FluentValidation: Check if one of two fields are empty

Question

I have this model

public class Person
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName         


        

Answer 1

Finally, this worked for me. I wanted to validate three properties where at least one is required. It returns an error message only once.

RuleFor(p => p).Cascade(CascadeMode.StopOnFirstFailure)
            .Must(p => !string.IsNullOrWhiteSpace(p.FirstName))
            .When(p => p.Id == 0 && string.IsNullOrWhiteSpace(p.LastName)).WithMessage("At least one is required (Id, FirstName, LastName).")
            .Must(p => !string.IsNullOrWhiteSpace(p.LastName))
            .When(p => p.Id == 0 && string.IsNullOrWhiteSpace(p.FirstName)).WithMessage("At least one is required (Id, FirstName, LastName).")
            .Must(p => p.Id != 0)
            .When(p => string.IsNullOrWhiteSpace(p.FirstName) && string.IsNullOrWhiteSpace(p.LastName)).WithMessage("At least one is required (Id, FirstName, LastName).");