Localization of data annotation attributes in Localization.StarterWeb

[xps]

I am looking for an example showing how to localize data annotation attributes. I feel like this should belong in the Localization.StarterWeb sample, but unfortunately it is not implemented. Here you can see English field names and validation messages when the French language is selected:

There are mostly 4 things that I would like to see:

1. Localization of messages of ValidationAttributes (e.g. [Required], "The {0} field is required.")
2. Localization of display names ([Display(Name = "Email Address")])
3. Localization of built-in validation messages ("The field {0} must be a number.")
4. Display of numbers and dates in a format matching the current culture, and properly taking that format into account in the built-in front-end validation (jquery.validate)

Ideally, in a way that is compatible with implementing a custom IStringLocalizer to retrieve the translations...

I would be willing to submit a PR given some pointers.

Any help appreciated.

[dougbu]

This sample might not use enough of the MVC support for it to be visible. Look at samples/Mvc.LocalizationSample.Web instead.

[xps]

Thanks, I had a look and indeed it shows how to address some of my points, namely:

• Localization of ValidationAttributes with explicit messages (e.g. [Required(ErrorMessage = "This field is required")])
• Localization of display names (e.g. [Display(Name = "Email Address")])

However, it does not show (and I don't know how to):

• Localize ValidationAttributes that use the default message (e.g. [Required] or [MaxLength(20)])
• Localize validation messages that appear automatically when there are no attributes. For instance, having a numeric property will cause the field to be required and the value will have to match a numeric format. When the value is invalid you will see a message like "The value '{0}' is not valid for {1}.". When the value is blank the message that is shown is "The value '' is invalid." which is different from the [Required] message and cannot seem to be overridden by adding a [Required] attribute.

Also there is the problem of the compatibility with the front-end validation, which I think should be supported since it is implemented in the default project templates, but let's leave that aside for now as it seems like a different subject.

[dougbu]

Localize ValidationAttributes that use the default message (e.g. [Required] or [MaxLength(20)])

The IStringLocalizer-based approach is explicitly opt-in. One of the reasons is ValidationAttributes are already localized in .NET.

If you're seeing .NET ValidationAttribute messages only in your system's default language, you either need to install .NET Framework (desktop) language packs or you need to wait for .NET Core language packs. (I'm not sure of the .NET Core language pack status but don't think they're available yet. @ryanbrandenburg is this correct?)

Localize validation messages that appear automatically when there are no attributes.

These messages are all retrieved from Funcs in ModelMetadata.ModelBindingMessageProvider. To override defaults, you can:

1. Make MvcOptions updates in your Startup class e.g.

services.AddMvc(options => options.ModelBindingMessageProvider.AttemptedValueIsInvalidAccessor =
    (value, name) => $"Hmm, '{value}' is not a valid value for '{name}'."));

2. Make similar updates in an IConfigureOptions<MvcOptions> implementation, perhaps using an IStringLocalizerFactory e.g.

var stringLocalizer = _stringLocalizerFactory.Create(typeof(MvcDataAnnotationsLocalizationOptionsSetup));
options.ModelBindingMessageProvider.AttemptedValueIsInvalidAccessor =
    (value, name) => stringLocalizer["The value '{0}' is not valid for {1}.", value, name];

3. Make type-specific changes in an IBindingMetadataProvider implementation e.g.

if (context.Key.MetadataKind == ModelMetadataKind.Property)
{
    var stringLocalizer = _stringLocalizerFactory.Create(typeof(DataAnnotationsMetadataProvider));
    context.BindingMetadata.ModelBindingMessageProvider.AttemptedValueIsInvalidAccessor =
        (value, name) => stringLocalizer[
            $"The value '{{0}}' is not valid for properties of type '{context.Key.ModelType.FullName}' like {context.Key.Name}.",
            value];
}

(Thought we had samples using at least one of these. But I'm not finding anything.)

[xps]

Thanks for the detailed info! With your help I managed to get everything localized.
Let me know if you'd like me to make some changes to the samples for the next guy.

[Eilon]

Closing because there are no plans to make this sample change.