Prest.SystemTextJson 0.5.0

Prest

Zero-allocation pooled collections for .NET with pluggable, zero-cost hash-table algorithms.

Highlights

• Pooled backing store. Arrays come from ArrayPool<T>.Shared; Dispose returns them. Single-consumer ownership — no locking, no concurrent mutation.
• Pluggable hash algorithm. PooledHashMap<K,V> and PooledHashSet<T> pick a probe strategy at the type-parameter level: Swiss (SIMD group scan; default, matches .NET 9's Dictionary), RobinHood (displacement-tracking linear, backward-shift on erase — no tombstones), Linear (simplest), Chained (dense entry storage, iteration-friendly). The algorithm is a struct generic — the JIT monomorphizes every call end-to-end. No virtual dispatch, no boxing.
• Short, first-class type names. PooledHashMap<K,V>, SwissHashMap<K,V>, RobinHoodHashMap<K,V,TFinalizer>, ComparerLinearHashMap<K,V>, … are real types with their own Create, so fields and parameters never need the full generic spelling.
• Auto-return-on-Dispose. PooledHashMap<K,V>.Create() rents from a per-thread cache; using disposes back into that cache so the next rental on the same thread skips allocation entirely.
• Concurrent pool for await code. Prest.ObjectPool ships PooledHashMapPool<K,V> / PooledHashSetPool<T> / PooledBufferWriterPool<T> backed by Microsoft.Extensions.ObjectPool.DefaultObjectPool — safe across threads, survives await, Dispose auto-returns to the pool.
• Zero-allocation enumeration. foreach, .Keys, .Values all return value-type enumerators — no IEnumerator<T> boxing.
• Serializer integrations. System.Text.Json converters and VYaml formatters for PooledArray<T> and PooledHashMap<K,V>.
• Native AOT-compatible. All shipping packages build with IsAotCompatible=true on .NET 8+. The reflection-based factory / resolver are flagged with RequiresDynamicCode so AOT analyzers point you at the concrete per-type converter/formatter as the AOT-safe alternative.

Packages

Package	Version	Source	Purpose
Prest		src/Prest	Core — PooledHashMap<K,V>, PooledHashSet<T>, algorithm/finalizer/hasher structs, PooledArray<T>, PooledList<T>, PooledStack<T>, PooledBufferWriter<T>
Prest.ObjectPool		src/Prest.ObjectPool	DefaultObjectPool-backed PooledHashMapPool<K,V>, PooledHashSetPool<T>, PooledBufferWriterPool<T> for await-safe rent/return
Prest.SystemTextJson		src/Prest.SystemTextJson	PooledJsonBufferWriter (pairs Utf8JsonWriter with PooledBufferWriter<byte>) + threadstatic cache
Prest.SystemTextJson.ObjectPool		src/Prest.SystemTextJson.ObjectPool	DefaultObjectPool-backed accessor for PooledJsonBufferWriter
Prest.Serializers.SystemTextJson		src/Prest.Serializers.SystemTextJson	JsonConverter implementations + PooledTypesJsonConverterFactory (one registration covers PooledArray<T>, PooledList<T>, PooledHashMap<K,V>, PooledHashSet<T> for any algorithm)
Prest.Serializers.VYaml		src/Prest.Serializers.VYaml	IYamlFormatter implementations for the same four types + PooledTypeFormatterResolver
Prest.Immutable		src/Prest.Immutable	PooledArray<T> ↔ ImmutableArray<T> conversion extensions

Install

Core package — pooled collections, PooledHashMap, PooledHashSet:

dotnet add package Prest

Optional packages, install only what you need:

# Await-safe object pools (PooledHashMapPool, PooledHashSetPool, PooledBufferWriterPool)
dotnet add package Prest.ObjectPool

# System.Text.Json integration — PooledJsonBufferWriter
dotnet add package Prest.SystemTextJson
dotnet add package Prest.SystemTextJson.ObjectPool   # + await-safe accessor

# JsonConverter / IYamlFormatter for pooled collection types
dotnet add package Prest.Serializers.SystemTextJson
dotnet add package Prest.Serializers.VYaml

# ImmutableArray<T> ↔ PooledArray<T> interop (net10.0+)
dotnet add package Prest.Immutable

All packages target net11.0, net10.0, net8.0, netstandard2.1, netstandard2.0 (except Prest.Serializers.SystemTextJson which is net8.0+ and Prest.Immutable which is net10.0+). See Target frameworks for the full matrix.

Quick start

using Prest;

// `using` disposes the map and returns pooled arrays to ArrayPool.Shared.
using var map = PooledHashMap<int, string>.Create(capacity: 16);

map.Add(1, "one");
map.Add(2, "two");
map.Add(3, "three");

Console.WriteLine($"Count: {map.Count}");          // 3
Console.WriteLine($"map[2] = {map[2]}");            // two

if (map.TryGetValue(3, out var three))
{
    Console.WriteLine($"TryGetValue(3) -> {three}");
}

map.Remove(1);
Console.WriteLine($"ContainsKey(1): {map.ContainsKey(1)}");  // False

Tour

Hash map / hash set

The default PooledHashMap<K,V> / PooledHashSet<T> close over SwissTable — the same algorithm .NET 9's Dictionary moved to, chosen as a reasonable starting point for mixed workloads. Like Dictionary, they take an optional comparer (see Custom equality comparer). Create(capacity) rents from a per-thread cache, Dispose returns to it. See Choosing an algorithm for when to pick a different one.

using var map = PooledHashMap<int, string>.Create(capacity: 16);
map.Add(1, "one");

using var set = PooledHashSet<string>.Create(capacity: 8);
set.Add("apple");
var wasNew = set.Add("apple");  // false — already present

Every alias also works with collection expressions. They go through the alias's own Create, so they rent from and return to the same per-thread cache:

using PooledHashMap<string, int> empty = [];
using PooledHashMap<string, int> stock = [new("apples", 3), new("pears", 5)];
using PooledHashMap<string, int> more = [.. stock, new("plums", 2)];
using PooledHashSet<string> tags = ["red", "green", "red"];  // 2 items — duplicates are ignored

// C# 15 collection arguments: pre-size the table.
using RobinHoodHashMap<int, int> sized = [with(capacity: 256)];

• [], [a, b] and spreads work on C# 12 and later. [with(...)] needs C# 15 (<LangVersion>preview</LangVersion> on the .NET 11 SDK).
• A key that appears twice in a map expression doesn't throw: the last entry wins.
• Map elements are KeyValuePair<K,V> values (new("apples", 3)). Dictionary expressions (["apples": 3]) aren't in the compiler yet.

Every alias's Create returns the alias itself, so the short name works for fields, parameters and return types:

sealed class WordCounter : IDisposable
{
    readonly PooledHashMap<string, int> _counts = PooledHashMap<string, int>.Create(capacity: 64);
    readonly ComparerSwissHashSet<string> _stopWords =
        ComparerSwissHashSet<string>.Create(StringComparer.OrdinalIgnoreCase);

    public void Dispose()
    {
        _counts.Dispose();
        _stopWords.Dispose();
    }
}

The generic form PooledHashMap<K,V,TAlgo> / PooledHashSet<T,TAlgo> is still there for code that is generic over the algorithm; every alias derives from it.

Pick an algorithm

Same API surface on every alias — swap the type name, nothing else:

using var swiss  = SwissHashMap<int, int>.Create(32);
using var robin  = RobinHoodHashMap<int, int>.Create(32);
using var linear = LinearHashMap<int, int>.Create(32);
using var chain  = ChainedHashMap<int, int>.Create(32);

PooledHashMap<K,V> and SwissHashMap<K,V> both use SwissTable, but they are separate types. PooledHashMap<K,V> takes an optional comparer, so every hash and equality check first tests whether it has one. SwissHashMap<K,V> takes no comparer and skips that test, which matters for integer keys: on ARM64, int-key hits measured 29–39% faster and misses 4–15% faster. For string keys the difference is small next to hashing the string (0–12%). See Choosing an algorithm below.

Each algorithm comes in five shapes (sets mirror maps, with T in place of K,V):

{Algo} is Swiss, RobinHood, Linear or Chained.

Custom equality comparer

The default PooledHashMap<K,V> / PooledHashSet<T> take a runtime IEqualityComparer<T> the way Dictionary does. Each algorithm also ships a Comparer{Algo}HashMap<K,V> / Comparer{Algo}HashSet<T> variant for a comparer with a specific algorithm or finalizer:

using var byName = PooledHashMap<string, int>.Create(StringComparer.OrdinalIgnoreCase);
using PooledHashSet<string> seen = [with(StringComparer.OrdinalIgnoreCase)];

using var map = ComparerSwissHashMap<string, int>.Create(
    StringComparer.OrdinalIgnoreCase, capacity: 8);

map.Add("Hello", 1);
Console.WriteLine(map.ContainsKey("HELLO"));  // True
Console.WriteLine(map.Comparer == StringComparer.OrdinalIgnoreCase);  // True

// Comparer + finalizer.
using var ids = ComparerRobinHoodHashMap<string, int, FibonacciFinalizer>.Create(
    StringComparer.Ordinal);

// Collection expressions (C# 15) take the comparer, and optionally a capacity, in with(...).
using ComparerSwissHashMap<string, int> m = [with(StringComparer.OrdinalIgnoreCase)];
using ComparerSwissHashSet<string> words =
    [with(StringComparer.OrdinalIgnoreCase, 16), "Hello", "HELLO"];  // 1 item

• On PooledHashMap<K,V> / PooledHashSet<T> the comparer is optional: Create(), [] and Create(null) use EqualityComparer<T>.Default.
• On the Comparer* aliases, Create(comparer) or [with(comparer), ...] creates one. Without a comparer (Create(), Create(capacity), [], [with(capacity)]) it's a compile error on these aliases. Passing null uses EqualityComparer<T>.Default.
• The {Algo}HashMap<K,V> / {Algo}HashMap<K,V,TFinalizer> aliases (and their sets) always use EqualityComparer<T>.Default. Create(comparer) or [with(comparer)] on them is a compile error that names the type to declare instead.
• Comparer returns the comparer the map or set uses.
• The per-thread cache only hands a disposed instance back to a Create call with the same comparer instance (reference equality — singletons like StringComparer.OrdinalIgnoreCase qualify). A different comparer gets a new instance. This applies to PooledHashMap<K,V> / PooledHashSet<T> too.

Custom hash finalizer

Bit-mixing stage applied after GetHashCode. Useful when keys have poor distribution (sequential ints, identity-hashed refs, single-field structs). Pick a finalizer via the second generic-arity alias:

// Sequential stride keys — Fibonacci spreads them out.
using var fib = SwissHashMap<int, int, FibonacciFinalizer>.Create(capacity: 64);
for (var i = 0; i < 32; i++) fib.Add(i * 1024, i);

// Adversarial input — XMX has the strongest mixing.
using var xmx = RobinHoodHashMap<int, int, XmxFinalizer>.Create(capacity: 64);

See Choosing a finalizer below.

Custom struct hasher

For custom equality without the virtual call of an IEqualityComparer<T>, implement IHasher<T> on a stateless readonly struct and plug it in with the four-argument alias (three for sets). The JIT inlines it like the default hasher:

readonly struct IgnoreCaseHasher : IHasher<string>
{
    public uint ComputeHash(string key) => (uint)string.GetHashCode(key, StringComparison.OrdinalIgnoreCase);
    public bool Equals(string x, string y) => string.Equals(x, y, StringComparison.OrdinalIgnoreCase);
}

using var headers = SwissHashMap<string, string, IgnoreCaseHasher, NoOpHashFinalizer>.Create();
using var seen = LinearHashSet<string, IgnoreCaseHasher, NoOpHashFinalizer>.Create();

The alias always uses default(THasher), so the hasher can't carry state. ComparerHasher<T> needs a comparer instance and is rejected with NotSupportedException; use a Comparer* alias instead.

Enumeration (zero allocation)

foreach, .Keys, and .Values return value-type enumerators — no boxing, no IEnumerator<T> allocation.

foreach (var kvp in map)             { /* kvp.Key, kvp.Value */ }
foreach (var key   in map.Keys)      { /* ... */ }
foreach (var value in map.Values)    { /* ... */ }

The heap-allocated collections also implement the read-only interfaces, so they can be passed to APIs and LINQ: PooledHashMap → IReadOnlyDictionary<K,V>, PooledList<T> → IReadOnlyList<T>, PooledHashSet and PooledStack<T> → IReadOnlyCollection<T>. Enumerating through an interface boxes the enumerator; foreach on the concrete type does not. Enumerators don't detect modification — don't add or remove items while enumerating (a hash table grow returns the old buffers to the pool).

Alternate-key lookup (.NET 10+)

Look up string keys by ReadOnlySpan<char> without allocating a string, the same way as Dictionary<TKey,TValue>.GetAlternateLookup:

using var map = PooledHashMap<string, int>.Create();
map.Add("apple", 1);

var lookup = map.GetAlternateLookup<ReadOnlySpan<char>>();
ReadOnlySpan<char> line = "apple=5";
var name = line[..line.IndexOf('=')];

lookup.TryGetValue(name, out var count);  // true, 1: no string allocated
lookup.TryAdd("pear".AsSpan(), 2);        // creates the "pear" string, because it is a new key
lookup.Remove(name);

The lookup has TryGetValue (also with the stored key), ContainsKey, the indexer getter, TryAdd and Remove. Sets have one too, with Add, Contains, Remove and TryGetValue. It works with the default hasher and with any comparer that implements IAlternateEqualityComparer<TAlternate, TKey>. All the built-in StringComparers do, so PooledHashMap<string, int>.Create(StringComparer.OrdinalIgnoreCase) supports span lookups as well. If the comparer doesn't, GetAlternateLookup throws InvalidOperationException and TryGetAlternateLookup returns false. As with Dictionary, each hash or equality check is one interface call to the comparer.

To skip those calls, or to look up by a type no comparer supports, pass a struct IAlternateHasher<TAlternate, TKey> and the JIT specializes the lookup for it. This one finds string keys by their UTF-8 bytes in a default map:

readonly struct Utf8Hasher : IAlternateHasher<ReadOnlySpan<byte>, string>
{
    // Hash and compare the UTF-16 form, so the hash matches the map's string hasher.
    public uint ComputeHash(ReadOnlySpan<byte> utf8)
    {
        Span<char> chars = utf8.Length <= 128 ? stackalloc char[128] : new char[utf8.Length];
        Utf8.ToUtf16(utf8, chars, out _, out var length);
        return (uint)string.GetHashCode(chars[..length]);
    }

    public bool Equals(ReadOnlySpan<byte> utf8, string key)
    {
        Span<char> chars = utf8.Length <= 128 ? stackalloc char[128] : new char[utf8.Length];
        Utf8.ToUtf16(utf8, chars, out _, out var length);
        return chars[..length].SequenceEqual(key);
    }

    public string Create(ReadOnlySpan<byte> utf8) => Encoding.UTF8.GetString(utf8);
}

var utf8 = map.GetAlternateLookup<ReadOnlySpan<byte>, Utf8Hasher>();
utf8.TryGetValue("apple"u8, out var count);

The hasher has to agree with the map's hasher: an alternate key must get the same raw hash (before the finalizer) as the key it stands for, and Create must return that key. Prest can't check this. If they disagree, lookups miss entries that are present and TryAdd can add duplicate keys. A map built on your own struct hasher has no comparer to ask, so it supports only this overload.

Alternate lookup is in the net10.0 and net11.0 builds. It relies on allows ref struct, which needs .NET 9+, so the net8.0 and netstandard builds don't have it.

Ref access to values

PooledCollectionsMarshal is the counterpart of CollectionsMarshal for maps. It has GetValueRefOrNullRef and GetValueRefOrAddDefault, which let you read or update a value in place with one lookup:

using var counts = PooledHashMap<string, int>.Create();
foreach (var word in words)
{
    PooledCollectionsMarshal.GetValueRefOrAddDefault(counts, word, out _)++;
}

ref var count = ref PooledCollectionsMarshal.GetValueRefOrNullRef(counts, "apple");
if (!Unsafe.IsNullRef(ref count))
{
    count = 0;
}

Both methods take any map type (PooledHashMap<K,V>, ComparerSwissHashMap<K,V>, …), including the stack-only maps. On net10.0+ they also take an alternate lookup, so ReadOnlyMemory<byte> keys can be found or added by a ReadOnlySpan<byte> (byteComparer implements IAlternateEqualityComparer<ReadOnlySpan<byte>, ReadOnlyMemory<byte>>):

using var symbols = new StackOnlyPooledHashMap<ReadOnlyMemory<byte>, Symbol>(64, byteComparer);
var bySpan = symbols.GetAlternateLookup<ReadOnlySpan<byte>>();
ref var symbol = ref PooledCollectionsMarshal.GetValueRefOrNullRef(bySpan, address.Span);

For stack-only maps, the plain-key overloads take the map by in, so they accept a using var local (which can't be passed by ref). They still work on that map, not a copy: GetValueRefOrAddDefault adds the entry to it. Pass the variable itself, because a method result would be copied first. The alternate-lookup overloads take the lookup by value, since it already refers to the map.

GetValueRefOrAddDefault creates the key (through the comparer or IAlternateHasher.Create) only when it adds the entry. As with CollectionsMarshal, the reference is valid only until the map changes. Adding or removing an entry, Clear, Dispose or Return can move entries or give the array back to the pool. After that, the reference points into memory someone else may be using.

Pooled arrays and lists

// Collection-expression literal over ArrayPool.Shared.
using PooledArray<int> numbers = [1, 2, 3];

// Growable, heap-allocated wrapper. Implements IReadOnlyList<T>, so it can be handed
// to APIs and LINQ; foreach still uses the allocation-free struct enumerator.
using var list = new PooledList<int>(initialCapacity: 16);
list.Add(42);
list.RemoveAt(0);

// Struct variant — zero wrapper allocation. Copies share rented buffers; Dispose one.
using var value = new ValuePooledList<int>(16);
value.Add(7);

// Ref-struct variant — compiler-enforced stack-only; an inline Span<T>
// can hold items before the pool rental happens.
Span<int> inline = stackalloc int[8];
using var stack = new StackOnlyPooledList<int>(inline);
stack.Add(99);

All three lists expose .Span (read-only) and AsSpan(), a writable Span<T> over [0, Count) — the counterpart of CollectionsMarshal.AsSpan(List<T>). Use it for in-place work the list has no method for:

using var words = new PooledList<string>();
words.AddRange(["banana", "fig", "kiwi"]);
words.AsSpan().Sort((a, b) => a.Length.CompareTo(b.Length)); // fig, kiwi, banana

Don't hold the span across calls that change the list (Add, AddRange, RemoveAt, Clear, Dispose, ToPooledArray, DetachArray) — get a new one afterwards. A grow returns the old array to ArrayPool<T>.Shared, and writing through a stale span would corrupt whatever code rented it next.

Pooled stacks

PooledStack<T> (class) and StackOnlyPooledStack<T> (ref struct) mirror Stack<T>: Push, Pop / TryPop, Peek / TryPeek. foreach walks top to bottom — the order Pop returns — while .Span exposes elements in push order (bottom at index 0).

// Iterative depth-first walk. The inline buffer covers typical depths,
// so the stack never touches the pool or the heap.
using var pending = new StackOnlyPooledStack<int>(stackalloc int[32]);
pending.Push(rootIndex);
while (pending.TryPop(out var index))
{
    Visit(index);
    foreach (var child in children[index])
    {
        pending.Push(child);
    }
}

// Heap-allocated variant — can live in a field or cross an await.
using var undo = new PooledStack<EditCommand>();
undo.Push(command);

Stack-only hash maps and sets

StackOnlyPooledHashMap<K,V> and StackOnlyPooledHashSet<T> are ref struct versions of the default map and set (SwissTable, default equality, same API core). The table lives inline in the struct, so there is no class instance and no per-thread cache; Dispose returns the arrays to ArrayPool<T>.Shared. default and new() rent on the first add.

// A temporary index inside one method.
using var firstSeen = new StackOnlyPooledHashMap<string, int>(16);
for (var i = 0; i < words.Length; i++)
{
    firstSeen.TryAdd(words[i], i); // keeps the first position of each word
}

using var seen = new StackOnlyPooledHashSet<int>();
foreach (var id in ids)
{
    if (!seen.Add(id))
    {
        Console.WriteLine($"duplicate {id}");
    }
}

// The defaults take an optional comparer, like PooledHashMap<K,V>.
using var ignoreCase = new StackOnlyPooledHashMap<string, int>(16, StringComparer.OrdinalIgnoreCase);

// The generic form takes any algorithm; pass it in when the hasher has state.
using var robin = new StackOnlyPooledHashMap<string, int,
    RobinHoodAlgorithm<KeyValueSlot<string, int>, string, ComparerHasher<string>, NoOpHashFinalizer>>(
    16, new(new ComparerHasher<string>(StringComparer.Ordinal), default));

Pick them over PooledHashMap / PooledHashSet for maps that live and die inside one synchronous method: nothing is allocated even when several are alive at once (the class's per-thread cache holds one instance per type), and there is no cache to return to. Pick the class types when the map has to cross an await, live in a field, be captured by a lambda, or be handed out as IReadOnlyDictionary<K,V> / IReadOnlyCollection<T> — a ref struct can do none of these. The saving is allocations, not time: renting and returning the arrays on every use costs about as much as reusing a cached PooledHashMap, and somewhat more for very small maps (StackOnlyPooledHashMapBenchmarks.cs).

Don't copy one: var b = a; gives two tables over the same arrays with separate counts, and disposing both returns the arrays twice. Pass it to helpers by ref (not in, which mutates a hidden copy). A using var local can't be passed by ref, so dispose in finally there:

var map = new StackOnlyPooledHashMap<int, int>();
try
{
    Fill(ref map);
}
finally
{
    map.Dispose();
}

On net10.0+ both types have GetAlternateLookup, with the same members as the class lookups. The lookup is a ref struct that refers to the map's table, so it works on a using var local and the compiler won't let it outlive the map. Unlike the map, it can be copied and passed by value. It also works with PooledCollectionsMarshal:

ReadOnlySpan<char> line = "apple,Pear,APPLE";
using var counts = new StackOnlyPooledHashMap<string, int>(16, StringComparer.OrdinalIgnoreCase);
var byName = counts.GetAlternateLookup<ReadOnlySpan<char>>();
foreach (var range in line.Split(','))
{
    PooledCollectionsMarshal.GetValueRefOrAddDefault(byName, line[range], out _)++; // a string only for new names
}

IBufferWriter<T>

Same-thread reuse via a threadstatic cache:

var writer = PooledBufferWriter<byte>.Rent();
try
{
    var span = writer.GetSpan(128);
    // ... write bytes, call writer.Advance(n)
    ReadOnlyMemory<byte> bytes = writer.WrittenMemory;
}
finally
{
    PooledBufferWriter<byte>.Return(writer);
}

For pooling that survives await, install Prest.ObjectPool:

using Prest.ObjectPool;

var writer = PooledBufferWriterPool<byte>.Rent();
try
{
    await Task.Yield();    // writer is still valid on any continuation thread
    // ...
}
finally
{
    PooledBufferWriterPool<byte>.Return(writer);
}

JSON

using System.Text.Json;
using Prest;
using Prest.Serializers.SystemTextJson;

// One registration covers every pooled type for any algorithm closure.
var options = new JsonSerializerOptions
{
    Converters = { new PooledTypesJsonConverterFactory() },
};

PooledArray<int> values = [1, 2, 3];
string json = JsonSerializer.Serialize(values, options);
using var roundTripped = JsonSerializer.Deserialize<PooledArray<int>>(json, options);

Alias-typed properties deserialize to the alias type — a PooledHashMap<string, int> property comes back as a PooledHashMap<string, int>, a RobinHoodHashSet<int, FibonacciFinalizer> as exactly that:

public sealed class Order
{
    public PooledHashMap<string, int>? Quantities { get; set; }
    public SwissHashSet<string>? Tags { get; set; }
}

var order = JsonSerializer.Deserialize<Order>(json, options);

Map keys are written and read as JSON property names through the key type's converter, so every key type System.Text.Json supports for Dictionary<K,V> keys works (string, numbers, Guid, enums, …).

Comparer* aliases serialize normally, but the JSON doesn't carry the comparer, so the factory can't deserialize them and throws NotSupportedException. Register an alias converter that supplies the comparer, ahead of the factory:

options.Converters.Insert(0, new PooledHashMapAliasConverter<
    ComparerSwissHashMap<string, int>, string, int,
    SwissTableAlgorithm<KeyValueSlot<string, int>, string, ComparerHasher<string>, NoOpHashFinalizer>>(
    capacity => ComparerSwissHashMap<string, int>.Create(StringComparer.OrdinalIgnoreCase, capacity)));

If you prefer to register converters individually — e.g. to restrict which closed types are supported — the factory-less form is still there:

options.Converters.Add(new PooledArrayConverter<int>());
options.Converters.Add(new PooledListConverter<int>());
options.Converters.Add(new PooledHashSetConverter<int,
    SwissTableAlgorithm<int, int, EqualityDefaultHasher<int>, NoOpHashFinalizer>>());
options.Converters.Add(new PooledHashMapConverter<int, string,
    SwissTableAlgorithm<KeyValueSlot<int, string>, int, EqualityDefaultHasher<int>, NoOpHashFinalizer>>());

// Alias types: pass the alias's own Create.
options.Converters.Add(new PooledHashMapAliasConverter<PooledHashMap<string, int>, string, int,
    SwissTableAlgorithm<KeyValueSlot<string, int>, string, EqualityDefaultHasher<string>, NoOpHashFinalizer>>(
    PooledHashMap<string, int>.Create));

YAML

using VYaml.Serialization;
using Prest;
using Prest.Serializers.VYaml;

var options = YamlSerializerOptions.Standard;
options.Resolver = CompositeResolver.Create(
    (IYamlFormatterResolver[])
    [
        PooledTypeFormatterResolver.Instance,
        StandardResolver.Instance,
    ]);

PooledArray<string> items = ["alpha", "beta"];
string yaml = YamlSerializer.SerializeToString(items, options);

The resolver covers PooledArray<T>, PooledList<T>, PooledHashMap<K,V>, and PooledHashSet<T> in one registration — no need to add each formatter individually. Alias types round-trip as themselves, as with JSON. Comparer* aliases need a PooledHashMapAliasFormatter / PooledHashSetAliasFormatter constructed with a comparer-supplying factory, placed ahead of the resolver (for example as the first argument of CompositeResolver.Create(formatters, resolvers)).

Immutable interop

using Prest;
using Prest.Immutable;

PooledArray<int> pooled = [1, 2, 3];
ImmutableArray<int> immutable = pooled.ToImmutableArray();

using var back = PooledArray.FromImmutableArray(immutable);

Choosing an algorithm

Quick rule of thumb:

• Start with the default PooledHashMap<K,V> (Swiss, optional comparer). For integer-keyed hot paths, use SwissHashMap<K,V>: the same algorithm without the comparer check. It is within a few percent of Dictionary on lookup workloads and roughly 10–20% faster than Dictionary on Add (thanks to pooled arrays — no allocation). The comparer check made PooledHashMap<int, int> hits 29–39% slower than SwissHashMap<int, int> on ARM64.
• If you do heavy add/remove churn on int/long keys, switch to RobinHood or keep Swiss and pair it with FibonacciFinalizer. Sequential or tightly-packed integer keys combined with identity hashing + tombstones is the one workload where Swiss can regress badly.
• If your app is read-mostly and iterates maps often, Chained gives you the densest enumeration path.

Choosing a finalizer

Per-thread cache vs. ObjectPool

Both PooledHashMap<K,V>.Create() (core) and PooledHashMapPool<K,V>.Create() (Prest.ObjectPool) return a ready-to-use map; both auto-return on Dispose. The difference is the storage backing the pool:

• Per-thread cache (core) — a [ThreadStatic] single-slot cache per thread and per type: PooledHashMap<K,V>, SwissHashMap<K,V>, the generic PooledHashMap<K,V,TAlgo> and every other alias each have their own slot, and Dispose returns an instance to the slot of its own type. Zero synchronisation. Ideal for synchronous request handlers and short-lived scopes. Do not keep a reference to the map after Dispose — another caller on this thread will rent it.
• Prest.ObjectPool — Microsoft.Extensions.ObjectPool.DefaultObjectPool (max 64 retained). Concurrent-safe; instances survive await boundaries. Use when the map must outlive a single synchronous scope.

You should never mix the two on the same instance — a map rented from PooledHashMapPool returns itself to that pool on Dispose, not to the per-thread cache.

Examples and benchmarks

examples/Prest.Examples/ — runnable mini-programs for each feature:

• BasicMap.cs, BasicSet.cs — minimal CRUD.
• Algorithms.cs — Swiss / RobinHood / Linear / Chained side-by-side.
• CustomComparer.cs — wiring an IEqualityComparer<T>.
• CustomHasher.cs — a stateless struct IHasher<T> through the hasher alias.
• TypedFields.cs — a class holding maps and sets in fields declared with the short alias types.
• Finalizers.cs — Fibonacci + XMX finalizers on clustered keys.
• Enumeration.cs — foreach, .Keys, .Values.

Run them:

dotnet run --project examples/Prest.Examples -c Release

benchmarks/Prest.Benchmarks/ — BenchmarkDotNet suites:

• PooledHashMapBenchmarks.cs — PooledHashMap<int,int> vs Dictionary<int,int> vs Faster.Map.DenseMap<int,int> across lookup (hit + miss), ContainsKey, insert-from-empty, and steady-state churn at N ∈ {16, 256, 4096, 65536}.
• PooledHashMapStringBenchmarks.cs — same comparisons on string-keyed maps.
• AlgorithmComparisonBenchmarks.cs — the four algorithms head-to-head.
• AlternateLookupBenchmarks.cs — ReadOnlySpan<char> and UTF-8 lookups on string-keyed maps vs Dictionary.GetAlternateLookup, through the comparer and through a struct hasher.
• FinalizerComparisonBenchmarks.cs — NoOp / Fibonacci / Lowbias32 / XMX on Swiss and RobinHood with sequential-int keys.
• PooledStackBenchmarks.cs — PooledStack<int> and StackOnlyPooledStack<int> vs Stack<int>: push N then pop N, on a fresh stack per call and on a reused one, at N ∈ {16, 256, 4096}.
• StackOnlyPooledHashMapBenchmarks.cs — StackOnlyPooledHashMap<int,int> and PooledHashMap<int,int>.Create vs Dictionary<int,int> as a method-local dedup index, with one map alive and with two alive at once, at N ∈ {16, 256, 4096}.

Latest results are committed under benchmarks/artifacts/results/ (-report-github.md files render directly on GitHub).

dotnet run --project benchmarks/Prest.Benchmarks -c Release -- --filter '*'

Native AOT

All shipping packages set <IsAotCompatible>true</IsAotCompatible> on their .NET 8+ TFMs, so trim / AOT / single-file analyzers run in-build and fail on any public API that isn't AOT-safe. The core types, algorithms, object pools, PooledBufferWriter, and every individual converter/formatter are all analyzer-clean.

Two convenience APIs are flagged with [RequiresDynamicCode] + [RequiresUnreferencedCode] because they reach the closed converter/formatter type via Type.MakeGenericType:

• Prest.Serializers.SystemTextJson.PooledTypesJsonConverterFactory
• Prest.Serializers.VYaml.PooledTypeFormatterResolver

If your project uses Native AOT or trimming, skip these two and register the concrete converters directly — each is AOT-safe when closed over concrete generic arguments:

options.Converters.Add(new PooledArrayConverter<int>());
options.Converters.Add(new PooledListConverter<int>());
options.Converters.Add(new PooledHashSetConverter<int,
    SwissTableAlgorithm<int, int, EqualityDefaultHasher<int>, NoOpHashFinalizer>>());
options.Converters.Add(new PooledHashMapConverter<int, string,
    SwissTableAlgorithm<KeyValueSlot<int, string>, int, EqualityDefaultHasher<int>, NoOpHashFinalizer>>());

// Alias types (PooledHashMap<K,V>, SwissHashSet<T>, …) take the alias's own Create.
options.Converters.Add(new PooledHashSetAliasConverter<SwissHashSet<string>, string,
    SwissTableAlgorithm<string, string, EqualityDefaultHasher<string>, NoOpHashFinalizer>>(
    SwissHashSet<string>.Create));

The same applies to VYaml: instead of adding PooledTypeFormatterResolver.Instance to your resolver chain, construct the concrete PooledArrayFormatter<T> / PooledListFormatter<T> / PooledHashMapFormatter<K,V,TAlgo> / PooledHashSetFormatter<T,TAlgo> (or PooledHashMapAliasFormatter<TMap,K,V,TAlgo> / PooledHashSetAliasFormatter<TSet,T,TAlgo> for alias types) you need at compile time and compose them into your own IYamlFormatterResolver.

Target frameworks

Prest.Serializers.SystemTextJson requires net8.0+ for IUtf8SpanFormattable. Prest.Immutable requires net10.0+ (uses C# extension members).

License

MIT © kerem-acer. See LICENSE.