.NET 8/9 `FrozenDictionary<TKey, TValue>` & `FrozenSet<T>`

1Concept

`FrozenDictionary` and `FrozenSet` optimize read-only lookup datasets. At creation, the compiler constructs a perfect hash table or jump table, providing near-instantaneous O(1) lookups with zero lock contention.

2Architecture Diagram

Immutable Read-Heavy Dataset (Config / Route Maps)
       │
       ▼ .ToFrozenDictionary()
[ Perfect Hash Table / Direct Memory Jump Table ]
├── Zero write locks
└── 30-50% faster read lookups than standard Dictionary!

3Code Example

using System;
using System.Collections.Frozen;
using System.Collections.Generic;

public class FrozenCollectionDemo
{
    public static void Main()
    {
        var rawRoutes = new Dictionary<string, string>
        {
            ["/api/v1/auth"] = "AuthHandler",
            ["/api/v1/users"] = "UserHandler",
            ["/api/v1/metrics"] = "MetricsHandler"
        };

        // Construct FrozenDictionary (.NET 8/9)
        FrozenDictionary<string, string> frozenRoutes = rawRoutes.ToFrozenDictionary(StringComparer.Ordinal);

        bool exists = frozenRoutes.TryGetValue("/api/v1/users", out var handler);
        Console.WriteLine($"Frozen Route Matched: {exists} -> {handler}");
        Console.WriteLine($"Is Frozen Read-Only: True");
    }
}

4Expected Output

Frozen Route Matched: True -> UserHandler
Is Frozen Read-Only: True

5Key Takeaways

• ✓Use `ToFrozenDictionary()` for routing tables, DI singletons, and lookup caches initialized at startup.
• ✓Building a frozen collection is slightly more expensive upfront to make all subsequent reads ultra-fast.
• ✓Thread-safe by design since mutation is prohibited.