How to translate "Errormessage" from a "Custom Attribute"

Question

I created a Custom Attribute which is valid only if a property CPF is a valid CPF, but when locating the application I noticed that my Custom Attribute were not having their messages located by the Framework as opposed to Data Attribute Required that has your message located correctly:

Example of the use of attributes being that Required is correctly located

[Required(ErrorMessage = "CPF Requerido")]
[CPF(ErrorMessage = "CPF Inválido")]
public string CPF { get; set; }

Setting location in Startup.Cs file

services
    .AddMvc()
    .SetCompatibilityVersion(CompatibilityVersion.Version_2_1)
    .AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix)
    .AddDataAnnotationsLocalization(options =>
    {
        options.DataAnnotationLocalizerProvider = (type, factory) =>
        {
             return factory.Create(typeof(SharedResource));
        };
    });

Custom validation class:

public class CPFAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext context)
    {
        //Omitido por não fazer parte do context
    }
}

Versions:

Microsoft.AspNetCore.App (2.1.1)

Microsoft.NETCore.App (2.1)

Example in English:

Example in Portuguese:

Answer 1

You need to add the settings in the constructor, or in the attribute configuration, in the constructor:

//base.ErrorMessageResourceName = "/*nome da chave*/" ou
base.ErrorMessageResourceType = typeof(/*ResourceName*/);

or in the attribute:

[CPFAttribute(ErrorMessageResourceType = typeof(/*ResourceName*/).

This part is only the server validation, you need to send the message with ValidationResult(ErrorMessage) which is what you typed in the configuration ErrorMessage, example:

public class CPFAttribute : ValidationAttribute, IClientModelValidator
{
    public CPFAttribute()
    {
        base.ErrorMessageResourceName = "/*nome da chave*/"
        base.ErrorMessageResourceType = typeof(/*ResourceName*/);
    }
    protected override ValidationResult IsValid(object value, ValidationContext context)
    {
        if (cpf for válido) //se for válido
        {
             return ValidationResult.Success;
        }   
        //se for inválido passe o valor da mensagem
        return new ValidationResult(ErrorMessage);
    }
    public void AddValidation(ClientModelValidationContext context)
    {
        if (context == null)
        {
            throw new ArgumentNullException(nameof(context));
        }
        context.Attributes.Add("data-val","true");
        context.Attributes.Add("data-val-cpf", 
                 !string.IsNullOrEmpty(ErrorMessage)) 
                 ? ErrorMessage 
                 : "CPF invalid");
    }
}

I also went a little further and put the implementation of the client side part that now needs to also create the function on jQuery.Validation to work as well:

(function ($) {
    $.validator.addMethod('data-val-cpf',
        function (value, element, params) {                        
            cpf = value.replace(/[^\d]+/g,'');  
            if(cpf == '') return false;                 
            if (cpf.length != 11 || 
                cpf == "00000000000" || 
                cpf == "11111111111" || 
                cpf == "22222222222" || 
                cpf == "33333333333" || 
                cpf == "44444444444" || 
                cpf == "55555555555" || 
                cpf == "66666666666" || 
                cpf == "77777777777" || 
                cpf == "88888888888" || 
                cpf == "99999999999")
                    return false;       
            // Valida 1o digito 
            add = 0;    
            for (i=0; i < 9; i ++)      
                add += parseInt(cpf.charAt(i)) * (10 - i);  
                rev = 11 - (add % 11);  
                if (rev == 10 || rev == 11)     
                    rev = 0;    
                if (rev != parseInt(cpf.charAt(9)))     
                    return false;       
            // Valida 2o digito 
            add = 0;    
            for (i = 0; i < 10; i ++)       
                add += parseInt(cpf.charAt(i)) * (11 - i);  
            rev = 11 - (add % 11);  
            if (rev == 10 || rev == 11) 
                rev = 0;    
            if (rev != parseInt(cpf.charAt(10)))
                return false;       
            return true;
        }, function (params, element) {            
            var msgCompare = $(element).attr('data-val-cpf');
            if (!msgCompare) {
                msgCompare = 'CPF inválido';
            }
            return msgCompare;
        });
    $.validator.unobtrusive.adapters.addBool('data-val-cpf');
})(jQuery);

this way you have the two validations with the messages you configured in the code.

Ref. C# using Resource file for validation messages