Modern collections (C# 13/14, .NET 10)

Collection choice is an API and performance decision, not a style preference. Pick the narrowest type that fits how the data is built, read, and exposed.

Collection expressions

[...] is target-typed — the compiler picks the concrete type from context, not from the syntax:

int[] arr = [1, 2, 3];              // T[]
List<int> list = [1, 2, 3];         // List<T>
Span<int> span = [1, 2, 3];         // stack-allocated for a ref struct target
int[] a = [1, 2];
int[] combined = [.. a, 5, 6];      // spread — concatenates/copies

For interface targets, the compiler synthesizes a read-only wrapper over an array, not a List<T>:

IEnumerable<int> ie = [1, 2, 3];      // compiler-generated read-only array wrapper
IReadOnlyList<int> irl = [1, 2, 3];   // same — not a List<T>, don't downcast to one

Don't cast a collection-expression result to List<T> expecting it to succeed — the concrete type is an implementation detail. If you need List<T> specifically, target-type to it explicitly.

The spread element .. works on any type enumerable and lets the compiler pick a single-allocation, correctly-sized result — prefer it over Concat().ToArray() chains.

.NET 11 / C# 15 — collection expression arguments

Check TargetFramework/LangVersion first (see csharp-language-versions) — needs C# 15. It is a compiler feature with no runtime dependency, so it also compiles on a net10.0 target when LangVersion is latest on SDK 11 (verified); at the default C# 14 it fails with CS9327. A collection expression can now take a leading with(...) argument list to configure the collection being built — capacity, comparer, or both — instead of falling back to imperative construction:

List<string> names = [with(capacity: 100), .. values];              // pre-sized List<T>
HashSet<string> set = [with(StringComparer.OrdinalIgnoreCase), "a", "A"]; // custom comparer, still target-typed
Dictionary<byte[], int> map = [with(new ByteArrayComparer())];            // comparer only, no elements

with(...) must be the first element and only applies when the target type has a matching constructor/factory shape (e.g. List<T>(int capacity), HashSet<T>(IEqualityComparer<T>)) — it's resolved the same way collection-expression construction already is, just with arguments threaded through instead of a bare new(). Prefer this over new List<T>(capacity) { ... }/new HashSet<T>(comparer) { ... } boilerplate once the project targets net11.0+.

.NET 11 library additions worth knowing

• EqualityComparer<T>.Create<TKey>(Func<T,TKey> keySelector, IEqualityComparer<TKey>? keyComparer = null) — build a comparer from a key projection without hand-writing IEqualityComparer<T>:

  var byLength = EqualityComparer<string>.Create(s => s.Length);
  var set = new HashSet<string>(byLength);

  (There's also a Create(Func<T?,T?,bool> equals, Func<T,int>? getHashCode = null) overload for a fully custom comparison.)
• LINQ FullJoin — outer join that keeps unmatched rows from both sides (filling the missing side with default/a supplied value), where Join only keeps matches and GroupJoin groups matches per outer element.
• Tuple-returning Join/GroupJoin overloads — skip the result-selector lambda when you just want the pairing: left.Join(right, l => l.Key, r => r.Key) returns (TLeft, TRight) tuples directly; left.GroupJoin(right, l => l.Key, r => r.Key) returns IEnumerable<IGrouping<TKey, TRight>> directly, no selector needed.
• BitArray span constructors — new BitArray(ReadOnlySpan<bool> values) alongside the existing bool[]/int[]/byte[] constructors, so a stackalloc/slice can seed a BitArray without an intermediate array. The ReadOnlySpan<byte> form is LSB first: byte 0x01 sets bit 0 and 0x80 sets bit 7. Formats that count bits from the most significant bit (PDF and TIFF bitmaps) need shifts instead. See dotnet-11-libraries.

params collections and spans (C# 13)

params is no longer array-only. Prefer params ReadOnlySpan<T> for hot-path APIs — the compiler stack-allocates the backing storage for small, fixed call sites instead of heap-allocating an array:

static int Sum(params ReadOnlySpan<int> values)
{
    var total = 0;
    foreach (var v in values) total += v;
    return total;
}

Sum(1, 2, 3);          // no array allocation
Sum([4, 5, 6]);         // collection expression also binds

Rules:

• params ReadOnlySpan<T> / params Span<T> cannot be used in async methods or iterators (spans can't cross an await or be captured in state machines) — keep those params T[].
• A type only needs a GetEnumerator(), indexer, and Count/Length (a "collection-like" shape) to support params now — you can author your own params-friendly collection types.
• Don't change an existing public params T[] to a span overload casually — it's a binary-breaking, source-compatible-only change; add an overload if both call shapes matter.

Choosing a collection type

• FrozenDictionary/FrozenSet are rarely the right choice — treat them as an exception, not a default. Construction analyses the actual key set to build a specialized layout (perfect-hash-style branching tuned for that data), which is meaningfully more expensive to build than a Dictionary/HashSet. That cost is only worth paying when the collection is built exactly once in a long-running process and then queried a very large number of times afterward — e.g. a server's static routing table or a compiler's fixed keyword set, built at startup and read for the life of the process.
• Do not reach for Frozen* for: short-lived processes, CLI tools, tests, analyzers/generators, request/response-scoped data, anything rebuilt per request or per compilation, or "it's read-only so it should be Frozen" reasoning. In all of these the construction cost is paid on (or near) every use, which makes it slower than Dictionary/HashSet, not faster.
• Default to Dictionary<K,V>/HashSet<T> (or a plain array/switch for a handful of fixed cases) for lookup tables. Only move to Frozen* after profiling shows construction is amortized over enough reads to matter — measure with BenchmarkDotNet including construction cost, not just steady-state lookup time, before switching.
• ImmutableArray<T>/ImmutableDictionary<K,V> give you real structural immutability and safe sharing across threads without copy-on-write bookkeeping in your own code — ImmutableDictionary edits are O(log n) persistent-tree operations, not full copies, but that means it is slower to read than a plain Dictionary. Don't reach for ImmutableDictionary as a "safe default" for something only ever read — that's still just Dictionary territory unless you need the structural-sharing/edit semantics.
• Plain Dictionary<K,V>/List<T>/arrays remain correct and fastest for anything mutable or short-lived. Don't add Frozen/Immutable ceremony to a collection nobody shares or freezes.
• ImmutableArray<T> is a thin struct wrapper over T[] — cheap to pass around, but a default instance is not the same as Empty (it's null-backed and throws on iteration). Always initialize it.

// ✅ default choice — cheap to build, fast enough for almost everything
static readonly Dictionary<string, Command> Commands =
    new() { ["run"] = Run, ["stop"] = Stop };

// ✅ Frozen only once measurement justifies it: built once at process startup,
// read millions of times over a long-running server's lifetime
static readonly FrozenDictionary<string, Route> Routes =
    LoadRoutesFromConfig().ToFrozenDictionary();

// ❌ rebuilding per call pays the (larger) construction cost on every use —
// this is slower than a plain Dictionary, not faster
FrozenDictionary<string, Command> Lookup() => _raw.ToFrozenDictionary(); // called per request

Use concrete collection types, not collection interfaces

Declare collections with their concrete types: List<T>, Dictionary<TKey, TValue>, HashSet<T>, T[], ImmutableArray<T>. Don't use the mutable collection interfaces (IList<T>, ICollection<T>, IDictionary<TKey, TValue>, ISet<T>) for fields, locals, parameters or return types. This matches the .NET analyzer guidance (CA1859, "use concrete types when possible for improved performance"):

• Speed: calls through a concrete type are direct and can be inlined; through an interface they're virtual dispatch. foreach over a List<T> uses its struct enumerator; over IList<T> the enumerator is boxed on older runtimes.
• Clarity: the concrete type tells the reader exactly what they have: its complexity, ordering, whether it's a copy.
• Honesty: the mutable interfaces promise operations that often throw (IList<T>.Add on an array, a read-only wrapper), so the type guarantees nothing.

// ✅ concrete types throughout
private readonly Dictionary<string, Order> _byId = [];
public List<Order> FindOpen(HashSet<string> ids) { ... }

// ❌ interface types that hide what the collection is and cost dispatch
private readonly IDictionary<string, Order> _byId = new Dictionary<string, Order>();
public IList<Order> FindOpen(ICollection<string> ids) { ... }

Narrow exceptions:

• IEnumerable<T> for a genuinely lazy or streaming sequence (an iterator, a query you don't want to materialise). Anything already materialised goes out as its concrete type.
• IReadOnlyList<T> / IReadOnlyDictionary<TKey, TValue> only when a public API deliberately exposes a read-only view of internal state without copying it. The view is a contract, not protection: a caller can cast back to List<T>. Return ImmutableArray<T> or a copy when the guarantee matters. Reach for FrozenDictionary<TKey, TValue> only for the long-lived, build-once table discussed above, not by default.

private readonly List<Order> _orders = [];
public IReadOnlyList<Order> Orders => _orders;   // read-only view of internal state, no copy

Span<T> / ReadOnlySpan<T> / stackalloc for local work

Use spans for slicing and short-lived buffers that never escape the method:

ReadOnlySpan<char> csv = "a,bb,ccc";
var comma = csv.IndexOf(',');
ReadOnlySpan<char> first = csv[..comma];      // slice, no allocation

Span<int> scratch = stackalloc int[16];        // small, fixed, stack-only

• Spans are ref struct — cannot be a field of a non-ref struct class, cannot be boxed, cannot cross await/yield. That's a feature: it keeps them from escaping.
• stackalloc is for small, bounded sizes known not to blow the stack (tens to low hundreds of elements) — never with an unbounded/user-controlled length.
• Slice instead of Substring/ToArray/Skip().Take() when you're only reading a subrange.

CollectionsMarshal — controlled escape hatches

For hot paths where the collection API forces an extra lookup or copy:

// mutate a List<T>'s backing storage in place, no copy
Span<int> span = CollectionsMarshal.AsSpan(list);
span[0] = 42;

// one lookup instead of ContainsKey + indexer/Add
ref int slot = ref CollectionsMarshal.GetValueRefOrAddDefault(dict, key, out var existed);
if (!existed) slot = ComputeInitial();
slot++;

Caveats: AsSpan is invalidated by any operation that resizes the list (Add past capacity, RemoveAt, etc.) — don't mutate the list's shape while holding the span. GetValueRefOrAddDefault returns a ref into the dictionary's internal storage — it's invalidated by any subsequent resize of the dictionary, so use it immediately, not stashed across other mutations.

LINQ in hot paths

LINQ is fine on cold/startup paths and for readability — the iterator and delegate overhead is real but usually irrelevant off the hot path. On measured hot loops (per-frame, per-request, per-notification), prefer a manual for/foreach — see csharp-performance for the allocation reasoning.

Newer LINQ methods worth knowing (avoid hand-rolled equivalents):

words.CountBy(w => w[0]);                          // group + count in one pass
words.AggregateBy(w => w[0], 0, (acc, w) => acc + w.Length); // group + fold, no intermediate groupings
words.Index();                                      // (int Index, T Item) pairs, replaces .Select((w, i) => (i, w))

Avoid multiple enumeration. An IEnumerable<T> from a LINQ query or a yield return iterator re-runs its whole pipeline (including side effects and I/O) on every enumeration:

// ❌ query runs twice
IEnumerable<int> Query() => data.Where(x => x > 0);
var q = Query();
if (q.Any()) Process(q.ToList());     // enumerated once for Any, again for ToList

// ✅ materialize once, reuse
var results = Query().ToList();
if (results.Count > 0) Process(results);

If a method returns IEnumerable<T> and the caller might enumerate more than once, materialize (ToList/ToArray) at the boundary rather than letting each call site guess.

Checklist

• Collection-expression target type is the one you actually want (IReadOnlyList<T>/IEnumerable<T> are not List<T>)
• params ReadOnlySpan<T> on hot-path variadic APIs, not async/iterator methods
• On C# 15 (net11.0, or net10.0 with LangVersion=latest on SDK 11): [with(capacity: n), ...]/[with(comparer), ...] used instead of new List<T>(n) { ... } boilerplate where the TFM allows it
• Dictionary/HashSet by default; Frozen* only in a long-running process, with a benchmark that includes construction cost
• Immutable* only where structural sharing/immutability is actually needed
• Collections declared as concrete types (List<T>, Dictionary<TKey, TValue>, HashSet<T>, T[]), never IList<T>/ICollection<T>/IDictionary<TKey, TValue>/ISet<T>
• IEnumerable<T> only for lazy sequences; IReadOnlyList<T>/IReadOnlyDictionary<TKey, TValue> only for a deliberate read-only view of internal state
• Spans/stackalloc stay local, bounded, and never cross await
• CollectionsMarshal refs/spans used immediately, not held across a resize
• LINQ avoided on measured hot loops; no query re-enumerated by accident