การสร้างคอลเลกชัน (ToList, ToDictionary) และข้อควรรู้ด้านประสิทธิภาพ

เหตุผลที่ควรสร้างคอลเลกชันจริง

ToList / ToArray

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

public class Program
{
  public static void Main(string[] args)
  {
    List<int> data = new List<int>(new int[] { 1, 2, 3, 4 });

    var evensQuery = data.Where(x => x % 2 == 0);
    int[] evensArray = evensQuery.ToArray(); // snapshot now
    List<int> evensList = evensQuery.ToList(); // snapshot now

    data.Add(6); // changes after materialization

    Console.WriteLine("Array:");
    foreach (int x in evensArray) Console.WriteLine(x); // 2,4
    Console.WriteLine("List:");
    foreach (int x in evensList) Console.WriteLine(x);  // 2,4
  }
}

พื้นฐาน ToDictionary

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

public sealed class City
{
  public string Name;
  public int Population;
  public City(string name, int pop){ Name = name; Population = pop; }
}

public class Program
{
  public static void Main(string[] args)
  {
    var cities = new City[]
    {
      new City("Oslo", 634000),
      new City("Rome", 2873000),
      new City("Riga", 605000)
    };

    Dictionary<string, int> byName = cities
      .ToDictionary(c => c.Name, c => c.Population);

    Console.WriteLine(byName["Rome"]); // 2873000
  }
}

รายการซ้ำ: ToLookup/GroupBy

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

public sealed class Item { public string Category; public string Name; public Item(string c, string n){ Category=c; Name=n; } }

public class Program
{
  public static void Main(string[] args)
  {
    var items = new Item[]
    {
      new Item("Food","Apple"),
      new Item("Food","Bread"),
      new Item("Tool","Hammer"),
      new Item("Tool","Wrench")
    };

    // If keys may repeat, use ToLookup (multi-value) or GroupBy
    var lookup = items.ToLookup(i => i.Category, i => i.Name);

    foreach (var g in lookup)
    {
      Console.WriteLine(g.Key + ":");
      foreach (var name in g) Console.WriteLine(" - " + name);
    }
  }
}

หลีกเลี่ยงการวนอ่านสมาชิกหลายครั้ง

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

public static class Expensive
{
  public static IEnumerable<int> Numbers()
  {
    // Simulate expensive enumeration (prints when enumerated)
    for (int i = 1; i <= 3; i++)
    {
      Console.WriteLine("Produce " + i);
      yield return i;
    }
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    var query = Expensive.Numbers().Select(x => x * x);

    Console.WriteLine("First pass:");
    foreach (int x in query) Console.WriteLine(x);

    Console.WriteLine("Second pass (re-enumerates work):");
    foreach (int x in query) Console.WriteLine(x);

    Console.WriteLine("Cache with ToList:");
    List<int> cached = query.ToList(); // runs once
    foreach (int x in cached) Console.WriteLine(x); // no extra production logs
  }
}

รายการตรวจสอบการสร้างคอลเลกชันจริง

ข้อกำหนดของ ToDictionary

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