Example of a custom AsyncPredicateValidator using Nito.AsyncEx

MainWindow.xaml.cs

namespace WpfApplication1
{
	using System.Threading;
	using System.Threading.Tasks;
	using System.Windows;
	using FluentValidation;
	using FluentValidation.Validators;
	using Nito.AsyncEx;

	public class TestModel {
		public string Name { get; set; }
	}

	public class TestModelValidator : AbstractValidator<TestModel> {
		public TestModelValidator() {
			RuleFor(m => m.Name)
				.MustAsync(async (s, token) => {
					await Task.Yield(); // Here we will avoid a deadlock in a single threaded context by using our custom predicate validator
					return true;
				});
		}
	}

	public class AsyncPredicateValidatorEx : AsyncPredicateValidator {
		public AsyncPredicateValidatorEx(AsyncPredicate predicate) : base(predicate) {
		}

		protected override bool IsValid(PropertyValidatorContext context) {
			return AsyncContext.Run(async () => await IsValidAsync(context, new CancellationToken()));
		}
	}

	public partial class MainWindow : Window {
		public MainWindow() {
			InitializeComponent();

			// Setting our custom async predicate validator that uses Nito.AsyncEx AsyncContext to avoid deadlocks
			ValidatorOptions.AsyncValidatorFactory = predicate => new AsyncPredicateValidatorEx(predicate);
			var model = new TestModel {Name = "test"};
			var result = new TestModelValidator().Validate(model);
		}
	}
}