Generics vs Non-Generics Collections ใน C#

C# มี Collections 2 แบบ: Non-Generic (Legacy) และ Generic (Modern)

Non-Generic Collections (System.Collections)

เก็บ object → Boxing/Unboxing, No Type Safety

Class	Description
`ArrayList`	Dynamic Array of Objects
`Hashtable`	Key-Value (Object-Object)
`Queue`	FIFO Queue of Objects
`Stack`	LIFO Stack of Objects
`SortedList`	Sorted Key-Value (Object-Object)
`BitArray`	Compact Bit Vector

Generic Collections (System.Collections.Generic) — แนะนำ

Type-Safe, No Boxing, Better Performance

Generic	Non-Generic Equivalent	Description
`List<T>`	ArrayList	Dynamic Array
`Dictionary<TKey, TValue>`	Hashtable	Key-Value Map
`Queue<T>`	Queue	FIFO
`Stack<T>`	Stack	LIFO
`SortedList<TKey, TValue>`	SortedList	Sorted Key-Value
`SortedDictionary<TKey, TValue>`	-	BST-based Sorted Map
`HashSet<T>`	-	Unique Set (Hash)
`SortedSet<T>`	-	Unique Set (Sorted)
`LinkedList<T>`	-	Doubly Linked List
`ConcurrentBag<T>` etc.	-	Thread-Safe Collections

Comparison: ArrayList vs List<int>

// Non-Generic: Boxing Everywhere
ArrayList nonGeneric = new ArrayList();
nonGeneric.Add(10);      // int → object (Boxing)
int val = (int)nonGeneric[0]; // Unboxing + Cast

// Generic: Type-Safe, No Boxing
List generic = new List();
generic.Add(10);         // int Stays int
int val2 = generic[0];   // Direct, No Cast

Performance Impact

Operation	ArrayList (int)	List<int>
Add 1M Items	~150ms + GC Pressure	~30ms
Memory	High (Boxed Objects)	Low (Contiguous Int[])
Type Safety	Runtime Error	Compile Error

Generic Methods และ Classes

// Generic Method
public T Max(T a, T b) where T : IComparable {
    return a.CompareTo(b) > 0 ? a : b;
}

// Generic Class
public class Repository where T : class, new() {
    public T GetById(int id) { /* ... */ }
    public void Add(T entity) { /* ... */ }
}

// Usage
var repo = new Repository();
var max = Max(10, 20); // Type Inference

Constraints (where T : ...)

Constraint	Meaning
`class`	Reference Type Only
`struct`	Value Type Only (Non-nullable)
`new()`	Parameterless Constructor
`BaseClass`	Derived from BaseClass
`Interface`	Implements Interface
`notnull`	Non-nullable (C# 8+)
`unmanaged`	Unmanaged Type (Blittable)
`delegate` / `enum`	Delegate / Enum Type

สรุป

Always Use Generic Collections — Type Safety, Performance, IntelliSense
Non-Generics: Legacy Only (Pre-.NET 2.0), COM Interop
เขียน Generic Classes/Methods เองได้ — Reusable, Type-Safe Algorithms
System.Collections.Immutable สำหรับ Immutable Collections
System.Collections.Concurrent สำหรับ Thread-Safe