Dodo.Unique

Time-bounded string canonicalization for .NET. Returns one canonical string instance per equal value so repeated reads share memory and become reference-equal.

Inspired by Go's unique package. .NET String.Intern is not used — it pins for process lifetime and offers no eviction. This pool keeps each entry for at least minRetention after its last access, then evicts via a two-tier hot/cold rotation.

Packages

Package	Contents	Depends on
Dodo.Unique	UniqueStringPool + UniqueStringConverter (System.Text.Json)	BCL only
Dodo.Unique.NewtonsoftJson	UniqueJsonStringConverter for Newtonsoft.Json	Dodo.Unique, Newtonsoft.Json

Target frameworks: net9.0, net10.0. The hot path relies on ConcurrentDictionary / FrozenDictionary GetAlternateLookup<ReadOnlySpan<char>>(), which were added in .NET 9.

Usage

Direct

using Dodo.Unique;

var pool = new UniqueStringPool(minRetention: TimeSpan.FromMinutes(30));

var a = pool.Make("Pepperoni");
var b = pool.Make("Pepperoni");
Debug.Assert(ReferenceEquals(a, b));

// span overload for zero-allocation lookup
ReadOnlySpan<char> span = "Pepperoni".AsSpan();
var c = pool.Make(span);
Debug.Assert(ReferenceEquals(a, c));

Or via the options class when readability or extra knobs matter:

var pool = new UniqueStringPool(new UniqueStringPoolOptions
{
    MinRetention = TimeSpan.FromMinutes(30),
    MaxLength = 256,
});

System.Text.Json

using System.Text.Json;
using Dodo.Unique;

var pool = new UniqueStringPool(TimeSpan.FromMinutes(30));
var options = new JsonSerializerOptions
{
    Converters = { new UniqueStringConverter(pool) },
};

var items = JsonSerializer.Deserialize<MenuItem[]>(json, options);
// repeating string field values now share a canonical instance

UniqueStringConverter also has a convenience ctor that builds a private pool from UniqueStringPoolOptions:

var converter = new UniqueStringConverter(new UniqueStringPoolOptions
{
    MinRetention = TimeSpan.FromMinutes(30),
    MaxLength = 256,
});

The converter decodes JSON strings directly into a stack-allocated buffer (default 256 chars) and falls back to Utf8JsonReader.GetString() for longer values. The buffer size is configurable up to a 1024-char (2 KB) hard ceiling — raise it only if your pool's MaxLength is higher and you have measured a real benefit:

var pool = new UniqueStringPool(TimeSpan.FromMinutes(30), maxLength: 512);
var converter = new UniqueStringConverter(pool, stackBufferLength: pool.MaxLength);

Both Read and Write are wired (Read canonicalizes; Write passes through unchanged), so the converter is safe to attach to round-trip serializer options.

Newtonsoft.Json

using Dodo.Unique;
using Dodo.Unique.NewtonsoftJson;
using Newtonsoft.Json;

var pool = new UniqueStringPool(TimeSpan.FromMinutes(30));
var settings = new JsonSerializerSettings
{
    Converters = { new UniqueJsonStringConverter(pool) },
};

var items = JsonConvert.DeserializeObject<MenuItem[]>(json, settings);

ReadJson canonicalizes via the pool; WriteJson passes the value through unchanged, so the converter is safe on round-trip settings.

Retention semantics

minRetention is a lower bound, not an upper bound. A value returned by Make is guaranteed to remain reference-equal to subsequent equal-value Make calls for at least minRetention from the moment of last access. Idle entries are evicted between minRetention and 2 * minRetention later.

This is the opposite of MemoryCache / Caffeine / Guava semantics, but right for canonicalization: if entries are evicted too eagerly, two reads of the same logical value return different string instances and the canonicalization benefit is lost.

Worst-case live entry count is bounded by unique inserts during 2 * minRetention. Bound per-entry cost with the maxLength parameter (default 256).

Why TTL, not WeakReference<string>?

Go's unique package uses weak refs + GC. The same approach in .NET has three problems that this pool avoids:

• Reclaim is GC-coupled, not access-coupled. WeakReference<T> clears only when a GC condemns the target's generation and no strong root remains. A string promoted to Gen 2 survives until the next full GC, regardless of when the cache last touched it. Timing also depends on the runtime's GC mode and configuration (workstation vs server, background vs blocking, GCLatencyMode, region/segment layout), so steady-state retention shifts across deployments and load patterns instead of tracking access.
• Per-entry cost is structural. Each entry adds a managed WeakReference wrapper, a slot in the native GC handle table, and per-collection scan work inside the GC pause. The cost scales with pool size and isn't amortizable.
• Literal / interned / FOH strings are immortal. Since .NET 8, string literals live on the Frozen Object Heap and are strongly rooted forever; WeakReference to them never clears. A WR-backed pool silently behaves differently for runtime-built vs literal strings.

This pool trades GC-coupled eviction for a predictable two-tier rotation: a bounded dictionary footprint, no handle-table tax, and behavior that doesn't depend on generation aging or GC configuration.

Build & test

dotnet build Dodo.Unique.slnx

Tests use TUnit (Microsoft.Testing.Platform). Run each test project as an executable:

dotnet run --project tests/Dodo.Unique.Tests -c Release
dotnet run --project tests/Dodo.Unique.NewtonsoftJson.Tests -c Release