.NET 10 LTS / C# 14: current state and migration

As of: 2026-10

Current versions and support

• .NET 10 (LTS) was released 2025-11-11. The latest patch is 10.0.12 (2026-09-08), and support ends 2028-11-14.
• .NET 8 (LTS) and .NET 9 (STS) both reach end of support on 2026-11-10. STS releases now get 2 years of support.
• .NET 11 is in preview: 11.0.0-rc.1 was published 2026-09-08. Do not target it in production yet.

C# 14 (ships with .NET 10)

• field keyword in property accessors: public string Name { get; set => field = value.Trim(); }
• extension blocks support extension properties and static extension members:

public static class Ext {
    extension(string s) { public bool IsBlank => string.IsNullOrWhiteSpace(s); }
}

• Null-conditional assignment: customer?.Order = order;
• nameof(List<>), implicit Span<T>/ReadOnlySpan<T> conversions, and lambda parameter modifiers without types.
• Partial constructors and events, and user-defined compound assignment and ++/-- operators.
• Gotcha: C# 14 span overload resolution can choose different overloads than before (a listed breaking change).

SDK changes an older model will get wrong

• File-based apps: dotnet run app.cs with #:package, #:project and #:property directives and shebang support. dotnet publish app.cs works, and file-based apps target Native AOT by default (#:property PublishAot=false to opt out). Double quotes are not allowed in directives.
• dotnet tool exec <tool> and dnx <tool> run a tool once without installing it.
• Noun-first commands: dotnet package add|list|remove and dotnet reference add|list|remove. The old forms still work.
• dotnet new sln creates .slnx by default.
• --interactive defaults to true in interactive terminals. In CI, pass --interactive false if prompts hang.
• dotnet restore audits transitive packages, so expect new NuGet audit warnings. A PackageReference without a version is an error.
• dotnet test supports Microsoft.Testing.Platform via global.json ("test": {"runner": "Microsoft.Testing.Platform"}).
• Default container images use Ubuntu. Console apps can dotnet publish /t:PublishContainer without EnableSdkContainerSupport.

ASP.NET Core 10 breaking changes

• Cookie login redirects are disabled for known API endpoints. They return 401/403 instead of a 302 to the login page.
• WithOpenApi is deprecated.
• WebHostBuilder, IWebHost and WebHost are obsolete. Use WebApplication.CreateBuilder.
• Razor runtime compilation is obsolete.
• IActionContextAccessor is obsolete.
• IPNetwork / ForwardedHeadersOptions.KnownNetworks are obsolete.
• New in ASP.NET Core 10: passkey support in Identity, and Blazor WebAssembly preloading.

Runtime and library behavior changes

• BackgroundService runs all of ExecuteAsync as a Task, including the synchronous prefix.
• Null configuration values are preserved.
• The default trace context propagator is W3C.
• System.Linq.AsyncEnumerable is in the core libraries, which can conflict with the System.Linq.Async package.
• The runtime no longer installs default SIGTERM handlers.
• OpenSSL 1.1.1+ is required on Unix.

What to do now

1. Move .NET 8 and .NET 9 apps to .NET 10 before 2026-11-10: <TargetFramework>net10.0</TargetFramework>.
2. Pin the SDK in global.json.
3. Review the ASP.NET Core auth redirect change and remove the System.Linq.Async package if it conflicts.
4. Re-check NuGet audit output.

Sources

• https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-core
• https://learn.microsoft.com/en-us/dotnet/core/whats-new/dotnet-10/overview
• https://learn.microsoft.com/en-us/dotnet/core/whats-new/dotnet-10/sdk
• https://learn.microsoft.com/en-us/dotnet/core/compatibility/10
• https://learn.microsoft.com/en-us/aspnet/core/breaking-changes/10/overview
• https://github.com/dotnet/core/releases