ความแตกต่างระหว่าง List และ Array ใน C#

Array = Fixed-size, Contiguous Memory
List<T> = Dynamic Array (Wrapper ของ Array) — Resize ได้อัตโนมัติ

Array (T[])

// Declaration
int[] arr = new int[5];                    // Size 5, Zero-initialized
int[] arr2 = new int[5] { 23, 14, 11, 78, 56 }; // Init with Values
int[] arr3 = { 23, 14, 11, 78, 56 };       // Implicit Size

// Access
arr[0] = 100;
int val = arr[2];

// Fixed Size — Cannot Add/Remove
// arr.Add(10); // ❌ No Add Method

List<T>

using System.Collections.Generic;

var list = new List();
list.Add("Audi");
list.Add("BMW");
list.Add("Chevrolet");
list.Add("Hyundai");

// Access
list[0] = "Tesla"; // Indexer
string first = list[0];

// Dynamic Operations
list.Insert(1, "Ford");   // Insert at Index
list.Remove("BMW");       // Remove by Value
list.RemoveAt(2);         // Remove by Index
list.AddRange(new[] { "VW", "Volvo" });

// Capacity vs Count
Console.WriteLine($"Count: {list.Count}, Capacity: {list.Capacity}");

Comparison

Feature	Array (T[])	List<T>
Size	Fixed	Dynamic (Auto-resize)
Add/Remove	❌	✅ Add, Insert, Remove, RemoveAt, Clear
Memory	Contiguous, Exact Size	Contiguous, Capacity ≥ Count (Over-allocates)
Performance (Index)	Fastest (Direct)	Fast (Bounds Check + 1 Indirection)
Performance (Add)	N/A	Amortized O(1), Occasional O(n) Resize
LINQ	✅ (AsEnumerable)	✅ Native
Interfaces	IList<T>, IReadOnlyList<T>	IList<T>, ICollection<T>, IEnumerable<T>
Span Support	✅ `arr.AsSpan()`	✅ `CollectionsMarshal.AsSpan(list)` (.NET 5+)

Internal: List<T> Resizing

// Simplified Internal Logic
private T[] _items;
private int _size;

public void Add(T item) {
    if (_size == _items.Length) {
        // Resize: Double Capacity (or 4 if empty)
        int newCapacity = _items.Length == 0 ? 4 : _items.Length * 2;
        Array.Resize(ref _items, newCapacity);
    }
    _items[_size++] = item;
}

เมื่อไหร่ใช้ไหน?

Array: ขนาดทราบล่วงหน้า, Performance Critical, Interop, Stackalloc, Numeric Computing
List<T>: ขนาดไม่ทราบ, ต้อง Add/Remove บ่อย, General Purpose Collection
ImmutableArray<T> / IReadOnlyList<T>: Read-only View, Thread Safety

Conversion

// List → Array
int[] array = list.ToArray();

// Array → List
List list = new List(array);
// หรือ
List list = array.ToList(); // ต้อง using System.Linq

สรุป

ส่วนใหญ่ใช้ List<T> ไปเลย — สะดวก ปลอดภัย เร็วพอ ใช้ Array เฉพาะเมื่อมีเหตุผลชัดเจน (Performance, Interop, Fixed Size)