using System;

using FluentValidation;

public class Entity

{

    public string StartPage { get; set; }

    public Guid CustomTargetGuid {get;set;}

}

​

public class EntityValidator : AbstractValidator<Entity>

{

    public EntityValidator()

    {

        RuleFor(p => p.CustomTargetGuid)

            .NotEmpty()

            .When(p => p.StartPage == "10F5F529-4858-448E-8B62-1478DAB7232A");

    }

}

​

public class Program

{

    public static void Main()

    {

        var entityValidator = new EntityValidator(); 

        var check = new Entity(){

            StartPage = "10F5F529-4858-448E-8B62-1478DAB7232A",

            CustomTargetGuid = Guid.Empty

        };

        var result = entityValidator.Validate(check);

        Console.WriteLine(result.IsValid);

        foreach (var error in result.Errors)

        {

            Console.WriteLine("   Error: " + error.ErrorMessage);

        }

    }

}