[Proposal]: Collection expressions

[CyrusNajmabadi]

Collection expressions

• Specification: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/collection-expressions.md
• Discussion: [Proposal]: Collection expressions (VS 17.7, .NET 8) #8651

Summary

Collection expressions introduce a new terse syntax, [e1, e2, e3, etc], to create common collection values. Inlining other collections into these values is possible using a spread operator .. like so: [e1, ..c2, e2, ..c2].

Design meetings

• Nov 1, 2021: First look
• Feb 9, 2022: Triage
• Mar 9, 2022: Ambiguity of ..
• Sep 28, 2022: Triage
• Apr 3, 2023: Spreads and compiler optimizations
• Apr 26, 2023: Review, targeting IEnumerable<T>, and dictionary elements
• May 3, 2023: Natural type
• May 31, 2023: Review, construction strategies
• Jun 19, 2023: Type inference
• Jul 12, 2023: Create methods, extension methods
• Jul 17, 2023: Compiler check-in
• Aug 9, 2023: Targeting interfaces, loosening Create return type requirements
• Aug 14, 2023: Span betterness, type inference, targeting interfaces or Memory<T> or inline arrays
• Aug 16, 2023: Ref-safety scope
• Sep 18, 2023: Capacity optimization, buffering when length is known
• Sep 20, 2023: Spread type inference, overload resolution
• Sep 25, 2023: Well-defined behavior
• Sep 27, 2023: Allowed optimizations
• Oct 2, 2023: Conversions requiring Add, non-generic interfaces, strings
• Oct 11, 2023: Preferring ReadOnlySpan over Span
• Nov 15, 2023: Nullability analysis
• Jan 8, 2024: Determining iteration type
• Jan 10, 2024: Requiring construction APIs for convertibility
• Jan 31, 2024: Relax enumerable requirement
• Feb 5, 2024: Inline collections
• Feb 26, 2024: Collections with iteration type of object with more specific Add
• Apr 15, 2024: Non-enumerable collections, relax Add requirement
• Apr 17, 2024: Relax Add requirement

See also:

• Better conversion from collection expression
• Collection expression arguments
• Dictionary Expressions

Working group meetings

• Oct 6, 2022: Initial goals, dictionaries
• Oct 14, 2022: Dictionaries, efficiency
• Oct 21, 2022: Spans and stack allocation, review
• Apr 5, 2023: Efficient construction for array-backed collections
• Apr 28, 2023: Natural type, spread type inference, dictionary add vs overwrite
• May 26, 2023: Construction API calls, dictionary sigils, C# 12 cut line
• Jun 12, 2023: API shape for BCL collections to participate
• Jun 26, 2023: Lifetimes, BCL APIs, breaking changes, extension methods, syntax disambiguation
• Jul 26, 2023: Create methods with derived type, extension GetEnumerator, synthesized inline arrays
• Aug 3, 2023: Escape scope, blittable data
• Aug 10, 2023: Behavior on runtimes without inline arrays
• Aug 11, 2023: Concrete types used when targeting interface types
• Jan 23, 2024: Design restart