---
name: microsoft-extensions-configuration
description: Configuration patterns using Microsoft.Extensions.Configuration. Covers configuration providers, binding, validation, and best practices for .NET applications. Use when setting up configuration in .NET applications, implementing configuration validation with IValidateOptions, or managing settings across different environments.
---

# 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": "user@example.com",
    "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": "Server=primary;..."
    },
    "Replica": {
      "ConnectionString": "Server=replica;..."
    }
  }
}

// Registration
builder.Services.AddOptions<DatabaseSettings>("Primary")
    .BindConfiguration("Databases:Primary")
    .ValidateDataAnnotations()
    .ValidateOnStart();

builder.Services.AddOptions<DatabaseSettings>("Replica")
    .BindConfiguration("Databases:Replica")
    .ValidateDataAnnotations()
    .ValidateOnStart();

// Consumption
public class DataService
{
    private readonly DatabaseSettings _primary;
    private readonly DatabaseSettings _replica;

    public DataService(IOptionsSnapshot<DatabaseSettings> options)
    {
        _primary = options.Get("Primary");
        _replica = options.Get("Replica");
    }
}
```

### Named Options Validator

```csharp
public class DatabaseSettingsValidator : IValidateOptions<DatabaseSettings>
{
    public ValidateOptionsResult Validate(string? name, DatabaseSettings options)
    {
        var failures = new List<string>();
        var prefix = string.IsNullOrEmpty(name) ? "" : $"[{name}] ";

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

        // Name-specific validation
        if (name == "Primary" && options.ReadOnly)
        {
            failures.Add("Primary database cannot be read-only");
        }

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

---

## Pattern 6: Options Lifetime

Understanding the three options interfaces:

| Interface | Lifetime | Reloads on Change | Use Case |
|-----------|----------|-------------------|----------|
| `IOptions<T>` | Singleton | No | Static config, read once |
| `IOptionsSnapshot<T>` | Scoped | Yes (per request) | Web apps needing fresh config |
| `IOptionsMonitor<T>` | Singleton | Yes (with callback) | Background services, real-time updates |

### IOptionsMonitor for Background Services

```csharp
public class BackgroundWorker : BackgroundService
{
    private readonly IOptionsMonitor<WorkerSettings> _optionsMonitor;
    private WorkerSettings _currentSettings;

    public BackgroundWorker(IOptionsMonitor<WorkerSettings> optionsMonitor)
    {
        _optionsMonitor = optionsMonitor;
        _currentSettings = optionsMonitor.CurrentValue;

        // Subscribe to configuration changes
        _optionsMonitor.OnChange(settings =>
        {
            _currentSettings = settings;
            _logger.LogInformation("Worker settings updated: Interval={Interval}",
                settings.PollingInterval);
        });
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            await DoWorkAsync();
            await Task.Delay(_currentSettings.PollingInterval, stoppingToken);
        }
    }
}
```

---

## Pattern 7: Post-Configuration

Modify options after binding but before validation:

```csharp
builder.Services.AddOptions<ApiSettings>()
    .BindConfiguration("Api")
    .PostConfigure(options =>
    {
        // Ensure BaseUrl ends with /
        if (!string.IsNullOrEmpty(options.BaseUrl) && !options.BaseUrl.EndsWith('/'))
        {
            options.BaseUrl += '/';
        }

        // Set defaults based on environment
        options.Timeout ??= TimeSpan.FromSeconds(30);
    })
    .ValidateDataAnnotations()
    .ValidateOnStart();
```

### PostConfigure with Dependencies

```csharp
builder.Services.AddOptions<ApiSettings>()
    .BindConfiguration("Api")
    .PostConfigure<IHostEnvironment>((options, env) =>
    {
        if (env.IsDevelopment())
        {
            options.Timeout = TimeSpan.FromMinutes(5); // Longer timeout for debugging
        }
    });
```

---

## Pattern 8: Complete Example - Production Settings Class

```csharp
using System.ComponentModel.DataAnnotations;
using Microsoft.Extensions.Options;

public class AkkaSettings
{
    public const string SectionName = "AkkaSettings";

    [Required]
    public string ActorSystemName { get; set; } = "MySystem";

    public AkkaExecutionMode ExecutionMode { get; set; } = AkkaExecutionMode.LocalTest;

    public bool LogConfigOnStart { get; set; } = false;

    public RemoteOptions RemoteOptions { get; set; } = new();

    public ClusterOptions ClusterOptions { get; set; } = new();

    public ClusterBootstrapOptions ClusterBootstrapOptions { get; set; } = new();
}

public enum AkkaExecutionMode
{
    LocalTest,   // No remoting, no clustering
    Clustered    // Full cluster with sharding, distributed pub/sub
}

public class AkkaSettingsValidator : IValidateOptions<AkkaSettings>
{
    private readonly IHostEnvironment _environment;

    public AkkaSettingsValidator(IHostEnvironment environment)
    {
        _environment = environment;
    }

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

        // Basic validation
        if (string.IsNullOrWhiteSpace(options.ActorSystemName))
        {
            failures.Add("ActorSystemName is required");
        }

        // Mode-specific validation
        if (options.ExecutionMode == AkkaExecutionMode.Clustered)
        {
            ValidateClusteredMode(options, failures);
        }

        // Environment-specific validation
        if (_environment.IsProduction() && options.ExecutionMode == AkkaExecutionMode.LocalTest)
        {
            failures.Add("LocalTest execution mode is not allowed in production");
        }

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

    private void ValidateClusteredMode(AkkaSettings options, List<string> failures)
    {
        if (string.IsNullOrEmpty(options.RemoteOptions.PublicHostName))
        {
            failures.Add("RemoteOptions.PublicHostName is required in Clustered mode");
        }

        if (options.RemoteOptions.Port is null or < 0)
        {
            failures.Add("RemoteOptions.Port must be >= 0 in Clustered mode");
        }

        if (options.ClusterBootstrapOptions.Enabled)
        {
            ValidateClusterBootstrap(options.ClusterBootstrapOptions, failures);
        }
        else if (options.ClusterOptions.SeedNodes?.Length == 0)
        {
            failures.Add("Either ClusterBootstrap must be enabled or SeedNodes must be specified");
        }
    }

    private void ValidateClusterBootstrap(ClusterBootstrapOptions options, List<string> failures)
    {
        if (string.IsNullOrEmpty(options.ServiceName))
        {
            failures.Add("ClusterBootstrapOptions.ServiceName is required");
        }

        if (options.RequiredContactPointsNr <= 0)
        {
            failures.Add("ClusterBootstrapOptions.RequiredContactPointsNr must be > 0");
        }

        switch (options.DiscoveryMethod)
        {
            case DiscoveryMethod.Config:
                if (options.ConfigServiceEndpoints?.Length == 0)
                {
                    failures.Add("ConfigServiceEndpoints required for Config discovery");
                }
                break;

            case DiscoveryMethod.AzureTableStorage:
                if (options.AzureDiscoveryOptions == null)
                {
                    failures.Add("AzureDiscoveryOptions required for Azure discovery");
                }
                break;
        }
    }
}

// Registration
builder.Services.AddOptions<AkkaSettings>()
    .BindConfiguration(AkkaSettings.SectionName)
    .ValidateDataAnnotations()
    .ValidateOnStart();

builder.Services.AddSingleton<IValidateOptions<AkkaSettings>, AkkaSettingsValidator>();
```

---

## Anti-Patterns to Avoid

### 1. Manual Configuration Access

```csharp
// BAD: Bypasses validation, hard to test
public class MyService
{
    public MyService(IConfiguration configuration)
    {
        var host = configuration["Smtp:Host"]; // No validation!
    }
}

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

### 2. Validation in Constructor

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

// GOOD: Validation at startup
builder.Services.AddOptions<Settings>()
    .ValidateDataAnnotations()
    .ValidateOnStart();
```

### 3. Forgetting ValidateOnStart

```csharp
// BAD: Validation only runs when first accessed
builder.Services.AddOptions<Settings>()
    .ValidateDataAnnotations(); // Missing ValidateOnStart!

// GOOD: Fails immediately if invalid
builder.Services.AddOptions<Settings>()
    .ValidateDataAnnotations()
    .ValidateOnStart();
```

### 4. Throwing in IValidateOptions

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

    return ValidateOptionsResult.Success;
}

// GOOD: Return failure result
public ValidateOptionsResult Validate(string? name, Settings options)
{
    if (options.Value < 0)
        return ValidateOptionsResult.Fail("Value cannot be negative");

    return ValidateOptionsResult.Success;
}
```

---

## Testing Configuration Validators

```csharp
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_WithMissingHost_ReturnsFail()
    {
        var settings = new SmtpSettings { Host = "" };

        var result = _validator.Validate(null, settings);

        result.Succeeded.Should().BeFalse();
        result.FailureMessage.Should().Contain("Host is required");
    }

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

        var result = _validator.Validate(null, settings);

        result.Succeeded.Should().BeFalse();
        result.FailureMessage.Should().Contain("Password is required");
    }
}
```

---

## Summary

| Principle | Implementation |
|-----------|----------------|
| Fail fast | `.ValidateOnStart()` |
| Strongly-typed | Bind to POCO classes |
| Simple validation | Data Annotations |
| Complex validation | `IValidateOptions<T>` |
| Cross-property rules | `IValidateOptions<T>` |
| Environment-aware | Inject `IHostEnvironment` |
| Testable | Validators are plain classes |