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How do environment variables map to nested configuration keys in ASP.NET Core, given environment variables can't contain colons on some platforms?

Intermediate

Answer

The environment variable configuration provider uses double underscores (__) as the hierarchy separator instead of colons, since some shells and operating systems don't support colons in environment variable names -- ASP.NET Core automatically translates ConnectionStrings__DefaultConnection into the equivalent "ConnectionStrings:DefaultConnection" configuration key internally.
# Setting a nested configuration value via environment variable
export ConnectionStrings__DefaultConnection="Server=prod;Database=MyApp;"

# Equivalent to this in appsettings.json:
# { "ConnectionStrings": { "DefaultConnection": "Server=prod;Database=MyApp;" } }
Real-world example A Kubernetes deployment manifest sets Logging__LogLevel__Default=Warning as a container environment variable, correctly overriding the nested Logging:LogLevel:Default key from the base appsettings.json without any code changes.

Common follow-ups: Does this convention apply consistently across Windows, Linux, and macOS?;How do you configure array values via environment variables?

Docker & Containerization;CI/CD Publishing & Deployment
All 15 Configuration & Options questions