.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<T>
csharp
// 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<T> (.NET 8+)
csharp
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
csharp
// ❌ 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
csharp
// 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> |