---
title: "Collection Expression Arguments in C# 15 are \"out\""
description: "The first preview of .NET 11 and C# 15 did just release. Some weeks back I did add already an article about that: [*\"Collection Expression Arguments in C# 15+\"*](https://steven-giesel.com/blogPost/352fe495-9cc4-4df8-8ad1-a3e26a64185c/collection-expression-arguments-in-c-15). Well quicker than anticipated that feature is now part of it!"
updated: 2026-02-12
tags: ["C#", "C# 15"]
canonical: "https://steven-giesel.com/blogPost/441e0a21-9869-49e4-beac-b30bb9f9494c/collection-expression-arguments-in-c-15-are-out"
---

# Collection Expression Arguments in C# 15 are "out"

The first preview of .NET 11 and C# 15 did just release. Some weeks back I did add already an article about that: [*"Collection Expression Arguments in C# 15+"*](https://steven-giesel.com/blogPost/352fe495-9cc4-4df8-8ad1-a3e26a64185c/collection-expression-arguments-in-c-15). Well quicker than anticipated that feature is now part of it!

## Slightly different syntax

To my original blog post they changed the syntax.

Here the "pseudo" syntax used in the GitHub issue:
```csharp
List<int> allNumbers = [args(capacity: 32), 0, ..evenNumbers, ..oddNumbers];
```

And here the final version:
```csharp
List<int> allNumbers = [with(capacity: 32), 0, ..evenNumbers, ..oddNumbers];
```

They reused the `with` operator. 

## Resources
 * More info also here: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-15#collection-expression-arguments