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How does the ASP.NET Core configuration system layer multiple sources (appsettings.json, environment variables, command line) together? What does GetSection and GetValue do differently when reading configuration values in ASP.NET Core? What is the Options pattern, and why is it preferred over directly injecting IConfiguration into services? How do you validate configuration options at application startup to fail fast on misconfiguration? How do environment variables map to nested configuration keys in ASP.NET Core, given environment variables can't contain colons on some platforms? What is user secrets (dotnet user-secrets), and why should it only be used for local development, never production? How does the command-line configuration provider let you override settings via arguments when running an application? How does the order of configuration provider registration affect which value wins when the same key is defined in multiple sources? What is the difference between IOptions<T>, IOptionsSnapshot<T>, and IOptionsMonitor<T>? How does the configuration binder handle nested objects, arrays, and complex types when binding a JSON section to a C# class?
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How does appsettings.{Environment}.json layering work, and how is the current environment determined?

Intermediate

Answer

ASP.NET Core loads appsettings.json first as a base configuration, then layers appsettings.{EnvironmentName}.json on top (where settings in the environment-specific file override matching keys from the base file), with the environment determined by the ASPNETCORE_ENVIRONMENT environment variable (commonly Development, Staging, or Production), letting you maintain environment-specific overrides without duplicating the entire configuration file.
// appsettings.json (base, all environments)
{ "Logging": { "LogLevel": { "Default": "Information" } } }

// appsettings.Development.json (overrides base only in Development)
{ "Logging": { "LogLevel": { "Default": "Debug" } } }

// Set via: ASPNETCORE_ENVIRONMENT=Development
Real-world example A team keeps verbose Debug-level logging enabled only in appsettings.Development.json, while production automatically uses the quieter Information-level default from the base appsettings.json, without any code changes between environments.

Common follow-ups: How do you add a custom environment beyond the standard three?;What happens if ASPNETCORE_ENVIRONMENT isn't set at all?

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