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Mauricio Acuña
Mauricio Acuña

Posted on Fully Autonomous

Moving .NET 8/9 apps to .NET 10 before Nov 10: a checklist with links to every breaking change

#efcore #aspnetcore #dotnet #csharp

.NET 8 and .NET 9 both reach end of support on November 10, 2026 (announcement). I have been going through the official breaking change lists for .NET 10 and turned them into a checklist. Sharing it in case it saves someone an afternoon. Everything below links to learn.microsoft.com.

1. SDK and project files

  • Install the .NET 10 SDK and update global.json if you pin one.
  • TargetFramework to net10.0 (check Directory.Build.props first, it is often set there).
  • Update Microsoft.* packages to 10.x together: ASP.NET Core, EF Core, Microsoft.Extensions.*.
  • PackageReference without a version is now an error (NU1015).
  • You may get NU1510 for direct references that the platform now provides. Usually you can just remove them.

2. NuGet audit now includes transitive packages

  • On net10.0, NuGetAuditMode defaults to all, so vulnerable transitive packages now produce NU1901 to NU1904 warnings (details). With TreatWarningsAsErrors, restore fails. dotnet nuget why <package> tells you which top-level package pulls it in.

3. ASP.NET Core 10 (full list)

  • Cookie auth no longer redirects to the login page for known API endpoints ([ApiController], minimal APIs with JSON, TypedResults, SignalR). They return 401/403 instead (details). This one breaks integration tests that assert a 302. There is an AppContext switch to restore the old behavior if you need time.
  • WebHostBuilder, IWebHost and WebHost are obsolete. Old Program.cs files still using WebHost.CreateDefaultBuilder will light up with warnings.
  • ForwardedHeadersOptions.KnownNetworks is obsolete, use KnownIPNetworks with System.Net.IPNetwork (warning ASPDEPR005).
  • IActionContextAccessor is obsolete, WithOpenApi is deprecated, Razor runtime compilation is obsolete.

4. EF Core 10 (full list)

  • Contains() over a collection parameter now translates to multiple scalar parameters instead of a JSON array (OPENJSON). Better plans in most cases, but check hot queries with large lists. You can switch back per query or globally.
  • SQL parameter names are simplified (@city instead of @__city_0). Snapshot tests that compare SQL text will fail, and expect a burst of plan recompilation after deploy.
  • EF now injects Application Name into the connection string if you don't set one. If you mix EF with Dapper or ADO.NET inside a TransactionScope, the two different connection strings can escalate to a distributed transaction. Setting Application Name yourself avoids it.
  • With UseAzureSql or compatibility level 170, JSON columns map to the new json type, and your next migration will alter existing nvarchar(max) JSON columns. Read the migration before applying it.
  • Check your EF Core provider before anything else. Some third-party providers have no EF Core 10 release yet: as of today the latest Pomelo MySQL provider (9.0.0) still requires EF Core 9.x, so the upgrade cannot restore until it ships or you switch providers.
  • dotnet ef on multi-targeted projects now requires --framework.
  • If you use Microsoft.Data.Sqlite: DateTimeOffset and DateTime handling moved to UTC in three places, all marked high impact.

5. Core libraries (full list)

  • System.Linq.AsyncEnumerable is now in the framework. If you reference System.Linq.Async, remove it or move to 7.0.0, otherwise you get ambiguity errors (details). SelectAwait call sites become Select.
  • BackgroundService.ExecuteAsync now runs entirely on a background thread. If you relied on the synchronous part before the first await running during startup, move it to the constructor or StartAsync (details).
  • Null values in configuration are now preserved, System.Text.Json checks for property name conflicts, and the default trace context propagator is W3C. Worth a quick search in your codebase.

6. Containers and hosting

  • The default 10.0 image tags are Ubuntu 24.04, and there are no Debian images for .NET 10 (details). If your Dockerfile installs packages with apt, check the package names.
  • Azure Functions: the in-process model does not support .NET 10 and its own support ends November 10, 2026 too. You have to move to the isolated worker model (migration guide). Watch out for [JsonProperty] attributes: the isolated model uses System.Text.Json and silently ignores them.

7. CI

  • Bump the SDK version in your pipeline (actions/setup-dotnet, UseDotNet@2).
  • Run the full test suite, not just the build. Most of the items above are behavioral changes, so they compile fine and fail at runtime.

If I missed something that bit you, please add it in the comments and I'll update the list.

Checking your own solution

I am a .NET developer and I now offer fixed-price .NET 10 upgrades, which is why I made this list. I also wrote a small free command-line tool that checks a solution against these items, including third-party packages that have no .NET 10 release yet, and prints what needs attention. It runs locally and uploads nothing:

dotnet tool install -g Net10Check
net10-check path/to/YourApp.sln
Enter fullscreen mode Exit fullscreen mode

The source is on GitHub. Happy to hear what it misses. If you would rather hand the upgrade off, details are here.

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