microsoft-extensions-configuration

What this skill does

# Microsoft.Extensions Configuration Patterns

## When to Use This Skill

Use this skill when:
- Binding configuration from appsettings.json to strongly-typed classes
- Validating configuration at application startup (fail fast)
- Implementing complex validation logic for settings
- Designing configuration classes that are testable and maintainable
- Understanding IOptions<T>, IOptionsSnapshot<T>, and IOptionsMonitor<T>

## Why Configuration Validation Matters

**The Problem:** Applications often fail at runtime due to misconfiguration - missing connection strings, invalid URLs, out-of-range values. These failures happen deep in business logic, far from where configuration is loaded, making debugging difficult.

**The Solution:** Validate configuration at startup. If configuration is invalid, the application fails immediately with a clear error message. This is the "fail fast" principle.

```csharp
// BAD: Fails at runtime when someone tries to use the service
public class EmailService
{
    public EmailService(IOptions<SmtpSettings> options)
    {
        var settings = options.Value;
        // Throws NullReferenceException 10 minutes into production
        _client = new SmtpClient(settings.Host, settings.Port);
    }
}

// GOOD: Fails at startup with clear error
// "SmtpSettings validation failed: Host is required"
```

---

## Pattern 1: Basic Options Binding

### Define a Settings Class

```csharp
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;
}
```

### Bind from Configuration

```csharp
// In Program.cs or service registration
builder.Services.AddOptions<SmtpSettings>()
    .BindConfiguration(SmtpSettings.SectionName);

// appsettings.json
{
  "Smtp": {
    "Host": "smtp.example.com",
    "Port": 587,
    "Username": "[email protected]",
    "Password": "secret",
    "UseSsl": true
  }
}
```

### Consume in Services

```csharp
public class EmailService
{
    private readonly SmtpSettings _settings;

    // IOptions<T> - singleton, read once at startup
    public EmailService(IOptions<SmtpSettings> options)
    {
        _settings = options.Value;
    }
}
```

---

## Pattern 2: Data Annotations Validation

For simple validation rules, use Data Annotations:

```csharp
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;

    [EmailAddress(ErrorMessage = "Username must be a valid email address")]
    public string? Username { get; set; }

    public string? Password { get; set; }

    public bool UseSsl { get; set; } = true;
}
```

### Enable Data Annotations Validation

```csharp
builder.Services.AddOptions<SmtpSettings>()
    .BindConfiguration(SmtpSettings.SectionName)
    .ValidateDataAnnotations()  // Enable attribute-based validation
    .ValidateOnStart();         // Validate immediately at startup
```

**Key Point:** `.ValidateOnStart()` is critical. Without it, validation only runs when the options are first accessed, which could be minutes or hours into application runtime.

---

## Pattern 3: IValidateOptions<T> for Complex Validation

Data Annotations work for simple rules, but complex validation requires `IValidateOptions<T>`:

### When to Use IValidateOptions

| Scenario | Data Annotations | IValidateOptions |
|----------|------------------|------------------|
| Required field | ✅ | ✅ |
| Range check | ✅ | ✅ |
| Regex pattern | ✅ | ✅ |
| Cross-property validation | ❌ | ✅ |
| Conditional validation | ❌ | ✅ |
| External service checks | ❌ | ✅ |
| Custom error messages with context | Limited | ✅ |
| Dependency injection in validator | ❌ | ✅ |

### Implementing IValidateOptions

```csharp
using Microsoft.Extensions.Options;

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

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

        // Range validation
        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 result
        return failures.Count > 0
            ? ValidateOptionsResult.Fail(failures)
            : ValidateOptionsResult.Success;
    }
}
```

### Register the Validator

```csharp
builder.Services.AddOptions<SmtpSettings>()
    .BindConfiguration(SmtpSettings.SectionName)
    .ValidateDataAnnotations()  // Run attribute validation first
    .ValidateOnStart();

// Register the custom validator
builder.Services.AddSingleton<IValidateOptions<SmtpSettings>, SmtpSettingsValidator>();
```

**Order matters:** Data Annotations run first, then IValidateOptions validators. All failures are collected and reported together.

---

## Pattern 4: Validators with Dependencies

IValidateOptions validators are resolved from DI, so they can have dependencies:

```csharp
public class DatabaseSettingsValidator : IValidateOptions<DatabaseSettings>
{
    private readonly ILogger<DatabaseSettingsValidator> _logger;
    private readonly IHostEnvironment _environment;

    public DatabaseSettingsValidator(
        ILogger<DatabaseSettingsValidator> logger,
        IHostEnvironment environment)
    {
        _logger = logger;
        _environment = environment;
    }

    public ValidateOptionsResult Validate(string? name, DatabaseSettings options)
    {
        var failures = new List<string>();

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

        // Environment-specific validation
        if (_environment.IsProduction())
        {
            if (options.ConnectionString?.Contains("localhost") == true)
            {
                failures.Add("Production cannot use localhost database");
            }

            if (!options.ConnectionString?.Contains("Encrypt=True") == true)
            {
                _logger.LogWarning("Production database connection should use encryption");
            }
        }

        // Validate connection string format
        if (!string.IsNullOrEmpty(options.ConnectionString))
        {
            try
            {
                var builder = new SqlConnectionStringBuilder(options.ConnectionString);
                if (string.IsNullOrEmpty(builder.DataSource))
                {
                    failures.Add("ConnectionString must specify a Data Source");
                }
            }
            catch (Exception ex)
            {
                failures.Add($"ConnectionString is malformed: {ex.Message}");
            }
        }

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

---

## Pattern 5: Named Options

When you have multiple instances of the same settings type (e.g., multiple database connections):

```csharp
// appsettings.json
{
  "Databases": {
    "Primary": {
      "ConnectionString"