法範圍內, 可以使用關鍵字 var 宣告變數, 這是有隱含的 "type", 也是強型別, 但是由編譯器來判斷型別, 例

// 以下宣告功能一樣
var i = 100;  // Implicitly typed
int i = 100;  // Explicitly typed