A lightweight library that brings expressive, fail-fast validation to .NET configuration options using the power of FluentValidation via the ExpressValidator library.

✨ Key Features

• Fail-fast validation on startup with the AddOptionsWithExpressValidation<MyOptions> method
• Validation failures are converted to a standard ValidateOptionsResult with detailed messages
• Detailed error reporting - OptionsValidationException or AggregateException can be handled at startup
• Configurable validation: stop on the first error or continue validating an option using ExpressValidator.OnFirstPropertyValidatorFailed.
• Zero boilerplate - no need to create separate AbstractValidator classes
• Fluent API - for property-level rules
• Uses IValidateOptions<TOptions> under the hood

⚡ Quick Start

// 1. Define Your Options Classes
public class MyOptions1
{
	public int Option1 { get; set; }
	public int Option2 { get; set; }
}

public class MyOptions2
{
	public int Option3 { get; set; }
	public int Option4 { get; set; }
}

// 2. Configure Validation in Program.cs
var builder = WebApplication.CreateBuilder(args);

builder.Services
	.AddOptionsWithExpressValidation<MyOptions1>(
	(eb) =>
			// Select the options property to validate
			eb.AddProperty(o => o.Option1)
			// Define FluentValidation rules
			.WithValidation(o => o.GreaterThan(10))
			.AddProperty(o => o.Option2)
			.WithValidation(o => o.GreaterThan(20))
	// Configuration section name
	, "MyOptions1"
	// Fail fast when validation fails (optional; default behavior is to collect all errors)
	, ExpressValidator.OnFirstPropertyValidatorFailed.Break)

	.AddOptionsWithExpressValidation<MyOptions2>(
	(eb) =>
			eb.AddProperty(o => o.Option3)
			.WithValidation(o => o.GreaterThan(30))
			.AddProperty(o => o.Option4)
			.WithValidation(o => o.GreaterThan(40))
	, "MyOptions2");

var loggerFactory = LoggerFactory.Create(lb => lb.AddConsole());
var logger = loggerFactory.CreateLogger<Program>();

 // 3. Handle Validation Errors
try
{
	var app = builder.Build();
	// This endpoint returns a message when all options are valid.
	app.MapGet("/",
		(IOptions<MyOptions1> options1, IOptions<MyOptions2> options2) => "The option values " +
			$"{options1.Value.Option1}, " +
			$"{options1.Value.Option2}, " +
			$"{options2.Value.Option3}, " +
			$"and {options2.Value.Option4} are correct!");

	await app.RunAsync();
}
// Single options type validation failure.
catch (OptionsValidationException ove)
{
	foreach (var failure in ove.Failures)
	{
		logger.LogCritical(
			"Options validation failure: {Failure}",
			failure
		);
	}

	logger.LogCritical(ove, "Options validation exception thrown");
}
// Multiple options types validation failures (e.g., both MyOptions1 and MyOptions2)
catch (AggregateException ae) when (ae.InnerExceptions.All(e => e is OptionsValidationException))
{
	foreach (var failure in ae
			.Flatten()
			.InnerExceptions
			.Cast<OptionsValidationException>()
			.SelectMany(ex => ex.Failures))
	{
		logger.LogCritical(
			"Options validation failure: {Failure}",
			failure
		);
	}

	logger.LogCritical(ae, "AggregateException thrown");
}
catch (Exception ex)
{
	logger.LogCritical(ex, "An unhandled exception occurred during application startup.");
}

In the appsettings.json

{
...
"MyOptions1": {
  "Option1": 9,
  "Option2": 19
},
"MyOptions2": {
  "Option3": 29,
  "Option4": 39
}
...
}

🧩 How It Works

AddOptionsWithExpressValidation<TOptions>

• Registers options
• Attaches express validation, and enables ValidateOnStart
• Binds configuration values using the provided section name

ExpressOptionsValidator<TOptions>

• Implements IValidateOptions<TOptions>
• Runs the express validator against your options.
• Converts validation results into ValidateOptionsResult
• Produces per-property error messages

🏆 Sample

See samples folder for concrete example.