c# - How to provide localized validation messages for validation attributes

Question

I am working on an ASP.NET Core application and I would like to override the default validation error messages for data-annotations, like Required, MinLength, MaxLength, etc. I read the documentation at Globalization and localization in ASP.NET Core, and it seems that it does not cover what I was looking for...

For instance, a validation error message for the Required attribute can always be the same for any model property. The default text just states: The {0} field is required, whereby the {0} placeholder will be filled up with the property’s display name.

In my view models, I use the Required attribute without any named arguments, like this...

class ViewModel
{
    [Required, MinLength(10)]
    public string RequiredProperty { get; set; }
}

Setting an ErrorMessage or ErrorMessageResourceName (and ErrorMessageResourceType) is unnecessary overhead, in my opinion. I thought I could implement something similar to IDisplayMetadataProvider allowing me to return error messages for applied attributes, in case the validation has failed. Is this possible?

Answer 1

For those that end up here, in search of a general solution, the best way to solve it is using a Validation Metadata Provider. I based my solution on this article: AspNetCore MVC Error Message, I usted the .net framework style localization, and simplified it to use the designed provider.

1. Add a Resource file for example ValidationsMessages.resx to your project, and set the Access Modifier as Internal or Public, so that the code behind is generated, that will provide you with the ResourceManager static instance.
2. Add a custom localization for each language ValidationsMessages.es.resx. Remember NOT to set Access Modifier for this files, the code is created on step 1.
3. Add an implementation of IValidationMetadataProvider
4. Add the localizations based on the Attributes Type Name like "RequiredAtrribute".
5. Setup your app on the Startup file.

Sample ValidationsMessages.es.resx

Sample for IValidatioMetadaProvider:

using Microsoft.AspNetCore.Mvc.ModelBinding.Metadata;
using System.ComponentModel.DataAnnotations;
using System.Reflection;

public class LocalizedValidationMetadataProvider : IValidationMetadataProvider
{
    public LocalizedValidationMetadataProvider()
    {
    }

    public void CreateValidationMetadata(ValidationMetadataProviderContext context)
    {
        if (context.Key.ModelType.GetTypeInfo().IsValueType && Nullable.GetUnderlyingType(context.Key.ModelType.GetTypeInfo()) == null && context.ValidationMetadata.ValidatorMetadata.Where(m => m.GetType() == typeof(RequiredAttribute)).Count() == 0)
            context.ValidationMetadata.ValidatorMetadata.Add(new RequiredAttribute());
        foreach (var attribute in context.ValidationMetadata.ValidatorMetadata)
        {
            var tAttr = attribute as ValidationAttribute;
            if (tAttr?.ErrorMessage == null && tAttr?.ErrorMessageResourceName == null)
            {
                var name = tAttr.GetType().Name;
                if (Resources.ValidationsMessages.ResourceManager.GetString(name) != null)
                {
                    tAttr.ErrorMessageResourceType = typeof(Resources.ValidationsMessages);
                    tAttr.ErrorMessageResourceName = name;
                    tAttr.ErrorMessage = null;
                }
            }
        }
    }
}

Add the provider to the ConfigureServices method on the Startup class:

services.AddMvc(options =>
{
     options.ModelMetadataDetailsProviders.Add(new LocalizedValidationMetadataProvider());
})