Fox.ConfigKit

Lightweight .NET configuration validation library with fail-fast startup validation

Fox.ConfigKit validates IOptions<T> configurations at application startup, catching errors before they cause runtime failures.

Installation

dotnet add package Fox.ConfigKit

NuGet Package Manager:

Install-Package Fox.ConfigKit

PackageReference:

<PackageReference Include="Fox.ConfigKit" Version="1.0.0" />

Core Concepts

Fail-Fast Validation

Configuration errors are caught at application startup, not at runtime:

builder.Services.AddConfigKit<DatabaseConfig>("Database") .NotEmpty(c => c.ConnectionString, "Connection string is required") .InRange(c => c.MaxPoolSize, 1, 1000, "Max pool size must be between 1 and 1000") .ValidateOnStartup(); // Application won't start if configuration is invalid!

Fluent API

Type-safe, chainable validation rules:

builder.Services.AddConfigKit<ApiConfig>("Api") .NotEmpty(c => c.BaseUrl, "API URL is required") .UrlReachable(c => c.BaseUrl, message: "API is not reachable") .InRange(c => c.TimeoutSeconds, 5, 300, "Timeout must be between 5 and 300 seconds") .ValidateOnStartup();

IOptions Integration

Seamlessly integrates with Microsoft.Extensions.Options:

public class MyService { private readonly ApiConfig config; public MyService(IOptions<ApiConfig> config) { this.config = config.Value; // Already validated at startup! } }

Key Validation Rules

String Validation

builder.Services.AddConfigKit<AppConfig>("App") .NotEmpty(c => c.Name, "Application name is required") .NotNull(c => c.Version, "Version is required") .MatchesPattern(c => c.Version, @"^\d+\.\d+\.\d+$", "Version must be X.Y.Z format") .ValidateOnStartup();

Comparable Value Validation

All comparison methods support any IComparable<T> type (int, decimal, DateTime, TimeSpan, etc.):

Integer validation:

builder.Services.AddConfigKit<ApiConfig>("Api") .GreaterThan(c => c.Port, 1024, "Port must be > 1024") .Maximum(c => c.Port, 65535, "Port must be <= 65535") .InRange(c => c.MaxRetries, 0, 10, "Max retries must be 0-10") .ValidateOnStartup();

Decimal validation:

builder.Services.AddConfigKit<ProductConfig>("Product") .Minimum(c => c.Price, 0.01m, "Price must be at least $0.01") .Maximum(c => c.Discount, 0.5m, "Discount cannot exceed 50%") .ValidateOnStartup();

DateTime validation:

builder.Services.AddConfigKit<CampaignConfig>("Campaign") .Minimum(c => c.StartDate, DateTime.Today, "Campaign must start today or later") .LessThan(c => c.EndDate, new DateTime(2025, 12, 31), "Campaign must end before 2025") .ValidateOnStartup();

TimeSpan validation:

builder.Services.AddConfigKit<ApiConfig>("Api") .GreaterThan(c => c.Timeout, TimeSpan.Zero, "Timeout must be positive") .Maximum(c => c.Timeout, TimeSpan.FromMinutes(5), "Timeout cannot exceed 5 minutes") .ValidateOnStartup();

Available methods:

• GreaterThan(selector, min) - Exclusive boundary (>)
• LessThan(selector, max) - Exclusive boundary (<)
• Minimum(selector, min) - Inclusive boundary (>=)
• Maximum(selector, max) - Inclusive boundary (<=)
• InRange(selector, min, max) - Inclusive boundaries (>=, <=)

File System Validation

builder.Services.AddConfigKit<LoggingConfig>("Logging") .NotEmpty(c => c.LogDirectory, "Log directory is required") .DirectoryExists(c => c.LogDirectory, message: "Log directory does not exist") .ValidateOnStartup();

Network Validation

builder.Services.AddConfigKit<ExternalApiConfig>("ExternalApi") .NotEmpty(c => c.BaseUrl, "API URL is required") .UrlReachable(c => c.BaseUrl, timeout: TimeSpan.FromSeconds(10), message: "API is not reachable") .ValidateOnStartup();

Security Validation

builder.Services.AddConfigKit<SecurityConfig>("Security") .NotEmpty(c => c.ApiKey, "API key is required") .NoPlainTextSecrets(c => c.ApiKey, "API key appears to be a plain-text secret") .ValidateOnStartup();

Common Scenarios

Database Configuration

public sealed class DatabaseConfig { public string ConnectionString { get; set; } = string.Empty; public int MaxPoolSize { get; set; } public int CommandTimeoutSeconds { get; set; } } builder.Services.AddConfigKit<DatabaseConfig>("Database") .NotEmpty(c => c.ConnectionString, "Connection string is required") .InRange(c => c.MaxPoolSize, 1, 1000, "Max pool size must be between 1 and 1000") .InRange(c => c.CommandTimeoutSeconds, 1, 600, "Timeout must be between 1 and 600 seconds") .ValidateOnStartup();

Conditional Validation

Apply validation rules based on environment or configuration values:

builder.Services.AddConfigKit<SecurityConfig>("Security") .NotEmpty(c => c.Environment, "Environment is required") .When(c => c.Environment == "Production", b => { // Rules only apply in production b.NotEmpty(c => c.CertificatePath, "Certificate is required in production") .FileExists(c => c.CertificatePath, message: "Certificate file not found"); }) .ValidateOnStartup();

Cross-Property Validation

builder.Services.AddConfigKit<DatabaseConfig>("Database") .NotEmpty(c => c.ConnectionString, "Connection string is required") .When(c => c.RequireSsl, b => { b.MatchesPattern(c => c.ConnectionString, "Encrypt=True|Encrypt=true", "SSL required but connection string does not specify Encrypt=True"); }) .ValidateOnStartup();

When to Use Fox.ConfigKit

Use Fox.ConfigKit when:

• You want to catch configuration errors at startup, not runtime
• You need to validate IOptions<T> configuration classes
• You want type-safe, fluent validation rules
• You need environment-specific validation rules
• You want to verify external dependencies (URLs, files) at startup
• You need to enforce configuration constraints across environments

Don't use Fox.ConfigKit when:

• You need runtime validation of user input (use FluentValidation, DataAnnotations)
• You need complex, multi-step validation workflows
• You want to validate DTOs or request models (not configuration)
• You need async validation beyond startup checks

Related Packages

• Fox.ConfigKit.ResultKit - Integration with Result pattern for functional configuration validation
• Fox.ResultKit - Lightweight Result pattern library for Railway Oriented Programming

Full Documentation

For comprehensive documentation, advanced scenarios, and API reference, see the GitHub repository.

License

This project is licensed under the MIT License - see the LICENSE.txt file for details.

Author

Károly Akácz

• GitHub: @akikari
• Repository: Fox.ConfigKit

Acknowledgments

Inspired by the following concepts:

• Railway Oriented Programming by Scott Wlaschin - Fail-fast error handling principles
• Options Pattern - Microsoft's recommended approach for configuration
• Fail-Fast Philosophy - Catch errors early to prevent cascading failures