dotnet "The framework Microsoft.NETCore.App was not found"

A built app tried to run but the .NET runtime version it targets is not installed. The host lists the frameworks it does have and exits, because it cannot run on a runtime that is absent.

What this error means

Running the app (or a tool, or dotnet test) fails with "The framework Microsoft.NETCore.App, version x was not found", listing the installed runtimes. Building may have worked; running does not.

Terminal

You must install or update .NET to run this application.

App: /app/MyApp.dll
Framework: 'Microsoft.NETCore.App', version '8.0.0' (x64)

The following frameworks were found:
  6.0.27 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]

Common causes

The target runtime is not installed

The app targets a runtime version (e.g. 8.0) but the runner or container only has an older one. Building with the SDK does not guarantee the matching runtime exists at run time.

Runtime image older than the build target

A multi-stage Docker build compiled against 8.0 but the final stage uses a 6.0 runtime image, so the published app has no matching framework to run on.

How to fix it

Install the matching runtime

Add the runtime version the app targets to the runner.

.github/workflows/ci.yml

- uses: actions/setup-dotnet@v4
  with:
    dotnet-version: '8.0.x'   # installs the 8.0 runtime + SDK

Align the runtime image to the build target

In Docker, make the final stage’s runtime image match the TargetFramework you built against.

Dockerfile

FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS runtime
WORKDIR /app
COPY --from=build /app/publish .
ENTRYPOINT ["dotnet", "MyApp.dll"]

Or publish self-contained

A self-contained publish bundles the runtime so no framework needs to be installed on the host.