BlazorEditForms/Model/Employee.cs

using FluentValidation;

namespace BlazorEditForms.Model;

public class Employee
{
    public string Code { get; set; } = string.Empty;
    public string Firstname { get; set; } = string.Empty;
    public string Lastname { get; set; } = string.Empty;
    public IList<PhoneNumber> PhoneNumbers { get; set; } = [];
}

public class EmployeeValidator : AbstractValidator<Employee>
{
    public EmployeeValidator()
    {
        RuleFor(x => x.Code).NotEmpty().MinimumLength(3);
        RuleForEach(x => x.PhoneNumbers).SetValidator(new PhoneNumberValidator());
        RuleFor(x => x.Firstname).MustAsync(async (value, ct) =>
        {
            await Task.Delay(500);
            return value == "Foo";
        });
    }
}