Collection expressions, introduced in C# 12.0 (.NET 8.0), provide a unified syntax for creating collections. Instead of new List<int> { ... } or new[] { ... }, you can use [...] directly, and the compiler infers the target collection type. The spread operator (..) allows combining collections easily. This feature makes collection initialization more readable and reduces ceremony.
Why it matters
- Cleaner, more consistent syntax across all collection types (arrays, lists, spans, etc.).
- The spread operator (
..) enables elegant composition and flattening of collections. - The compiler infers collection type from context, reducing explicit type declarations.
- Improves readability and aligns C# with syntax in modern languages like Python and JavaScript.
Cautions
- Collection expression syntax requires type inference context; always ensure the compiler can determine the target type from assignment or parameter type.
- The spread operator creates a copy of the collection, which has performance implications for large collections.
- Some older custom collection types may not support collection expression syntax; verify compatibility with your collection types.
- Empty collection expressions
[]are valid but require explicit type context to avoid ambiguity.
Unified syntax for array and list creation
Collection expressions provide a consistent syntax for creating arrays, lists, and other collections, making initialization cleaner and less verbose.
Valid since C# 12.0
Without var
// Array
int[] numbers = new int[] { 1, 2, 3, 4, 5 };
// List
List<string> names = new List<string> { "Alice", "Bob", "Charlie" };
// Span
Span<int> values = new Span<int>(new int[] { 10, 20, 30 });With var
// Array
int[] numbers = [1, 2, 3, 4, 5];
// List
List<string> names = ["Alice", "Bob", "Charlie"];
// Span
Span<int> values = [10, 20, 30];Spread operator for combining collections
The spread operator (`..`) makes it easy to combine existing collections into a new one without explicit loops or LINQ.
Valid since C# 12.0
int[] first = [1, 2, 3];
int[] second = [4, 5];
int[] third = [6];
// Combine collections with spread operator
int[] combined = [..first, ..second, ..third];
// Result: [1, 2, 3, 4, 5, 6]
Console.WriteLine(string.Join(", ", combined));