C# Academy · Lesson

Collection Expression Syntax

Initialize arrays, lists, and spans uniformly.

Lesson 1 of 413 steps

Collection Expression Syntax is a free C# Academy lesson on CoddyKit. This is lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.

Collection Expressions (C# 12)

C# 12 introduces collection expressions: a single square-bracket syntax to create arrays, lists, spans, and more. Instead of new int[] { 1, 2, 3 } you simply write [1, 2, 3].

using System;

int[] numbers = [1, 2, 3, 4, 5];
Console.WriteLine(string.Join(",", numbers)); // 1,2,3,4,5

One Syntax, Many Targets

The same [...] literal works whether the target is an array, a List<T>, or a Span<T>. The compiler picks the right construction.

using System;
using System.Collections.Generic;

int[] arr = [1, 2, 3];
List<int> list = [4, 5, 6];
Span<int> span = [7, 8, 9];
Console.WriteLine(arr.Length + " " + list.Count + " " + span.Length);

Empty Collections

An empty collection expression is just []. It is clearer than Array.Empty<T>() or new List<T>() and target-types automatically.

using System;
using System.Collections.Generic;

int[] empty = [];
List<string> names = [];
Console.WriteLine(empty.Length + " " + names.Count); // 0 0

Replacing Verbose Initializers

Collection expressions remove the ceremony of the old initializer syntax. Compare the two forms below.

using System;
using System.Collections.Generic;

// old
List<int> oldWay = new List<int> { 1, 2, 3 };
// new
List<int> newWay = [1, 2, 3];
Console.WriteLine(oldWay.Count == newWay.Count); // True

Nested Collections

You can nest collection expressions to build jagged arrays or lists of lists.

using System;

int[][] grid = [[1, 2], [3, 4], [5, 6]];
Console.WriteLine(grid[1][0]); // 3

Expressions as Elements

Elements can be any expression, including variables, method calls, and arithmetic.

using System;

int x = 10;
int[] values = [x, x * 2, x + 5, Math.Abs(-3)];
Console.WriteLine(string.Join(",", values)); // 10,20,15,3

Multidimensional Note

Collection expressions create jagged arrays (int[][]), not rectangular ones (int[,]). Rectangular arrays still use their own syntax.

using System;

int[][] jagged = [[1, 2, 3], [4, 5]];
Console.WriteLine(jagged[0].Length + " " + jagged[1].Length); // 3 2

Working with Span

Because collection expressions can target Span<T> and ReadOnlySpan<T>, the compiler may use stack allocation, avoiding heap garbage for short-lived data.

using System;

ReadOnlySpan<int> stackData = [1, 2, 3, 4];
int total = 0;
foreach (int n in stackData) total += n;
Console.WriteLine(total); // 10

Passing Collections to Methods

When a parameter type is a collection, you can pass a collection expression directly at the call site.

using System;

static int Sum(int[] xs) {
    int t = 0;
    foreach (int x in xs) t += x;
    return t;
}

Console.WriteLine(Sum([2, 4, 6])); // 12

Readability Wins

Collection expressions shine in data-heavy code, like configuration tables or test fixtures, where the brackets keep the focus on the values.

using System;

string[] days = ["Mon", "Tue", "Wed", "Thu", "Fri"];
Console.WriteLine(days[^1]); // Fri

Returning Collections

A method can return a collection expression, with the return type guiding the construction.

using System;

static int[] Primes() => [2, 3, 5, 7, 11];

Console.WriteLine(string.Join(",", Primes())); // 2,3,5,7,11

Quick Check

Check your understanding of collection expression basics.

Answer it in the interactive lesson

Recap

You learned the C# 12 collection expression syntax.

[1, 2, 3] replaces verbose initializers.
[] is an empty collection.
It is target-typed: arrays, lists, and spans all work.
Nested brackets build jagged arrays.

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Frequently Asked Questions

Is the “Collection Expression Syntax” lesson free?

Yes — the full text of “Collection Expression Syntax” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO. The C# Academy course includes 4 lessons in total.

What will I learn in “Collection Expression Syntax”?

Initialize arrays, lists, and spans uniformly. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C# Academy?

No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

How long does the “Collection Expression Syntax” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

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