ExpressValidator.Extensions.ValidationOnStart 0.1.0-preview

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

✨ Key Features

• Fail-fast validation on startup with ValidateOnStart
• Startup validation - catch configuration issues immediately when your app starts
• Fluent API - express validation rules in a clean, readable way
• Expressive syntax for property-level rules
• Seamless integration with IValidateOptions<TOptions> and the Microsoft Options pipeline
• Configurable validation mode - stop on first error or continue
• Detailed error reporting - with property names
• No AbstractValidator required – define rules inline, directly in your service registration

⚡ Quick Start

1. Define Your Options Class

public class MyOptions
{
	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);

//// All validation in one place, no extra classes needed
builder.Services
	.AddOptionsWithExpressValidation<MyOptions>(
	(eb) =>
		eb.AddProperty(o => o.Option1)
		.WithValidation(o => o.GreaterThan(10))
		.AddProperty(o => o.Option2)
		.WithValidation(o => o.GreaterThan(50)),
			"MyOptions"); // Configuration section name
	.AddOptionsWithExpressValidation<MyOptions2>(
		(eb) =>
		eb.AddProperty(o => o.Option3)
		.WithValidation(o => o.GreaterThan(5))
		.AddProperty(o => o.Option4)
		.WithValidation(o => o.GreaterThan(7)), 
			"MyOptions2");

3. Handle Validation Errors at Startup

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

try
{
	var app = builder.Build();
	app.MapGet("/", () => "Hello!");
	await app.RunAsync();
}
// Handles the standard exception thrown when a single set of options (e.g., MyOptions) fails validation.
catch (OptionsValidationException ove)
{
	foreach (var failure in ove.Failures)
	{
		logger.LogCritical(ove, failure);
	}
}
// Handles the case where validation errors from multiple option types (e.g., both MyOptions and MyOptions2)
catch (AggregateException ae) when (ae.InnerExceptions.All(e => e is OptionsValidationException))
{
	foreach (var failure in ae.Flatten().InnerExceptions)
	{
		logger.LogCritical(ae, "A critical exception occurred: {Message}", failure.Message);
	}
}
catch (Exception ex)
{
	logger.LogCritical(ex, "An unhandled exception occurred during application startup.");
}

🧩 How It Works

AddOptionsWithExpressValidation<TOptions>

Registers options, binds configuration, attaches express validation, and enables ValidateOnStart.

services.AddOptionsWithExpressValidation<MyOptions>(
	eb => eb.AddProperty(o => o.Option1).WithValidation(o => o.GreaterThan(10)),
	"MyOptions");

ExpressOptionsValidator<TOptions>

Implements IValidateOptions<TOptions> and runs the express validator against your options.

• Converts validation results into ValidateOptionsResult
• Produces per-property error messages
• Supports short-circuit or collect-all failure modes

IValidateOptions<TOptions>

The standard Microsoft interface for options validation.

This ensures fail-fast startup validation and protects against misconfiguration.

🏆 Sample

See samples folder for concrete example.