Configuration Patterns

Implement strongly-typed configuration with validation using Microsoft.Extensions.Options. Bind appsettings.json to POCO classes, validate at startup with Data Annotations or IValidateOptions<T>, and choose the correct options lifetime (IOptions<T>, IOptionsSnapshot<T>, IOptionsMonitor<T>) for each scenario.

Acronyms: DI (Dependency Injection), POCO (Plain Old CLR Object).

When to Use This Skill

• Binding configuration sections from appsettings.json to strongly-typed C# classes
• Validating configuration at application startup to fail fast on misconfiguration
• Implementing complex cross-property validation logic with IValidateOptions
• Choosing between IOptions, IOptionsSnapshot, and IOptionsMonitor lifetimes
• Designing configuration classes that are independently testable without ASP.NET hosting
• Replacing manual IConfiguration string-key access with type-safe Options pattern
• Adding environment-specific validation rules using injected IHostEnvironment

Related Skills

Core Principles

1. Fail Fast at Startup — Validate all configuration before the application serves requests. Why: runtime configuration failures are hard to debug and cause production incidents.
2. Strongly-Typed over String Keys — Bind configuration sections to POCO classes instead of accessing IConfiguration["key"]. Why: compile-time safety and IntelliSense catch typos before runtime.
3. Validate Constraints, Not Just Presence — Use cross-property and conditional validation to enforce business rules. Why: a non-null connection string can still be invalid.
4. Choose Lifetime Intentionally — Select IOptions<T> for static config, IOptionsSnapshot<T> for per-request reload, IOptionsMonitor<T> for background services. Why: wrong lifetime causes stale config or unnecessary overhead.
5. Separate Validation from Settings — Place validation logic in dedicated IValidateOptions<T> classes, not in constructors. Why: validators run at startup and are independently testable.

Values: 基礎と型の追求（Options パターンという「型」を徹底し、どのプロジェクトでも再利用可能な設定基盤を作る）, 温故知新（.NET の設定バインディング原則を正しく理解し、IValidateOptions の進化した検証機能と組み合わせる）

Workflow: Implement Validated Configuration

Step 1: Define a Settings Class

Create a POCO class with a SectionName constant matching the appsettings.json key. Use default values for optional properties. Why: co-locating the section name with the class makes binding discoverable.

public class SmtpSettings
{
    public const string SectionName = "Smtp";

    public string Host { get; set; } = string.Empty;
    public int Port { get; set; } = 587;
    public string? Username { get; set; }
    public string? Password { get; set; }
    public bool UseSsl { get; set; } = true;
}

Values: 基礎と型の追求（命名規則と定数パターンの「型」が、設定バインディングの発見可能性を支える）

Step 2: Bind and Register with Data Annotations

Use BindConfiguration to connect the POCO to a JSON section. Add ValidateDataAnnotations() for simple rules and always call ValidateOnStart(). Why: without ValidateOnStart(), validation only runs when options are first accessed — possibly hours into production.

using System.ComponentModel.DataAnnotations;

public class SmtpSettings
{
    public const string SectionName = "Smtp";

    [Required(ErrorMessage = "SMTP host is required")]
    public string Host { get; set; } = string.Empty;

    [Range(1, 65535, ErrorMessage = "Port must be between 1 and 65535")]
    public int Port { get; set; } = 587;
}

// In Program.cs
builder.Services.AddOptions<SmtpSettings>()
    .BindConfiguration(SmtpSettings.SectionName)
    .ValidateDataAnnotations()
    .ValidateOnStart();

Values: 継続は力（.ValidateOnStart() という地道な一行をコツコツ守ることで、本番障害を未然に防ぐ）

Step 3: Add Complex Validation with IValidateOptions

Implement IValidateOptions<T> for cross-property, conditional, or DI-dependent validation. Return ValidateOptionsResult.Fail(failures) instead of throwing exceptions. Why: returning failures preserves the validation chain and collects all errors at once.

using Microsoft.Extensions.Options;

public class SmtpSettingsValidator : IValidateOptions<SmtpSettings>
{
    public ValidateOptionsResult Validate(string? name, SmtpSettings options)
    {
        var failures = new List<string>();

        if (string.IsNullOrWhiteSpace(options.Host))
        {
            failures.Add("Host is required");
        }

        if (options.Port is < 1 or > 65535)
        {
            failures.Add($"Port {options.Port} is invalid. Must be between 1 and 65535");
        }

        // Cross-property validation
        if (!string.IsNullOrEmpty(options.Username) && string.IsNullOrEmpty(options.Password))
        {
            failures.Add("Password is required when Username is specified");
        }

        // Conditional validation
        if (options.UseSsl && options.Port == 25)
        {
            failures.Add("Port 25 is typically not used with SSL. Consider port 465 or 587");
        }

        return failures.Count > 0
            ? ValidateOptionsResult.Fail(failures)
            : ValidateOptionsResult.Success;
    }
}

Register the validator after options binding:

builder.Services.AddOptions<SmtpSettings>()
    .BindConfiguration(SmtpSettings.SectionName)
    .ValidateDataAnnotations()
    .ValidateOnStart();

// Register custom validator — runs after Data Annotations
builder.Services.AddSingleton<IValidateOptions<SmtpSettings>, SmtpSettingsValidator>();

Values: 成長の複利（バリデーターを独立クラスに分離することで、テストと検証ロジックが同時に成長する構造を作る）

Step 4: Choose the Correct Options Lifetime

Select the interface that matches how your service consumes configuration. Why: wrong lifetime causes stale data in long-running services or unnecessary overhead in singletons.

// ✅ Singleton service — use IOptions<T>
public class EmailService
{
    private readonly SmtpSettings _settings;
    public EmailService(IOptions<SmtpSettings> options)
    {
        _settings = options.Value;
    }
}

// ✅ Background service — use IOptionsMonitor<T>
public class HealthChecker : BackgroundService
{
    private readonly IOptionsMonitor<HealthCheckSettings> _monitor;
    public HealthChecker(IOptionsMonitor<HealthCheckSettings> monitor)
    {
        _monitor = monitor;
        _monitor.OnChange(s => _logger.LogInformation("Settings reloaded"));
    }
}

Values: ニュートラルな視点（ライフタイムの選択基準を明確にし、環境に依存しない判断基準を提供する）

Step 5: Test Validators Independently

Instantiate validators directly in unit tests without ASP.NET hosting. Create settings objects with known values and assert on ValidateOptionsResult. Why: validators are plain classes — no DI container or HTTP pipeline needed.

public class SmtpSettingsValidatorTests
{
    private readonly SmtpSettingsValidator _validator = new();

    [Fact]
    public void Validate_WithValidSettings_ReturnsSuccess()
    {
        var settings = new SmtpSettings
        {
            Host = "smtp.example.com", Port = 587,
            Username = "user@example.com", Password = "secret"
        };

        var result = _validator.Validate(null, settings);
        result.Succeeded.Should().BeTrue();
    }

    [Fact]
    public void Validate_WithUsernameButNoPassword_ReturnsFail()
    {
        var settings = new SmtpSettings
        {
            Host = "smtp.example.com",
            Username = "user@example.com", Password = null
        };

        var result = _validator.Validate(null, settings);
        result.Succeeded.Should().BeFalse();
        result.FailureMessage.Should().Contain("Password is required");
    }
}

Values: 成長の複利（テスト可能な設計が、実装と品質を同時に成長させる）

Good Practices

• ✅ Use ValidateOnStart() on every options registration to fail fast at startup
• ✅ Define SectionName as a const string inside the settings class for co-location
• ✅ Use Data Annotations for simple rules (Required, Range, EmailAddress)
• ✅ Use IValidateOptions<T> for cross-property and conditional validation logic
• ✅ Collect all failures in a List<string> instead of returning on first error
• ✅ Use IOptionsMonitor<T> in background services for live configuration reload
• ✅ Place validators near the settings class for discoverability
• ✅ Accept configuration section names as parameters for reusable extension methods

Common Pitfalls

1. Forgetting ValidateOnStart — Without .ValidateOnStart(), validation only runs when options are first accessed, possibly hours into runtime. Fix: always chain .ValidateOnStart() after validation registration.
2. Throwing in IValidateOptions — Throwing exceptions inside Validate() breaks the validation chain and loses other errors. Fix: return ValidateOptionsResult.Fail(message) instead.
3. Wrong Options Lifetime — Using IOptions<T> in a background service misses configuration changes. Using IOptionsSnapshot<T> in a singleton causes scope errors. Fix: match the interface to service lifetime.
4. Captive Configuration — Injecting IConfiguration directly and accessing config["Smtp:Host"] bypasses all validation and type safety. Fix: use IOptions<SmtpSettings> instead.
5. No Cross-Property Validation — Data Annotations cannot validate relationships between properties (e.g., Username requires Password). Fix: implement IValidateOptions<T> for multi-field rules.

Anti-Patterns

❌ Manual Configuration Access → ✅ Strongly-Typed Options

// ❌ BAD: Bypasses validation, hard to test, no IntelliSense
public class MyService
{
    public MyService(IConfiguration configuration)
    {
        var host = configuration["Smtp:Host"]; // No validation, no type safety
    }
}

// ✅ GOOD: Strongly-typed, validated at startup
public class MyService
{
    public MyService(IOptions<SmtpSettings> options)
    {
        var host = options.Value.Host; // Validated, typed, discoverable
    }
}

❌ Constructor Validation → ✅ Startup Validation

// ❌ BAD: Validation happens at runtime, not at startup
public class MyService
{
    public MyService(IOptions<Settings> options)
    {
        if (string.IsNullOrEmpty(options.Value.Required))
            throw new ArgumentException("Required is missing"); // Too late!
    }
}

// ✅ GOOD: Fails immediately at startup with clear error message
builder.Services.AddOptions<Settings>()
    .ValidateDataAnnotations()
    .ValidateOnStart();

❌ Throwing in Validator → ✅ Returning Failure Result

// ❌ BAD: Throws exception, breaks validation chain
public ValidateOptionsResult Validate(string? name, Settings options)
{
    if (options.Value < 0)
        throw new ArgumentException("Value cannot be negative");
    return ValidateOptionsResult.Success;
}

// ✅ GOOD: Returns failure, all validators run and errors are collected
public ValidateOptionsResult Validate(string? name, Settings options)
{
    if (options.Value < 0)
        return ValidateOptionsResult.Fail("Value cannot be negative");
    return ValidateOptionsResult.Success;
}

Quick Reference

Validation Strategy Decision Table

Options Lifetime Decision Table

Registration Checklist

builder.Services.AddOptions<MySettings>()
    .BindConfiguration(MySettings.SectionName)   // 1. Bind to JSON section
    .ValidateDataAnnotations()                    // 2. Attribute validation
    .ValidateOnStart();                           // 3. Fail fast at startup

// 4. Register complex validator (optional)
builder.Services.AddSingleton<IValidateOptions<MySettings>, MySettingsValidator>();

Resources

• Options Pattern in .NET
• Configuration in ASP.NET Core
• IValidateOptions
• references/detailed-patterns.md — Named Options, Options Lifetime, Post-Configuration, Akka.NET production example, testing validators