.NET Fast Lookup

A guide for fast lookup APIs leveraging O(1) time complexity.

Quick Reference: See QUICKREF.md for essential patterns at a glance.

1. Core APIs

API Time Complexity Features

HashSet<T>

O(1) Mutable, no duplicates

FrozenSet<T>

O(1) Immutable, .NET 8+

Dictionary<K,V>

O(1) Mutable, Key-Value

FrozenDictionary<K,V>

O(1) Immutable, .NET 8+

2. HashSet

// O(1) time complexity for existence check var allowedIds = new HashSet<int> { 1, 2, 3, 4, 5 };

if (allowedIds.Contains(userId)) { // Allowed user }

// Set operations setA.IntersectWith(setB); // Intersection setA.UnionWith(setB); // Union setA.ExceptWith(setB); // Difference

3. FrozenSet (.NET 8+)

using System.Collections.Frozen;

// Immutable fast lookup (read-only scenarios) var allowedExtensions = new[] { ".jpg", ".png", ".gif" } .ToFrozenSet(StringComparer.OrdinalIgnoreCase);

if (allowedExtensions.Contains(fileExtension)) { // Allowed extension }

4. Dictionary<K,V> Optimization

// ❌ Two lookups if (dict.ContainsKey(key)) { var value = dict[key]; }

// ✅ Single lookup if (dict.TryGetValue(key, out var value)) { // Use value }

// Lookup with default value var value = dict.GetValueOrDefault(key, defaultValue);

5. Comparer Optimization

// Case-insensitive string comparison var set = new HashSet<string>(StringComparer.OrdinalIgnoreCase); set.Add("Hello"); set.Contains("HELLO"); // true

6. When to Use

Scenario Recommended Collection

Frequently modified set HashSet<T>

Read-only configuration data FrozenSet<T>

Frequent existence checks HashSet<T> / FrozenSet<T>

Key-Value cache Dictionary<K,V>

Static mapping table FrozenDictionary<K,V>

7. References

• HashSet

• FrozenSet