using System;

using FluentValidation;

using System.Linq;

​

public class PersonDetailsViewModel

{

  public bool? HasDetails { get; set; }

  public BasicDetailsViewModel? BasicDetails { get; set; }

}

public class BasicDetailsViewModel

{

  public string? FirstName { get; set; }

  public string? Country { get; set; }

}

public class PersonDetailsViewModelValidator : AbstractValidator<PersonDetailsViewModel>

{

  public PersonDetailsViewModelValidator()

  {

    RuleFor(model => model.BasicDetails)

      .SetValidator(new BasicDetailsViewModelValidator())

      .When(model => model.HasDetails == true);

    RuleFor(model => model.BasicDetails)

      .SetValidator(new DifferentValidator())

      .When(model => model.HasDetails == false || model.HasDetails == null);

  }

}

public class BasicDetailsViewModelValidator : AbstractValidator<BasicDetailsViewModel?>

{

  public BasicDetailsViewModelValidator()

  {

​

    RuleFor(model => model.FirstName)

        .NotEmpty().WithMessage("Name cannot be empty");

​

    RuleFor(model => model.Country)

        .NotEmpty().WithMessage("Country cannot be empty");

  }

}

public class DifferentValidator : AbstractValidator<BasicDetailsViewModel?>

{

  public DifferentValidator()

  {

    RuleFor(model => model.FirstName)

        .Must(name => name == null || name.Length <= 10).WithMessage("First name must be 10 characters or less.");

​

    RuleFor(model => model.Country)

        .Must(country => country == null || country.All(char.IsUpper)).WithMessage("Country must be in uppercase.");

  }

}

​

public class Program

{

    public static void Main()

    {

        var fail1 = new PersonDetailsViewModel { HasDetails = false, BasicDetails = new BasicDetailsViewModel { FirstName = "LongFirstNameThatFails", Country = "UK" } };

        var fail2 = new PersonDetailsViewModel { HasDetails = false, BasicDetails = new BasicDetailsViewModel { FirstName = "John", Country = "lowercasecountry" } };

        var fail3 = new PersonDetailsViewModel { HasDetails = true, BasicDetails = new BasicDetailsViewModel { FirstName = "", Country = "" } };

        var fail4 = new PersonDetailsViewModel { HasDetails = false, BasicDetails = new BasicDetailsViewModel { FirstName = "LongFirstNameThatFails", Country = "lowercasecountry" } };

        var pass1 = new PersonDetailsViewModel { HasDetails = false, BasicDetails = new BasicDetailsViewModel { FirstName = "John", Country = "UK" } };

        var pass2 = new PersonDetailsViewModel { HasDetails = false, BasicDetails = null };

​

        var validator = new PersonDetailsViewModelValidator();

​

        PrintValidationResult(validator.Validate(fail1), "fail1:");

        PrintValidationResult(validator.Validate(fail2), "fail2:");

        PrintValidationResult(validator.Validate(fail3), "fail3:");

        PrintValidationResult(validator.Validate(fail4), "fail4:");

        PrintValidationResult(validator.Validate(pass1), "pass1:");

        PrintValidationResult(validator.Validate(pass2), "pass2:");

    }

​

    public static void PrintValidationResult(FluentValidation.Results.ValidationResult result,string objectName)

    {

        Console.WriteLine(objectName);

        if (result.IsValid)

        {

            Console.WriteLine("✅ Validation successful");

        }

        else

        {

            foreach (var error in result.Errors)

            {

                Console.WriteLine("❌ " + error.ErrorMessage);

            }

        }

        Console.WriteLine();

    }

}

​