localizing-viewmodel-netcore.md

View Model Localization

Sometimes it's also beneficial to provide proper error messages for the user while validating model on the client-side. Localization provider can help here as well.

Preparing ViewModel

To add support for viewmodel localization - there isn't really anything special. Just add [LocalizedModel] attribute to your viewmodel.

[LocalizedModel]
public class UserViewModel
{
    [Display(Name = "User name:", Description = "This is description of UserName field")]
    [Required(ErrorMessage = "Name of the user is required!")]
    public string UserName { get; set; }

    [Display(Name = "Password:")]
    [Required(ErrorMessage = "Password is kinda required :)")]
    [StringLength(15, MinimumLength = 5, ErrorMessage = "Please use longer password than 5 symbols!!")]
    public string Password { get; set; }
}

Markup for the page is the same standard way using tag helpers.

<form asp-controller="Home" asp-action="Index" method="post">
    <div>
        <label asp-for="UserName"></label>
        <input asp-for="UserName"/>
        <span asp-validation-for="UserName"></span>
    </div>
    <div>
        <label asp-for="Password"></label>
        <input asp-for="Password" type="password"/>
        <span asp-validation-for="Password"></span>
    </div>
    <button type="submit">Submit</button>
</form>

Error message for invalid property will be taken from localization provider in currently set language.

Where Is Translation?

When you need to change translation for validation message, you should look for {ViewModel-Type}.{Property}-{DataAnnotationAttribute} resource key. For example, username's Required validation error message is located under resource with key DbLocalizationProvider.Core.AspNetSample.Models.UserViewModel.UserName-Required.

Gotchas

My ViewModel's Property Is Not Localized!

After some inspection of Asp.Net Core built-in validation attribute adapters it's obvious that custom IStringLocalizer passed to IValidationAttributeAdapterProvider.GetAttributeAdapter() will be used only when data annotation attribute will have ErrorMessage property set.

For example, if you will have following model:

[LocalizedModel]
public class UserViewModel
{
    [Required]
    public string UserName { get; set; }
}

Error message for the UserName property will be not translated by LocalizedProvider.

You must set error message for the Required attribute:

[LocalizedModel]
public class UserViewModel
{
    [Required(ErrorMessage = "...")]
    public string UserName { get; set; }
}