C# Collection Expressions: The [ ] Syntax and Spread ..

One bracket syntax, and the compiler figures out whether you meant an array, a List, a Span, or something else entirely.

Quick: what's the syntax for creating a new array with three numbers in it? An empty List<string>? A Span<int> with the same three numbers? If you've written C# for a while, you know the honest answer is "three different syntaxes, and you'd better remember which is which" — new[] { 1, 2, 3 }, new List<string>(), stackalloc int[] { 1, 2, 3 }. Each collection type historically brought its own initializer dialect.

C# 12 introduces a single, unified syntax that works across nearly every collection type in .NET: square brackets. [1, 2, 3] means "a collection containing 1, 2, and 3" — and the compiler figures out from context whether that should become an array, a List<T>, a Span<T>, or something else, without you writing a different incantation for each one.

In this lesson, you'll learn collection expression syntax ([1, 2, 3]), the spread operator (..) for combining collections, how the same syntax adapts across arrays, List<T>, Span<T>, and other collection types, and why this replaces a decade of increasingly inconsistent initializer syntax.

What Is It?

The Simple Explanation

A collection expression is a bracket-enclosed list of values — [1, 2, 3] — that creates a collection. What kind of collection it becomes depends entirely on what you're assigning it to: assign it to an int[] and you get an array; assign it to a List<int> and you get a list. Same syntax, different result, chosen by context — exactly the way 0 can be an int, a double, or a byte depending on where it's used.

The Technical Definition

A collection expression (C# 12) is a target-typed expression of the form [e1, e2, ..., en] that the compiler converts into whatever collection type the expression is being assigned or passed to — an array, List<T>, Span<T>, ReadOnlySpan<T>, any type implementing the standard collection interfaces (like IEnumerable<T>), or any type exposing a compatible Create factory pattern recognized via the [CollectionBuilder] attribute. The spread element .. inlines the contents of another collection directly into a collection expression, at the position where it appears.

Why Does It Exist?

The Problem

Every collection type in .NET grew its own initializer syntax over the years, and none of them match:

int[] numbers = new int[] { 1, 2, 3 };
int[] shortForm = { 1, 2, 3 };                    // only legal at declaration
List<int> list = new List<int> { 1, 2, 3 };
List<int> listInferred = new() { 1, 2, 3 };        // target-typed new, C# 9
Span<int> span = stackalloc int[] { 1, 2, 3 };
int[] combined = numbers.Concat(new[] { 4, 5 }).ToArray(); // "add two arrays together"

Beyond the inconsistency, that last line highlights a real pain point: combining collections — "take everything from this array, then everything from that one" — required reaching for LINQ's Concat and ToArray, turning a conceptually simple operation into a multi-step chain.

The Need

Developers needed one consistent syntax for "here are the values in this collection" that worked the same way regardless of the target type, plus a direct, readable way to combine or inline other collections without a detour through LINQ.

The Solution

The same code, with collection expressions:

int[] numbers = [1, 2, 3];
List<int> list = [1, 2, 3];
Span<int> span = [1, 2, 3];
int[] combined = [.. numbers, 4, 5]; // spread numbers' contents in, then append 4 and 5

One bracket syntax across every target, and the spread operator makes combining collections a direct, readable part of the literal itself.

Big Picture

How It Works

int[] numbers = [1, 2, 3];
List<string> names = ["Alice", "Bob"];
Dictionary<string, int> scores = new() { ["Alice"] = 90 }; // dictionaries use their own syntax still

List<int> empty = []; // equivalent to new List<int>()
int[] emptyArray = [];

int[] first = [1, 2, 3];
int[] second = [4, 5];

int[] combined = [.. first, .. second];     // [1, 2, 3, 4, 5]
int[] withExtra = [0, .. first, 99];        // [0, 1, 2, 3, 99]

ReadOnlySpan<int> span = [1, 2, 3];  // no heap allocation for this one
HashSet<string> tags = ["c#", "dotnet"];
IEnumerable<int> sequence = [1, 2, 3];

Simple Example

int[] weekdays = [1, 2, 3, 4, 5];
int[] weekend = [6, 7];

int[] fullWeek = [.. weekdays, .. weekend];
Console.WriteLine(string.Join(", ", fullWeek)); // 1, 2, 3, 4, 5, 6, 7

List<string> baseTags = ["c#", "dotnet"];
List<string> extendedTags = [.. baseTags, "records", "patterns"];
Console.WriteLine(string.Join(", ", extendedTags)); // c#, dotnet, records, patterns

Code → Meaning → Result: [.. weekdays, .. weekend] reads almost like plain English — "everything from weekdays, then everything from weekend" — and produces a brand-new array holding all seven values, without a single call to Concat or ToArray.

Real-World Example

Building a product catalog page that combines a curated list of "featured" products with the regular catalog, while excluding anything currently out of stock:

public record Product(string Name, decimal Price, bool InStock);

List<Product> GetFeaturedProducts() =>
[
    new Product("Wireless Mouse", 24.99m, true),
    new Product("Mechanical Keyboard", 89.99m, true)
];

List<Product> GetCatalogProducts(List<Product> inventory)
{
    List<Product> featured = GetFeaturedProducts();
    List<Product> inStockInventory = inventory.Where(p => p.InStock).ToList();

    // Featured items always appear first, followed by the rest of the in-stock catalog
    return [.. featured, .. inStockInventory];
}

// ─── Usage: adding a promotional item on top of an existing list, without mutating it ───
List<Product> catalog = GetCatalogProducts(inventory);
List<Product> withPromo = [new Product("Flash Sale Item", 9.99m, true), .. catalog];

Each line reads as a direct description of what's being built — "featured, then in-stock inventory," or "a promo item, then the whole existing catalog" — with the spread operator doing the work that would otherwise need Concat, Prepend, or manual loop-and-add code.

Under the Hood

Common Confusion

1. [] needs a target type — it can't stand entirely on its own

Just like new() (target-typed new, C# 9), a collection expression needs to know what type it's becoming. var x = [1, 2, 3]; doesn't compile — the compiler has no target to infer from, since var alone gives it nothing to pick from. You need an explicit target: a declared type (int[] x = [1, 2, 3];), a parameter type, or a return type.

2. The spread operator .. here is unrelated to the range operator .. used in indexing

C# also uses .. for range expressions like array[1..3] (a slice from index 1 up to, but not including, index 3). Inside a collection expression, .. means something different — "spread this entire collection's elements in here" — not a range. Context (inside [...] as a standalone prefix before a collection, versus inside indexing brackets) disambiguates the two uses.

3. Dictionaries don't (yet) use collection-expression bracket syntax

You still initialize a Dictionary<TKey, TValue> with its existing collection-initializer or index-initializer syntax (new() { ["key"] = value }). Collection expressions currently target sequence-shaped collections (things you enumerate one element at a time), not key/value stores.

Common Mistakes

Mistake 1 — Trying to use a collection expression without a clear target type

var numbers = [1, 2, 3]; //  CS9176: there is no target type for this collection expression

Give the compiler a type to target:

int[] numbers = [1, 2, 3]; //  fine

Mistake 2 — Forgetting that spreading into an array still creates a brand-new array

Assuming [.. original, extraItem] somehow appends in place, mutating original:

int[] original = [1, 2, 3];
int[] extended = [.. original, 4];

original[0] = 99;
Console.WriteLine(extended[0]); // still 1 — extended is a completely separate array

Understand that a collection expression, spread or not, always constructs a new collection — it copies elements in, it never reaches back into the source collection to modify it.

Mistake 3 — Spreading a very large collection repeatedly in a hot loop

Each spread still has to enumerate and copy the source collection's elements. Spreading a large collection inside a loop that runs many times can add up to real, avoidable allocation and copying cost — the same caution that applies to any repeated collection-building code, collection expressions included.

When Should I Use It?

• Use collection expressions by default for array, list, and span initialization in new C# 12+ code — they're shorter, consistent across target types, and often compile to more efficient code than the older syntax.
• Use the spread operator whenever you'd otherwise reach for Concat, Prepend, Append, or manual "add these existing items too" loops.
• Keep using explicit new Dictionary<...>() initializer syntax for dictionaries, since collection expressions don't (currently) cover key/value collections.

Mental Model

Key Takeaway

Check Your Understanding

You've seen how one bracket syntax adapts across collection types, and how the spread operator inlines other collections. Let's check your understanding.

You can now build and combine collections with one consistent, readable syntax across arrays, lists, spans, and more.