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NuGet - Lock files

A feature I wasn’t aware of that it existed for NuGet is the concept of a lock file. This allows to lock the project dependency graph guaranteeing that the same packages are restored every time.

I was thinking that NuGet guarantees this by default but there are some situations where NuGet will not come up with the same dependency graph on every restore.

Some examples:

  1. nuget.config mismatch: Different package sources can be used in different nuget.config files resulting in different packages to be used

  2. Intermediate versions: A missing version of the package, matching PackageReference version requirements, is published

  3. Package deletion: Though nuget.org does not allow package deletions, not all package repositories have this constraint. Deletion of a package version results in NuGet finding the best match when it cannot resolve to the deleted version.

  4. Floating versions: When you use floating versions like <PackageReference Include="My.Sample.Lib" Version="4.*"/>, you might get different versions after new versions are available.

By introducing a lock file the problems above can be mitigated. A lock file contains  the package hash. If you download a corrupted or malicious package, NuGet can detect it and fail. It also guarantees a deterministic restore so always the same packages will be restored.

Remark: As nice bonus, you also get a performance improvement as nuget no longer needs to compute the dependency graph again when restoring packages.

To enable this feature you need to update your csproj file and add the following line:

By default NuGet does a quick check to see if there were any changes in the package dependencies as mentioned in the project file (or dependent projects’ files) and if there were no changes, it just restores the packages mentioned in the lock file.

If NuGet detects a change in the defined dependencies as enumerated in the project file(s), it re-evaluates the package graph and updates the lock file to reflect the new package graph applicable for the project. Of course this is a scenario that you don’t want on your build server in your CI/CD builds.

You can disable this by setting the RestoreLockedMode to true when running a CI build:

More information:

Enable repeatable package restores using a lock file - The NuGet Blog (microsoft.com)

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