Collection Expressions & Spread Operator (..)

1Concept

Introduced in C# 12 and enhanced in C# 13, Collection Expressions `[a, b, ..c]` provide a unified syntax for initializing arrays, `Span<T>`, `ReadOnlySpan<T>`, `List<T>`, and immutable sets with compiler-optimized memory pre-sizing.

2Architecture Diagram

[ item1, item2, ..existingList, item3 ]
       │
       ▼
[ Compiler Pre-calculates exact capacity: 2 + existingList.Count + 1 ]
       │
       ▼
[ Single Heap/Stack Allocation with Span Copy (Zero Resizing Overhead) ]

3Code Example

using System;
using System.Collections.Generic;

public class CollectionExpressionDemo
{
    public static void Main()
    {
        int[] baseline = [10, 20, 30];
        int[] extended = [0, ..baseline, 40, 50];

        Console.WriteLine($"Extended Array: [{string.Join(", ", extended)}]");
        Console.WriteLine($"Length: {extended.Length}");

        // Span creation with zero heap allocation!
        ReadOnlySpan<char> vowels = ['a', 'e', 'i', 'o', 'u'];
        Console.WriteLine($"Stack Span Vowels Length: {vowels.Length}");
    }
}

4Expected Output

Extended Array: [0, 10, 20, 30, 40, 50]
Length: 6
Stack Span Vowels Length: 5

5Key Takeaways

• ✓Collection expressions eliminate verbose `new int[] { }` or `new List<int>()` syntax.
• ✓The spread operator (`..`) enables zero-allocation span concatenation.
• ✓The compiler automatically selects the most efficient backend data structure.