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Configuration & appsettings.json in ASP.NET Core

💡 Concept Name

Configuration & appsettings.json in ASP.NET Core

📘 Quick Intro

ASP.NET Core uses a flexible configuration system that supports hierarchical key-value pairs from multiple sources like appsettings.json, environment variables, and command-line arguments.

🧠 Analogy / Short Story

Think of configuration like a personal assistant managing your preferences: one assistant reads your written notes (appsettings.json), another listens to your voice (environment variables), and another reads sticky notes you place (CLI). Whichever speaks last, wins.

🔧 Technical Explanation

ASP.NET Core builds configuration using the ConfigurationBuilder. It reads from sources in order and stores them as key-value pairs, which you can inject using the IConfiguration interface.

  • appsettings.json: Default file for settings.
  • appsettings.{Environment}.json: Overrides for specific environments.
  • Environment Variables: Useful in Docker/Kubernetes.
  • CLI Args: Highest priority if used.

🎯 Purpose & Use Case

  • ✅ Store app configuration values
  • ✅ Use different settings per environment
  • ✅ Read connection strings and feature toggles
  • ✅ Secure secrets via external providers
  • ✅ Simplify config management in DevOps pipelines

💻 Real Code Example

Sample appsettings.json:


{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning"
    }
  },
  "ConnectionStrings": {
    "DefaultConnection": "Server=.;Database=AppDb;Trusted_Connection=True;"
  }
}

Reading values in C# (Program.cs or controller):


var config = builder.Configuration;
var connectionString = config.GetConnectionString("DefaultConnection");
var logLevel = config["Logging:LogLevel:Default"];

🎯 Interview Q&A

  • Q: What is IConfiguration in ASP.NET Core?
    A: It provides a way to read config values from multiple sources like JSON, ENV, CLI, etc.
  • Q: How does ASP.NET Core load appsettings.json?
    A: It is added by default to the configuration pipeline in CreateDefaultBuilder.
  • Q: How to access strongly typed config in controllers?
    A: Bind a POCO class and inject it using IOptions<T>.
  • Q: What happens if multiple config sources have the same key?
    A: Last source added overrides previous ones (later wins).
  • Q: Can you change config without restarting the app?
    A: Yes, use IOptionsSnapshot for scoped services.
  • Q: How to secure secrets instead of putting in appsettings?
    A: Use environment variables or Secret Manager.
  • Q: Is appsettings.Production.json automatically used?
    A: Yes, if ASPNETCORE_ENVIRONMENT is set to "Production".
  • Q: What is ConfigureServices method used for?
    A: It registers services and configuration logic into DI.
  • Q: How to bind nested sections in config?
    A: Use GetSection("Section") multiple times or bind whole object.
  • Q: What is the use of AddJsonFile method?
    A: It explicitly adds JSON config files to the configuration builder.

📝 MCQs

📝 MCQs

Q1. What interface is used to access configuration in ASP.NET Core?

  • IConfig
  • IOptions
  • IConfiguration
  • IAppSettings

Q2. What file format is used for default config in ASP.NET Core?

  • XML
  • YAML
  • JSON
  • INI

Q3. How to access settings in a strongly typed way?

  • IValue
  • IAppConfig
  • IOptions
  • IBind

Q4. What happens if ENV and appsettings.json have the same key?

  • JSON wins
  • ENV value wins
  • Both fail
  • It throws error

Q5. Which method loads appsettings.json by default?

  • ConfigureBuilder
  • AddConfig
  • CreateDefaultBuilder
  • UseDefaults

Q6. Which config source is used for command line arguments?

  • EnvConfig
  • CommandLine
  • RuntimeArgs
  • ConsoleReader

Q7. How can secrets be securely stored?

  • appsettings.json
  • public.json
  • Environment.cs
  • Secret Manager

Q8. Which method binds config section to POCO?

  • UseConfig()
  • GetValue()
  • GetSection().Bind()
  • MapConfig()

Q9. What is the use of IOptionsSnapshot?

  • Singleton access
  • Scoped config updates
  • Logging
  • Debugging

Q10. How to use different config for prod & dev?

  • Multiple apps
  • Rewrite settings
  • Environment-specific JSON files
  • Change app.cs

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