All 15 Configuration & Options Pattern questions
More in Configuration & Options Pattern
How does ASP.NET Core's configuration system layer multiple sources together, and what is the default provider precedence? What is the Options pattern, and why is it preferred over injecting IConfiguration directly and reading string keys throughout an application? How does appsettings.{Environment}.json layering work, and how does ASP.NET Core determine the current environment? How does named options support configuring multiple distinct instances of the same options class? What is user secrets (dotnet user-secrets), and why should it be used only for local development, never production? How do you use IHostEnvironment/IWebHostEnvironment to conditionally configure services based on the current environment? How does the command-line configuration provider let you override settings via arguments when running an application, and what's its precedence? What is the difference between IOptions<T>, IOptionsSnapshot<T>, and IOptionsMonitor<T>, and which is safe to inject into a singleton? How do you validate configuration options at application startup to fail fast on misconfiguration, using ValidateDataAnnotations and ValidateOnStart? How does configuration hot-reload work with reloadOnChange for appsettings.json, and what are its limitations?
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ASP.NET Core › Configuration & Options Pattern

How do environment variables map to nested configuration keys, given some shells don't support colons in variable names?

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?

Hosting Models: Kestrel IIS & Reverse Proxies;CI/CD Publishing & Deployment
All 15 Configuration & Options Pattern questions