Configuration

// Options class with validation attributespublic class DatabaseOptions{    public const string SectionName = "Database";
    [Required]    public required string ConnectionString { get; init; }
    [Range(1, 100)]    public int MaxRetryCount { get; init; } = 3;
    [Range(1, 60)]    public int CommandTimeoutSeconds { get; init; } = 30;}
// Registration with validationbuilder.Services.AddOptions<DatabaseOptions>()    .BindConfiguration(DatabaseOptions.SectionName)    .ValidateDataAnnotations()    .ValidateOnStart(); // Fails at startup if configuration is invalid

// IOptions<T> — singleton, read once at startup, doesn't changepublic class OrderService(IOptions<DatabaseOptions> options){    private readonly DatabaseOptions _db = options.Value;}
// IOptionsSnapshot<T> — scoped, re-reads per request (for reloadable config)public class OrderService(IOptionsSnapshot<DatabaseOptions> options){    private readonly DatabaseOptions _db = options.Value;}
// IOptionsMonitor<T> — singleton, actively watches for changespublic class BackgroundWorker(IOptionsMonitor<WorkerOptions> options){    public void DoWork()    {        var current = options.CurrentValue; // Always latest    }}

builder.Services.AddOptions<JwtOptions>()    .BindConfiguration("Jwt")    .Validate(options =>    {        if (string.IsNullOrEmpty(options.Key) || options.Key.Length < 32)            return false;        if (options.ExpirationMinutes <= 0)            return false;        return true;    }, "JWT key must be at least 32 characters and expiration must be positive")    .ValidateOnStart();

// Program.cs — add Key Vault as a configuration sourceif (builder.Environment.IsProduction()){    var keyVaultUri = new Uri(builder.Configuration["KeyVault:Uri"]!);    builder.Configuration.AddAzureKeyVault(keyVaultUri, new DefaultAzureCredential());}

// Named options — different config per named instancebuilder.Services.AddOptions<SmtpOptions>("internal")    .BindConfiguration("Smtp:Internal");builder.Services.AddOptions<SmtpOptions>("customer")    .BindConfiguration("Smtp:Customer");
// Usagepublic class EmailService(IOptionsSnapshot<SmtpOptions> options){    public async Task SendInternalEmail(string to, string body)    {        var smtp = options.Get("internal");        // ...    }}

// BAD — stringly-typed, no validation, hard to testpublic class OrderService(IConfiguration config){    public void Process()    {        var timeout = int.Parse(config["Database:CommandTimeout"]!);    }}
// GOOD — strongly-typed optionspublic class OrderService(IOptions<DatabaseOptions> options){    public void Process()    {        var timeout = options.Value.CommandTimeoutSeconds;    }}

// BAD — misconfiguration discovered at runtimebuilder.Services.Configure<JwtOptions>(builder.Configuration.GetSection("Jwt"));
// GOOD — fail fast at startupbuilder.Services.AddOptions<JwtOptions>()    .BindConfiguration("Jwt")    .ValidateDataAnnotations()    .ValidateOnStart();