Localize DataAnnotations in Asp.net core

Question

In my asp.net core (.net5) application I have a form with a required field. When the field is empty I have

"The XX field is required"... In English... I want to translate it in French. I mean, I don't really want to translate , I want to use the French version of the message. I don't want to add Resource files, because I have any custom strings to translate, I just want to use existing messages, but in French.

I started to read here but did't really get the point if the article really proposes me to translate each message manually by myself.

I added this one in the Configure

var supportedCultures = new[] { "fr-FR" };
var localizationOptions = new RequestLocalizationOptions().SetDefaultCulture(supportedCultures[0])
    .AddSupportedCultures(supportedCultures)
    .AddSupportedUICultures(supportedCultures);

app.UseRequestLocalization(localizationOptions);

however this didn't change the message...

nor setting the culture params via URL, like this

Answer 1

If you just want one french version, you just need to define the error message in [Required]

attribute, like:

public class MyModel
{
    [Required(ErrorMessage = "Le champ Nom est obligatoire")]
    public string Nom { get; set; }
}

And if your shared code refers to the documention you shared, you need to add Resource file to

define the translated message, or the message will not changed.

Edit:

If you have other properties, you can define a customRequired attribute:

 public class CustomRequiredAttribute : ValidationAttribute
{
    public CustomRequiredAttribute()
    {         
    }
    
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value == null)
        {
            ErrorMessage = $"Le champ {validationContext.DisplayName} est obligatoire";
            return new ValidationResult(ErrorMessage);
        }
        return ValidationResult.Success;
    }
}

Model:

public class MyModel
{   
    [CustomRequired]  //custom attribute
    public string Nom { get; set; }        
    [CustomRequired]
    public string PreNom { get; set; }
}

To use your custom attribute, you should add ModelState.IsValid in your post action, so if invalid, it will return the input view to show the errorMessages,like:

[HttpPost]
    public IActionResult Privacy(MyModel myModel)
    {
        if (!ModelState.IsValid)
            return View("Index");
        return View();
    }