[*.cs]
dotnet_diagnostic.LC033.severity = error

LC033: Use FrozenSet for Read-Only Static Membership Caches

In Plain Terms

Imagine you made a VIP guest list and then laminated it forever. You do not need an editable whiteboard anymore. A laminated list is faster to check and nobody can accidentally scribble on it.

Goal

Detect private static readonly HashSet<T> membership caches that can be safely converted to FrozenSet<T> on .NET 8+.

The Problem

HashSet<T> is mutable and optimized for general-purpose set operations. When a cache is initialized once, never mutated, and only used for Contains(...), FrozenSet<T> is a better fit: it trades construction cost for faster steady-state lookups and lower ongoing overhead.

Example Violation

private static readonly HashSet<string> ElevatedRoles = new(StringComparer.OrdinalIgnoreCase)
{
    "admin",
    "ops"
};

static bool IsElevated(string role) => ElevatedRoles.Contains(role);

The Fix

Convert the cache to FrozenSet<T> and build it once with ToFrozenSet(...).

private static readonly FrozenSet<string> ElevatedRoles = new string[]
{
    "admin",
    "ops"
}.ToFrozenSet(StringComparer.OrdinalIgnoreCase);

Analyzer Logic

ID: LC033

Category: Performance

Severity: Info

Algorithm

1. Require a source-declared private static readonly HashSet<T> field with a single declarator and an inline initializer.
2. Require System.Collections.Frozen.FrozenSet<T> and the ToFrozenSet(...) extension to be available in the compilation: from the framework’s System.Collections.Frozen.FrozenSet class on .NET 8+ (or the System.Collections.Immutable 8.0+ package), or from a source-declared polyfill in the System.Collections.Frozen namespace. On frameworks without them (.NET 7 and older, .NET Standard without the package) the rule stays silent.
3. Accept only fixer-safe initializer shapes:
   • collection initializer forms (new HashSet<T>() { ... }, optionally with a comparer),
   • new HashSet<T>(source[, comparer]),
   • source.ToHashSet([comparer]).
4. Track every source reference to the field across the compilation and require every usage to be a direct Contains(...) call.
5. Skip any field used in IQueryable / expression-tree contexts, passed around through aliases, mutated, enumerated, or touched through any non-Contains member.

Notes

This rule is intentionally narrow. If there is any ambiguity about initialization, mutability, or usage shape, it stays silent instead of suggesting a speculative rewrite.