C# (.NET) Application Configuration: Libraries and Approaches

C# (.NET) Application Configuration: Complete Guide to the Microsoft.Extensions.Configuration Library

Modern C# and .NET applications require flexible settings management: database connection strings, external service URLs, logging parameters, etc. The Microsoft.Extensions.Configuration library provides a powerful and unified mechanism for working with configuration. It supports multiple data sources (JSON, XML, environment variables, command-line arguments) and allows combining them into a single hierarchy.

Key Features of the Library

• Multi-level merging — settings from different sources are combined into a single object.
• Source priority — the last added source overrides previous ones.
• Binding to classes (Options pattern) — automatic population of POCO objects from configuration.
• Dynamic updates — support for re-reading files without restarting the application.
• Security — sensitive data can be moved to environment variables or Azure Key Vault.

Basic Usage: Reading from appsettings.json

Create an appsettings.json file in the project root:

{  "Logging": {    "LogLevel": {      "Default": "Information",      "Microsoft": "Warning"    }  },  "ConnectionStrings": {    "DefaultConnection": "Server=localhost;Database=mydb;User Id=sa;Password=***;"  },  "AppSettings": {    "AppName": "MyApp",    "MaxRetries": 3,    "FeatureFlags": {      "EnableNewUI": true,      "EnableAnalytics": false    }  }}

Load the configuration in code:

using Microsoft.Extensions.Configuration;

var configuration = new ConfigurationBuilder()    .SetBasePath(Directory.GetCurrentDirectory())    .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)    .Build();

// Reading simple valuesstring appName = configuration["AppSettings:AppName"];int maxRetries = int.Parse(configuration["AppSettings:MaxRetries"]);

// Reading nested sectionsvar loggingSection = configuration.GetSection("Logging:LogLevel");string defaultLevel = loggingSection["Default"];

// Reading a connection stringstring connString = configuration.GetConnectionString("DefaultConnection");

Console.WriteLine($"Application: {appName}, Retries: {maxRetries}");

Source Priority and Merging

You can combine multiple sources. For example, first load JSON, then override individual parameters via environment variables:

var config = new ConfigurationBuilder()    .AddJsonFile("appsettings.json")    .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true) // for Development/Production    .AddEnvironmentVariables() // environment variables have higher priority    .AddCommandLine(args)      // command-line arguments have the highest priority    .Build();

Binding to Strongly Typed Classes (Options Pattern)

Create a class for the configuration section:

public class AppSettings{    public string AppName { get; set; }    public int MaxRetries { get; set; }    public FeatureFlags FeatureFlags { get; set; }}

public class FeatureFlags{    public bool EnableNewUI { get; set; }    public bool EnableAnalytics { get; set; }}

Then bind it in Program.cs using the DI container:

// Registering servicesbuilder.Services.Configure<AppSettings>(    builder.Configuration.GetSection("AppSettings"));

// Usage in a class (e.g., in a controller)public class HomeController : Controller{    private readonly AppSettings _settings;        public HomeController(IOpti