configuration

---
name: configuration
description: >
  Configuration patterns for .NET 10 applications. Covers the Options pattern,
  IOptionsSnapshot vs IOptions, secrets management, and environment-based
  configuration.
  Load this skill when setting up application configuration, managing secrets,
  binding configuration sections, or when the user mentions "configuration",
  "appsettings", "Options pattern", "IOptions", "IOptionsSnapshot", "secrets",
  "user secrets", "environment variables", "connection string", or "config binding".
---

# Configuration

## Core Principles

1. **Options pattern always** — Never read `IConfiguration` directly in services. Bind configuration sections to strongly-typed classes with validation.
2. **Validate on startup** — Use `ValidateDataAnnotations()` and `ValidateOnStart()` to catch misconfiguration before the first request.
3. **Secrets never in source** — Use user secrets in development, Azure Key Vault or environment variables in production. Never commit secrets to git.
4. **Configuration layering** — `appsettings.json` → `appsettings.{Environment}.json` → environment variables → user secrets. Later sources override earlier ones.

## Patterns

### Options Pattern

```csharp
// Options class with validation attributes
public 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 validation
builder.Services.AddOptions<DatabaseOptions>()
    .BindConfiguration(DatabaseOptions.SectionName)
    .ValidateDataAnnotations()
    .ValidateOnStart(); // Fails at startup if configuration is invalid
```

```json
// appsettings.json
{
  "Database": {
    "ConnectionString": "",
    "MaxRetryCount": 3,
    "CommandTimeoutSeconds": 30
  }
}
```

### Injecting Options

```csharp
// IOptions<T> — singleton, read once at startup, doesn't change
public 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 changes
public class BackgroundWorker(IOptionsMonitor<WorkerOptions> options)
{
    public void DoWork()
    {
        var current = options.CurrentValue; // Always latest
    }
}
```

### Custom Validation (Complex Rules)

```csharp
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();
```

### Azure Key Vault (Production)

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

### Configuration for Multiple Environments

```csharp
// Named options — different config per named instance
builder.Services.AddOptions<SmtpOptions>("internal")
    .BindConfiguration("Smtp:Internal");
builder.Services.AddOptions<SmtpOptions>("customer")
    .BindConfiguration("Smtp:Customer");

// Usage
public class EmailService(IOptionsSnapshot<SmtpOptions> options)
{
    public async Task SendInternalEmail(string to, string body)
    {
        var smtp = options.Get("internal");
        // ...
    }
}
```

## Anti-patterns

### Don't Read IConfiguration Directly

```csharp
// BAD — stringly-typed, no validation, hard to test
public class OrderService(IConfiguration config)
{
    public void Process()
    {
        var timeout = int.Parse(config["Database:CommandTimeout"]!);
    }
}

// GOOD — strongly-typed options
public class OrderService(IOptions<DatabaseOptions> options)
{
    public void Process()
    {
        var timeout = options.Value.CommandTimeoutSeconds;
    }
}
```

### Don't Put Secrets in appsettings.json

```json
// BAD — committed to source control
{
  "Jwt": { "Key": "super-secret-key" },
  "Database": { "ConnectionString": "Server=prod;Password=secret" }
}

// GOOD — appsettings.json has defaults/structure only
{
  "Jwt": { "Key": "", "Issuer": "myapp", "Audience": "myapp" },
  "Database": { "ConnectionString": "" }
}
// Secrets provided via user-secrets (dev) or env vars / Key Vault (prod)
```

### Don't Skip Startup Validation

```csharp
// BAD — misconfiguration discovered at runtime
builder.Services.Configure<JwtOptions>(builder.Configuration.GetSection("Jwt"));

// GOOD — fail fast at startup
builder.Services.AddOptions<JwtOptions>()
    .BindConfiguration("Jwt")
    .ValidateDataAnnotations()
    .ValidateOnStart();
```

## Decision Guide

| Scenario | Recommendation |
|----------|---------------|
| Binding config to class | Options pattern with `BindConfiguration` |
| Simple, immutable config | `IOptions<T>` |
| Config that changes per request | `IOptionsSnapshot<T>` |
| Background service watching config | `IOptionsMonitor<T>` |
| Development secrets | `dotnet user-secrets` |
| Production secrets | Azure Key Vault or environment variables |
| Validating config | `ValidateDataAnnotations()` + `ValidateOnStart()` |
| Multiple configs of same type | Named options with `IOptionsSnapshot<T>.Get(name)` |