Last modified: Oct 12, 2026

Upgrading the app to .NET 10

How to upgrade a v8 app from .NET 8 to .NET 10.

When you upgrade the app to a new major version, such as v9, it also gets a newer version of .NET. If you need to change the .NET version before then, you can move a v8 app to .NET 10 as described below. This may be relevant when the .NET version the app uses no longer gets security updates from Microsoft. You can find the dates in Microsoft’s .NET support policy.

The app must use Altinn.App 8.12.9 or later. That is the first v8 version that supports .NET 10.

Upgrade the app

You make the changes in the app’s repository.

  1. Open App/App.csproj and update Altinn.App.Api and Altinn.App.Core to the latest 8.x version. You can find the versions on NuGet.
  2. Set TargetFramework to net10.0 in the same file. If the app has a test project, make the same change there.
  3. Delete the lines for packages that Altinn.App already includes, such as Swashbuckle.AspNetCore, to avoid error NU1605 (“Detected package downgrade”).
  4. Open Dockerfile and replace 8.0 with 10.0 in both FROM lines. If the version is followed by @sha256: and a long code, delete that part. If you want to keep such a code, use the one for .NET 10 and update it every month, or the app will not get security updates.
  5. Build the app, for example with dotnet build App/App.csproj.
  6. Publish the app to TT02 and test the whole process before you publish it to production.

This is how App/App.csproj looks after the changes:

<PropertyGroup>
  <TargetFramework>net10.0</TargetFramework>
</PropertyGroup>
<ItemGroup>
  <PackageReference Include="Altinn.App.Api" Version="8.12.11" />
  <PackageReference Include="Altinn.App.Core" Version="8.12.11" />
</ItemGroup>

This is how the FROM lines in Dockerfile look:

FROM mcr.microsoft.com/dotnet/sdk:10.0-alpine AS build
FROM mcr.microsoft.com/dotnet/aspnet:10.0-alpine AS final

If the app has a lot of custom code, read Microsoft’s lists of breaking changes in .NET 9 and .NET 10.

New warnings after the upgrade

The build may show new warnings. They do not stop the build:

  • NU1510: the app references a package that is now part of .NET 10, such as Microsoft.Extensions.Logging.Debug. You can delete the line.
  • CS0618: the code uses a method that newer v8 versions mark as obsolete. The method still works, but replace it with the one the warning suggests when you have the chance.