[Proposal]: Collection Expression Arguments (VS 18.8, .NET 11)

Collection Expression Arguments

• Specification: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-15.0/collection-expression-arguments.md
• Discussion: https://github.com/dotnet/csharplang/discussions/8886

Summary

Introduce a way for users to create collection expressions (e.g. [a, b, c]) while also passing along arguments to the respective creation construction for that collection.

An example strawman syntax for this would be:

List<string> values = [args(capacity: 32); a, b, c];

Design meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-12-09.md#collection-expression-arguments
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-01-22.md#collection-expression-arguments
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-03-12.md#collection-builder-method-parameter-order
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-03-17.md#collection-expression-arguments
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-04-09.md#dictionary-expressions
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-04-09.md#collection-expression-arguments
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-04-14.md
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-05-07.md#collection-expression-arguments
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-05-12.md#collection-expression-arguments
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-12-10.md#collection-expression-arguments
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-12-17.md#collection-expression-arguments-parsing-question