.NET Magazine國際中文電子雜誌
作 者：許薰尹
審 稿：張智凱
文章編號： N180419402
出刊日期： 2018/4/18
在這篇文章中，將延續《LINQ語法簡介 - 1》、《LINQ語法簡介 - 2》、《LINQ語法簡介 - 3》與《LINQ語法簡介 - 4》文章的情境，介紹常用的LINQ運算子（Operator），以透過更簡易的語法來查詢陣列或集合中的內容，這一篇主要介紹轉換運算子，包含：「ToArray」、「ToList」、「ToDictionary」、「ToLookup」、「Cast」、「AsEnumerable」與「OfType」。
轉換運算子 – ToArray
轉型運算子可以根據需求將序列轉換成集合或陣列，「ToArray」方法用來轉換成陣列，參考以下範例程式碼，將「Customer」集合轉換成陣列，然後透過「foreach」迴圈印出陣列中「Customer」物件的客戶名稱「CustomerName」：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace LINQDemo {
class Customer {
public int CustomerID { get; set; }
public string CustomerName { get; set; }
public string ContactName { get; set; }
public string City { get; set; }
public int PostalCode { get; set; }
public string Country { get; set; }
}
class Program {
static void Main( string [] args ) {
List<Customer> customers = new List<Customer> {
new Customer(){ CustomerID = 1, CustomerName ="Mary" , ContactName = "Maria Anders" , City = "Berlin", PostalCode = 12209 , Country = "Germany" },
new Customer(){ CustomerID = 2, CustomerName ="Ann" , ContactName = "Ana Trujillo" , City = "México ", PostalCode = 05021 , Country = "Mexico" },
new Customer(){ CustomerID = 3, CustomerName ="Lili" , ContactName="Futterkiste" , City = "México ", PostalCode = 05023 , Country = "UK" }
};
Customer [] customersArray = customers.ToArray( );
foreach ( var item in customersArray ) {
Console.WriteLine(item.CustomerName);
}
}
}
}
這個範例程式的執行結果參考如下：
圖 1：轉換算子 - 「ToArray」。
轉換運算子 – ToList
「ToList」用來將序列轉換成集合，參考以下範例程式碼，叫用「ToList」方法，將「Customer」陣列轉換成集合：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace LINQDemo {
class Customer {
public int CustomerID { get; set; }
public string CustomerName { get; set; }
public string ContactName { get; set; }
public string City { get; set; }
public int PostalCode { get; set; }
public string Country { get; set; }
}
class Program {
static void Main( string [] args ) {
Customer [] customers = new Customer [] {
new Customer(){ CustomerID = 1, CustomerName ="Mary" , ContactName = "Maria Anders" , City = "Berlin", PostalCode = 12209 , Country = "Germany" },
new Customer(){ CustomerID = 2, CustomerName ="Ann" , ContactName = "Ana Trujillo" , City = "México ", PostalCode = 05021 , Country = "Mexico" },
new Customer(){ CustomerID = 3, CustomerName ="Lili" , ContactName="Futterkiste" , City = "México ", PostalCode = 05023 , Country = "UK" }
};
List<Customer> customersArray = customers.ToList( );
foreach ( var item in customersArray ) {
Console.WriteLine(item.CustomerName);
}
}
}
}
這個範例程式的執行結果參考如下：
![clip_image002[1]](http://blogs.uuu.com.tw/Articles/image.axd?picture=clip_image002%5B1%5D_thumb.jpg "clip_image002[1]")
圖 2：轉換算子 - 「ToList」。
轉換運算子 – ToDictionary
「ToDictionary」可以將來源序列轉換成Dictionary類型的物件，叫用此方法時，需要利用Func委派指定分組鍵值（Key），參考以下範例程式碼，指定鍵值為「CustomerID」：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace LINQDemo {
class Customer {
public int CustomerID { get; set; }
public string CustomerName { get; set; }
public string ContactName { get; set; }
public string City { get; set; }
public int PostalCode { get; set; }
public string Country { get; set; }
}
class Program {
static void Main( string [] args ) {
List<Customer> customers = new List<Customer> {
new Customer(){ CustomerID = 1, CustomerName ="Mary" , ContactName = "Maria Anders" , City = "Berlin", PostalCode = 12209 , Country = "Germany" },
new Customer(){ CustomerID = 2, CustomerName ="Ann" , ContactName = "Ana Trujillo" , City = "México ", PostalCode = 05021 , Country = "Mexico" },
new Customer(){ CustomerID = 3, CustomerName ="Lili" , ContactName="Futterkiste" , City = "México ", PostalCode = 05023 , Country = "UK" }
};
IDictionary<int , Customer> dic = customers.ToDictionary( c => c.CustomerID );
foreach ( var item in dic ) {
Console.WriteLine( $" {item.Key} - {item.Value.CustomerName}" );
}
}
}
}
這個範例程式的執行結果參考如下：
圖 3：轉換算子 - 「ToDictionary」。
轉換運算子 – ToLookup
「ToLookup」的用法和「GroupBy」一樣應用在資料分組，唯一的不同點是「GroupBy」是延遲執行查詢，而「ToLookup」是馬上執行查詢，參考以下範例程式碼，根據「Customer」的「City」做資料分組：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace LINQDemo {
class Customer {
public int CustomerID { get; set; }
public string CustomerName { get; set; }
public string ContactName { get; set; }
public string City { get; set; }
public int PostalCode { get; set; }
public string Country { get; set; }
}
class Program {
static void Main( string [] args ) {
List<Customer> customers = new List<Customer> {
new Customer(){ CustomerID = 1, CustomerName ="Mary" , ContactName = "Maria Anders" , City = "Berlin", PostalCode = 12209 , Country = "Germany" },
new Customer(){ CustomerID = 2, CustomerName ="Ann" , ContactName = "Ana Trujillo" , City = "México ", PostalCode = 05021 , Country = "Mexico" },
new Customer(){ CustomerID = 3, CustomerName ="Lili" , ContactName="Futterkiste" , City = "México ", PostalCode = 05023 , Country = "UK" }
};
ILookup<string , Customer> customerGroup = customers.ToLookup( c => c.City );
foreach ( var g in customerGroup ) {
Console.WriteLine(g.Key);
foreach ( var item in g ) {
Console.WriteLine( $" {item.CustomerName}" );
}
}
}
}
}
這個範例程式的執行結果參考如下：
圖 4：轉換算子 - 「ToLookup」。
轉換運算子 – Cast
「Cast」用來將序列轉換成「IEnumerable<T>」型別，參考以下範例程式碼展示如何轉換「Customer」陣列：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace LINQDemo {
class Customer {
public int CustomerID { get; set; }
public string CustomerName { get; set; }
public string ContactName { get; set; }
public string City { get; set; }
public int PostalCode { get; set; }
public string Country { get; set; }
}
class Program {
static void Main( string [] args ) {
Customer [] customers = new Customer [] {
new Customer(){ CustomerID = 1, CustomerName ="Mary" , ContactName = "Maria Anders" , City = "Berlin", PostalCode = 12209 , Country = "Germany" },
new Customer(){ CustomerID = 2, CustomerName ="Ann" , ContactName = "Ana Trujillo" , City = "México ", PostalCode = 05021 , Country = "Mexico" },
new Customer(){ CustomerID = 3, CustomerName ="Lili" , ContactName="Futterkiste" , City = "México ", PostalCode = 05023 , Country = "UK" }
};
IEnumerable<Customer> customersArray = customers.Cast<Customer>( );
foreach ( var item in customersArray ) {
Console.WriteLine( item.CustomerName );
}
}
}
}
這個範例程式的執行結果參考如下：
圖 5：轉換算子 - 「Cast」。
轉換運算子 – AsEnumerable
「AsEnumerable」的用途和「Cast」一樣，用來將序列轉換成「IEnumerable<T>」型別，參考以下範例程式碼展示如何轉換「Customer」陣列：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace LINQDemo {
class Customer {
public int CustomerID { get; set; }
public string CustomerName { get; set; }
public string ContactName { get; set; }
public string City { get; set; }
public int PostalCode { get; set; }
public string Country { get; set; }
}
class Program {
static void Main( string [] args ) {
Customer [] customers = new Customer [] {
new Customer(){ CustomerID = 1, CustomerName ="Mary" , ContactName = "Maria Anders" , City = "Berlin", PostalCode = 12209 , Country = "Germany" },
new Customer(){ CustomerID = 2, CustomerName ="Ann" , ContactName = "Ana Trujillo" , City = "México ", PostalCode = 05021 , Country = "Mexico" },
new Customer(){ CustomerID = 3, CustomerName ="Lili" , ContactName="Futterkiste" , City = "México ", PostalCode = 05023 , Country = "UK" }
};
IEnumerable<Customer> customersArray = customers.AsEnumerable();
foreach ( var item in customersArray ) {
Console.WriteLine( item.CustomerName );
}
}
}
}
這個範例程式的執行結果參考如下：
圖 6：轉換算子 - 「AsEnumerable」。
轉換運算子 – OfType
「OfType」運算子可以從陣列或集合中，找出指定型別的物件，放到集合中回傳，參考以下範例程式碼，找出集合中所有的字串：
List<object> list = new List<object> { 10 , "hello" , DateTime.Now , true , 'c' , 20 , "world" };
var result = from item in list.OfType<string>( )
select item;
foreach ( var item in result ) {
Console.WriteLine(item);
}
這個範例程式的執行結果參考如下：
圖 7：轉換算子 - 「OfType」。
若改成擴充方法語法，則寫法如下：
List<object> list = new List<object> { 10 , "hello" , DateTime.Now , true , 'c' , 20 , "world" };
var result = list.OfType<string>( );
foreach ( var item in result ) {
Console.WriteLine( item );
}
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