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.NET Conf 2026 roundup, day 1: .NET 11, C# 15 and what to do this week

.NET 11 shipped on the first day of .NET Conf 2026. What changes in your code, what is worth testing and what to do this week.

By 2 min read
  • dotnet
  • csharp
  • dotnetconf
  • dotnet11

.NET 11 shipped today, on the first day of .NET Conf 2026. My summary of what changes in your code and in your planning 👇

⚠️ .NET 8 and .NET 9 reached end of support today.

On 8 or 9? You need to move now. For production, the safest path is .NET 10 (LTS, until 11/2028).

On 10? No need to rush. 11 is STS (2 years): test it on a branch and decide based on what it brings.

🧩 C# 15: union types

Visual summary: C# 15: union types. A value is exactly one of N types, and the compiler checks whether the switch covers all cases, without the "_ =>" at the end.

A value is exactly one of N types, and the compiler checks whether the switch covers all cases, without the "_ =>" at the end. Added a new type? Every switch that does not handle it stops compiling. Anyone who uses the Result Pattern or OneOf will recognize this: now it is part of the language.

🔒 C# 15: closed hierarchies

Visual summary: C# 15: closed hierarchies. A closed class can only be inherited within its own assembly, so pattern matching is also exhaustive.

A closed class can only be inherited within its own assembly, so pattern matching is also exhaustive.

Practical rule: union to group types that are not related; closed when the hierarchy is yours.

⚡ Performance and CLI

Visual summary: Performance and CLI. Native AOT host for simple CLI commands, file-based apps with multiple files (#:include) and dotnet watch integrated with Aspire.

Native AOT host for simple CLI commands, file-based apps with multiple files (#:include) and dotnet watch integrated with Aspire.

🌐 ASP.NET Core and Blazor

Visual summary: ASP.NET Core and Blazor. Asynchronous validation with DataAnnotations (await on the database without blocking a thread), OpenAPI 3.2 and Blazor AI components (experimental, AG-UI protocol).

Asynchronous validation with DataAnnotations (await on the database without blocking a thread), OpenAPI 3.2 and Blazor AI components (experimental, AG-UI protocol).

🗄️ EF Core 11

Visual summary: EF Core 11. Vector indexes and approximate vector search in SQL Server (experimental).

Vector indexes and approximate vector search in SQL Server (experimental). For anyone building RAG in .NET, this is similarity search inside the database you already run in production.

🤖 AI and agents, the keynote theme

Visual summary: AI and agents, the keynote theme. Microsoft.Extensions.AI to talk to models, the MCP server template in the SDK (dotnet new mcpserver), Microsoft Agent Framework and .NET Skills, the .NET team's guides for coding agents.

Microsoft.Extensions.AI to talk to models, the MCP server template in the SDK (dotnet new mcpserver), Microsoft Agent Framework and .NET Skills, the .NET team's guides for coding agents.

📱 MAUI

Visual summary: MAUI. CoreCLR becomes the default runtime on all platforms, replacing Mono.

CoreCLR becomes the default runtime on all platforms, replacing Mono.

✅ What I would do this week

Visual summary: What I would do this week. List what still runs on .NET 8 or 9 and plan the move to 10.
  1. List what still runs on .NET 8 or 9 and plan the move to 10.
  2. Install the SDK 11 side by side and test union types wherever you use a base class or OneOf today.
  3. Run dotnet new mcpserver and plug it into the agent you already use.

Tomorrow (11/11) there is the Visual Studio keynote, the MCP C# SDK and EF Core 11. Free and live at dotnetconf.net.

Which of these updates goes into your project first?

#dotnet #csharp #dotnetconf #dotnet11

Original source: dotnetconf.net

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