When it is not valid, the ValidationMessage For is not displayed

Question

I am using custom validation within Blazor, the validation is performed correctly, however when it is not valid the ValidationMessage For is not displayed, I tried adding ValidationSummary and here it does appear ErrorMessage. I hope someone can support me

 public class Domicilio
    {
        public int Id { get; set; }
        public DateTimeOffset FechaRegistro { get; set; }
        [Required(ErrorMessage ="Favor de capturar la calle")]
        public string Calle { get; set; }
        [Required(ErrorMessage = "Favor de capturar el número exterior")]
        public string NumeroExt { get; set; }
        public string NumeroInt { get; set; }
        [Range(1,int.MaxValue, ErrorMessage="Favor de seleccionar la colonia")]
        public int CodigoPostalId { get; set; }
        public CatCodigosPostales CodigoPostal { get; set; }
        [OtroCPValidation]
        public string OtroCodigoPostal { get; set; }
        public string OtraColonia { get; set; }
        public string OtraCiudad { get; set; }
        public string OtroEstado { get; set; }
        [Range(1,int.MaxValue,ErrorMessage="Favor seleccione el país")]
        public int PaisId { get; set; }
        public CatPaises Pais { get; set; }
        public string Observaciones { get; set; }
    }

public class OtroCPValidation : ValidationAttribute
    {
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            var domicilio = (Domicilio)validationContext.ObjectInstance;
            if (domicilio.Pais.Pais != "México" && domicilio.OtroCodigoPostal == null)
            {
                return new ValidationResult($"Captura el código postal extranjero");
            }
                
            else
                return ValidationResult.Success;
        }
    }

<EditForm Model="Domicilio" OnValidSubmit="OnValidSubmit" style="height:75vh" class="row justify-content-center align-items-center">
    <DataAnnotationsValidator />
<div class="form-group">
                            <label>Código postal</label>
                            <div>
                                <SfMaskedTextBox Placeholder="Código postal" Mask="[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]" @bind-Value="@Domicilio.OtroCodigoPostal"></SfMaskedTextBox>
                                <ValidationMessage For="@(() => Domicilio.OtroCodigoPostal)" />
                            </div>
                        </div>
</EditForm>

Answer 1

I was able to solve the problem I had, I was missing an overload in the return of the IsValid method, I leave the return as it should be, I just needed that change.

return new ValidationResult(ErrorMessage, new[] { validationContext.MemberName});

Answer 2

Two things to check:

1. Do a simple validator to always return an error.

using System.ComponentModel.DataAnnotations;

namespace StackOverflow.Answers
{
    public class AlwaysInvalidValidation : ValidationAttribute
    {
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            return new ValidationResult($"Error Message");
        }
    }
}

2. Test with the basic Blazor Input controls.

3. Refactor the logic so you return an error unless the value passes the logic test.

I've tested a version of your code and it works.