Localize standard DataAnnotations in ASP.NET Core MVC

The problem

As far as I see from the docs, DataAnnotation localization asks additional resx files, or hardcoded translation strings in [Attributes], or specific code customization, that seem not really fair versus non-English projects.

In order to make "The XX field is required" string to appear as "Le champ XX est obligatoire" we need to write it hardcoded on all decorated with [Required] properties, or write custom code to implement it on all the [Required] properties, and also for all other DataAnnotation attributes, that seem strange…

DataAnnotation should be translated in a similar way another localized strings are, depend on the current culture, be available in localized .json or res files or similar, without the need for each developer to translate by itself the same “Is required” string with same text in their projects.

Further technical details

• ASP.NET Core version: 5.0.202
• VS 2019

Linked to

• https://github.com/dotnet/aspnetcore/issues/4848
• https://github.com/dotnet/aspnetcore/issues/2631
• https://github.com/aspnet/Localization/issues/286
• SO question

[ianbrian]

Localization should be baked in. I’ve been spending ages replacing hard-coded strings in Identity razor pages. Millions of developers must have done that. What a waste of time.

[LazZiya]

Many developers prefers official solutions, but sometimes when the wait is too long, you will start developing it yourself or find a community solution.

I’ve developed a nuget (XLocalizer) for simplifing localization in all aspects including auto adding missing keys and online translation support.

In terms of validation there are three main categories:

• DataAnnotations errors
• Model binding errors
• Identity describer errors

Basically there is no need to provide any error message manually, XLocalizer will take care of localizing all automatically. But if you want to customize any error message you can do it simply in startup or json.

• Option A : In startup

services.AddRazorPages()
        .AddXLocalizer<...>(ops =>
        {
            // Data annotation error messages
            ops.ValidationErrors = new ValidationErrors 
            {
                RequiredAttribute_ValidationError = "The {0} field is required.",
                CompareAttribute_MustMatch = "'{0}' and '{1}' do not match.",
                StringLengthAttribute_ValidationError = "The field {0} must be a string with a maximum length of {1}.",
                // ...
            };

            // Model binding error messages
            ops.ModelBindingErrors = new ModelBindingErrors
            {
                AttemptedValueIsInvalidAccessor = "The value '{0}' is not valid for {1}.",
                MissingBindRequiredValueAccessor = "A value for the '{0}' parameter or property was not provided.",
               // ...
            };

            // Identity Errors
            ops.IdentityErrors = new IdentityErrors
            {
                DuplicateEmail = "Email '{0}' is already taken.",
                DuplicateUserName = "User name '{0}' is already taken.",
                // ...
            }
        });

• Option B - Json:

{
  "XLocalizerOptions": {

    ....

    "ValidationErrors": {
      "CompareAttribute_MustMatch": "'{0}' and '{1}' do not match. They should not be different!",
      "CreditCardAttribute_Invalid": "The {0} field is not a valid credit card number.",
      "CustomValidationAttribute_ValidationError": "{0} is not valid.",
      ...
    },
    "IdentityErrors": {
      "DuplicateEmail": "Email '{0}' is already taken.",
      "DuplicateUserName": "User name '{0}' is already taken. Please try another one.",
      "InvalidEmail": "Email '{0}' is invalid.",
      ...
    },
    "ModelBindingErrors": {
      "AttemptedValueIsInvalidAccessor": "The value '{0}' is not valid for {1}.",
      "MissingBindRequiredValueAccessor": "A value for the '{0}' parameter or property was not provided.",
      ...
    }
  }
}

• XLocalizer Repo
• Docs
• Samples