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Why does copying only the .csproj file before the rest of the source code in a Dockerfile improve build caching?

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Answer

Docker caches each instruction's layer based on its inputs; by copying only the .csproj (which changes infrequently) and running dotnet restore before copying the rest of the source code (which changes on every commit), Docker can reuse the cached restore layer across builds as long as dependencies haven't changed, avoiding a full package restore on every single build even when only application code (not dependencies) changed -- significantly speeding up iterative CI builds.
# Optimized layer caching:
COPY *.csproj .
RUN dotnet restore          # cached as long as .csproj is unchanged
COPY . .                    # this layer invalidates on every code change
RUN dotnet publish -c Release -o /app/publish  # but restore step above stays cached
Real-world example A CI pipeline's Docker build time drops from 3 minutes to 45 seconds after restructuring the Dockerfile to copy project files separately, since NuGet restore (the slowest step) now hits the Docker layer cache on every commit that doesn't touch dependencies.

Common follow-ups: How does this pattern extend to multi-project solutions with several .csproj files?;What invalidates the restore layer's cache even without dependency changes?

.NET CLI SDK & Project Structure (csproj);CI/CD Publishing & Deployment
All 16 Docker & Containerization questions