Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using FluentValidation;

namespace ConsoleApplication11
{
    public class Entity
    {
        public string Type { get; set; }
    }

    public class EntityValidator : AbstractValidator<Entity>
    {
        public EntityValidator()
        {

            this.RuleFor(a => a.Type)
                .NotEmpty()
                .WithMessage("Type cannot be null or empty");

        }
    }

    internal class Program
    {
        private static void Main(string[] args)
        {
            Entity entity = null;


            var validator = new EntityValidator();


            validator.Validate(entity);
        }
    }

}