.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+

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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

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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
}

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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);

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5. Comparer Optimization

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

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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>

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7. References

• HashSet
• FrozenSet