C#: What are frozen collections in .NET 8?

Frozen collections — immutable, optimized for read-heavy workloads.

FrozenDictionary:

1var dictionary = new Dictionary<string, int>
2{
3    ["apple"] = 1,
4    ["banana"] = 2
5};
6
7// Build once, read many times
8var frozen = dictionary.ToFrozenDictionary();
9
10// O(1) lookups, memory-optimized layout
11var value = frozen["apple"];

FrozenSet:

1var frozenSet = new HashSet<string> { "a", "b", "c" }
2    .ToFrozenSet();

vs Dictionary:

• Build: O(n) — slower than dictionary creation.
• Lookup: O(1) — same speed.
• Memory: 10-20% less due to optimized layout.
• No more additions after freezing.

Best for:

• Configuration loaded at startup.
• Lookup tables.
• Route matching.