gistfile1.cs

using System;
using FluentValidation;
using Xunit;

namespace Tests
{
    public class UnitTest1
    {
        [Theory]
        [InlineData("ben",11)]
        [InlineData("be",11)]
        [InlineData("ben",9)]
        [InlineData("be",9)]
        public void TestValidation(string name, int age)
        {
            var parent = new Parent(name, new Child(age));
            var validator = new ParentValidator();
            validator.ValidateAndThrow(parent);
        }

        [Theory]
        [InlineData("VALID")]
        [InlineData("No")]
        public void TestNullChild(string name)
        {   
            var parent = new Parent(name, null);
            var validator = new ParentValidator();
            validator.ValidateAndThrow(parent);
        }
    }

    public class Parent
    {
        public Parent(string name, Child child)
        {
            Name = name;
            Child = child;
        }
        public string Name { get; set; }
        public Child Child { get; set; }
    }

    public class Child
    {
        public Child(int age)
        {
            Age = age;
        }
        public int Age { get; set; }
    }

    public class ChildValidator : AbstractValidator<Child>
    {
        public ChildValidator()
        {
            RuleFor(x => x.Age)
                .GreaterThan(10)
                .WithMessage("Age is too young");
            
        }
    }

    public class ParentValidator : AbstractValidator<Parent>
    {
        public ParentValidator()
        {
            RuleFor(x => x.Name)
                .MinimumLength(3);
            RuleFor(x=>x.Child)
                .NotNull()
                .WithErrorCode("not null code")
                .WithMessage("not null message")
                .SetValidator(new ChildValidator());
        }
    }
}