FluentValidation filter for ASP.NET Core Minimal APIs

AddValidationFilterExtension.cs

public static class AddValidationFilterExtension
{
	public static TBuilder AddValidationFilter<TBuilder>(
		this TBuilder builder
	) where TBuilder : IEndpointConventionBuilder
	{
		return builder.AddEndpointFilterFactory((factoryCtx, next) =>
		{
			var sp = factoryCtx.ApplicationServices.GetRequiredService<IServiceProviderIsService>();
			var validatableParameters = factoryCtx.MethodInfo.GetParameters()
				.Select(p => new
				{
					p.Position,
					ValidatorType = typeof(IValidator<>).MakeGenericType(p.ParameterType),
				})
				.Where(p => sp.IsService(p.ValidatorType))
				.ToArray();

			if (!validatableParameters.Any())
				throw new InvalidOperationException($"No validatable parameters on '{factoryCtx.MethodInfo.Name}'.");

			return async ctx =>
			{
				var validationResults = await Task.WhenAll(
					validatableParameters.Select(p =>
					{
						var validator = (IValidator)ctx.HttpContext.RequestServices
							.GetRequiredService(p.ValidatorType);

						var objToValidate = ctx.Arguments[p.Position];

						return validator.ValidateAsync(
							new ValidationContext<object?>(objToValidate),
							ctx.HttpContext.RequestAborted
						);
					})
				);

				if (validationResults.Any(r => !r.IsValid))
				{
					return Results.BadRequest(new
					{
						Errors = validationResults
							.SelectMany(r => r.Errors)
							.Select(e => e.ErrorMessage),
					});
				}

				return await next(ctx);
			};
		});
	}
}

Program.cs

// ...

app.MapGet("/", YourHandler)
	.AddValidationFilter();
  
app.Run();