--- title: Explore new features available in C# 15 preview | daily.dev description: C# 15, shipping with .NET 11 in November, is available now in .NET 11 preview 7. New features include union types (constraining a value to a specified set of... canonical: https://daily.dev/posts/explore-new-features-available-in-c-15-preview-plkkrytfs twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Explore new features available in C# 15 preview | daily.dev og:description: C# 15, shipping with .NET 11 in November, is available now in .NET 11 preview 7. New features include union types (constraining a value to a specified set of... og:url: https://daily.dev/posts/explore-new-features-available-in-c-15-preview-plkkrytfs og:image: https://api.daily.dev/og/posts/PLkKRYtfs.png og:image:alt: Explore new features available in C# 15 preview og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Explore new features available in C# 15 preview **[.NET Blog](https://daily.dev/sources/dotnet)** · 6 min read · 13 upvotes · 0 comments ## Summary C# 15, shipping with .NET 11 in November, is available now in .NET 11 preview 7. New features include union types (constraining a value to a specified set of case types with exhaustive switch support), closed hierarchies (restricting class derivation to the declaring assembly), a preview redesign of the unsafe model that allows pointer declarations outside unsafe contexts while still gating dereferencing operations, collection expression arguments via a new with(...) syntax, extension indexers building on C# 14's extension members, and labeled break/continue for exiting nested loops directly. The unsafe model changes require explicit opt-in via project file settings and may still change before C# 16. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### What are union types in C# 15? Union types in C# 15 let you declare a type as one of a fixed set of case types, such as 'public union Pet(Cat, Dog, Bird)'. Each case type implicitly converts to the union, and a switch expression over a non-null union value is exhaustive without needing a discard or default arm, since the compiler knows the complete set of possible types. _daily.dev surfaces language updates like this for developers deciding when to adopt new C# type-safety features._ ### How does the unsafe code model change in C# 15? C# 15 previews a redesign where pointer types, address-of operations, fixed statements, stackalloc-to-pointer conversions, and sizeof on unmanaged types no longer require an unsafe context, while dereferencing pointers still does. It's opt-in via 'updated-memory-safety-rules' and 'preview', and adding the unsafe modifier to a member signature is a breaking change restricting who can call it. _Developers planning an unsafe-code migration can track this preview's evolution on daily.dev before it lands in C# 16._ ### How do I pass constructor arguments in a C# collection expression? C# 15 adds a with(...) element written first in a collection expression to forward arguments to the underlying constructor or Create method. For example, 'List names = [with(capacity: values.Length * 2), .. values];' or 'HashSet set = [with(StringComparer.OrdinalIgnoreCase), "Hello", "HELLO"];' This also underpins the upcoming dictionary expressions syntax. _Teams adopting new collection expression syntax can follow C# language changes as they ship on daily.dev._ ## Similar posts on daily.dev - [Explore union types in C\# 15](https://daily.dev/posts/explore-union-types-in-c-15-oygraauhf) · DevBlogs · 64 upvotes · 3 comments --- Tags: [#.net](https://daily.dev/tags/.net), [#c#](https://daily.dev/tags/c#), [#type-systems](https://daily.dev/tags/type-systems) [View this post on daily.dev](https://daily.dev/posts/explore-new-features-available-in-c-15-preview-plkkrytfs) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Explore new features available in C# 15 preview","url":"https://daily.dev/posts/explore-new-features-available-in-c-15-preview-plkkrytfs","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/explore-new-features-available-in-c-15-preview-plkkrytfs"},"datePublished":"2026-08-24T17:03:33.501Z","dateModified":"2026-09-14T06:55:26.096Z","description":"C# 15, shipping with .NET 11 in November, is available now in .NET 11 preview 7. 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Each case type implicitly converts to the union, and a switch expression over a non-null union value is exhaustive without needing a discard or default arm, since the compiler knows the complete set of possible types. daily.dev surfaces language updates like this for developers deciding when to adopt new C# type-safety features."}},{"@type":"Question","name":"How does the unsafe code model change in C# 15?","acceptedAnswer":{"@type":"Answer","text":"C# 15 previews a redesign where pointer types, address-of operations, fixed statements, stackalloc-to-pointer conversions, and sizeof on unmanaged types no longer require an unsafe context, while dereferencing pointers still does. It's opt-in via 'updated-memory-safety-rules' and 'preview', and adding the unsafe modifier to a member signature is a breaking change restricting who can call it. 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