จำนวน Elements ใน HashSet C#

HashSet<T> - Count Property

HashSet<T> (ใน System.Collections.Generic) เป็น Collection ที่เก็บ Unique Values (ไม่ซ้ำ) ใช้ Count Property ดึงจำนวน Elements

ตัวอย่างที่ 1: HashSet Basic Operations

using System;
using System.Collections.Generic;

public class Demo
{
    public static void Main()
    {
        HashSet set1 = new HashSet();
        set1.Add(25);
        set1.Add(50);
        set1.Add(75);
        set1.Add(100);
        set1.Add(125);
        set1.Add(150);
        
        Console.WriteLine("Elements in HashSet1:");
        foreach (int val in set1) Console.WriteLine(val);
        
        Console.WriteLine($"\nNumber of elements in HashSet1 = {set1.Count}");  // 6
        
        HashSet set2 = new HashSet();
        set2.Add(30);
        set2.Add(60);
        set2.Add(100);   // Duplicate with set1
        set2.Add(150);   // Duplicate with set1
        set2.Add(200);
        set2.Add(250);
        
        Console.WriteLine("\nElements in HashSet2:");
        foreach (int val in set2) Console.WriteLine(val);
        
        Console.WriteLine($"Number of elements in HashSet2 = {set2.Count}");  // 6
        
        // Set Operations
        Console.WriteLine($"\nOverlap (Common Elements)? {set1.Overlaps(set2)}");  // True
        Console.WriteLine($"Set1 Contains 60? {set1.Contains(60)}");  // False
        Console.WriteLine($"Set2 Contains 60? {set2.Contains(60)}");  // True
    }
}

ผลลัพธ์:

Elements in HashSet1:
25
50
75
100
125
150

Number of elements in HashSet1 = 6

Elements in HashSet2:
30
60
100
150
200
250

Number of elements in HashSet2 = 6

Overlap (Common Elements)? True
Set1 Contains 60? False
Set2 Contains 60? True

ตัวอย่างที่ 2: HashSet กับ Duplicate Handling

using System;
using System.Collections.Generic;

public class Demo
{
    public static void Main()
    {
        HashSet set1 = new HashSet();
        set1.Add("AB");
        set1.Add("CD");
        set1.Add("EF");
        set1.Add("AB");  // Duplicate - Ignored!
        set1.Add("IJ");
        set1.Add("KL");
        set1.Add("EF");  // Duplicate - Ignored!
        set1.Add("OP");
        
        Console.WriteLine("Elements in HashSet1:");
        foreach (string val in set1) Console.WriteLine(val);
        
        Console.WriteLine($"\nNumber of elements in HashSet1 = {set1.Count}");  // 6 (ไม่ใช่ 8)
        
        HashSet set2 = new HashSet();
        set2.Add("EF");
        set2.Add("KL");
        
        Console.WriteLine($"\nNumber of elements in HashSet2 = {set2.Count}");  // 2
        Console.WriteLine("Elements in HashSet2:");
        foreach (string val in set2) Console.WriteLine(val);
        
        // Set Operations
        Console.WriteLine($"\nIs Set1 Superset of Set2? {set1.IsSupersetOf(set2)}");  // True
        Console.WriteLine($"Is Set1 Subset of Set2? {set1.IsSubsetOf(set2)}");  // False
        Console.WriteLine($"Set Equals? {set1.SetEquals(set2)}");  // False
    }
}

ผลลัพธ์:

Elements in HashSet1:
AB
CD
EF
IJ
KL
OP

Number of elements in HashSet1 = 6

Number of elements in HashSet2 = 2
Elements in HashSet2:
EF
KL

Is Set1 Superset of Set2? True
Is Set1 Subset of Set2? False
Set Equals? False

HashSet Operations Cheat Sheet

Performance Characteristics

• O(1) Average สำหรับ Add, Remove, Contains (Hash-based)
• O(n) สำหรับ Union, Intersect, Except (ต้อง Iterate)
• No Ordering - Iteration Order ไม่รับประกัน (แตกต่างจาก SortedSet)
• No Duplicates - Add ค่าซ้ำ Return False, ไม่ Add

HashSet vs List vs Dictionary

สรุป

• HashSet<T>.Count - ดึงจำนวน Elements (Unique)
• Add Duplicate → Ignored (Return False)
• Contains O(1) - เร็วมากกว่า List O(n)
• Set Operations: Union, Intersect, Except, SymmetricExcept
• Subset/Superset/Overlaps/SetEquals สำหรับ Logic Check
• เหมาะสำหรับ: Unique Collection, Fast Lookup, Set Math, Deduplication