MSBuild "MSB3644: reference assemblies were not found"

MSBuild could not find the reference assemblies (the targeting pack) for the .NET Framework version your project targets. On Linux/CI runners these packs are frequently absent.

What this error means

Build fails with MSB3644 naming a .NETFramework,Version=vX.Y whose reference assemblies are missing, suggesting you install the targeting pack or retarget. Common when building classic .NET Framework projects on hosted runners.

MSBuild output

error MSB3644: The reference assemblies for .NETFramework,Version=v4.7.2 were not found.
To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework
version or retarget your application.

Common causes

Targeting pack not installed

The reference assemblies for the targeted .NET Framework version are a separate developer pack that is not present on the runner.

Building .NET Framework on Linux

Classic .NETFramework targeting packs are Windows-oriented. A Linux runner without Mono/reference-assembly packages cannot resolve them.

How to fix it

Build .NET Framework on a Windows runner

Classic Framework projects build most reliably on windows-latest where the targeting packs ship.

.github/workflows/ci.yml

jobs:
  build:
    runs-on: windows-latest
    steps:
      - uses: actions/checkout@v4
      - run: dotnet build -c Release

Install the targeting pack

On Windows runners, install the developer pack for the framework version, or reference the targeting-pack NuGet package.

.csproj

<PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies.net472" Version="1.0.3">
  <PrivateAssets>all</PrivateAssets>
</PackageReference>

Or retarget to a modern framework

If the project can move, retarget <TargetFramework> to net8.0 and build cross-platform.
Confirm dependencies support the new target before switching.
Keep Framework-only projects on Windows runners until they are migrated.

How to prevent it

Build classic .NET Framework projects on Windows runners.
Add the Microsoft.NETFramework.ReferenceAssemblies package for Framework targets.
Migrate to net8.0+ where possible for cross-platform CI.