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What is a NuGet lock file (packages.lock.json), and what problem does it solve for reproducible builds?

Intermediate

Answer

A lock file records the exact resolved version of every direct and transitive dependency at restore time, ensuring that subsequent restores (across different machines, CI runs, or dates) produce bit-for-bit identical dependency trees rather than potentially resolving to newer compatible versions that happen to be available -- critical for reproducible builds and catching unexpected dependency changes in code review.
<PropertyGroup>
  <RestorePackagesWithLockFile>true</RestorePackagesWithLockFile>
</PropertyGroup>

# Generates packages.lock.json, committed to source control
# dotnet restore --locked-mode  fails the build if resolution would differ from the lock file
Real-world example A CI pipeline runs `dotnet restore --locked-mode` to guarantee that a build today resolves the exact same dependency versions as it did when the lock file was last committed, catching any unexpected drift immediately.

Common follow-ups: How do you update the lock file when you intentionally want a new version?;How does this compare to npm's package-lock.json conceptually?

CI/CD Publishing & Deployment;.NET CLI SDK & Project Structure (csproj)
All 15 Assemblies & NuGet questions