1 Multilingual form validation error strings

A separate task is how to handle in Asp.Net MVC application a localization of form validation error strings, or so-called Data Annotation Localization. That is the focus of this article.

2 Other articles in this series

Articles in this series are:

• ASP.NET 8 – Multilingual Application with single Resx file – Part 1

• ASP.NET 8 – Multilingual Application with single Resx file – Part 2 – Alternative Approach

• ASP.NET 8 – Multilingual Application with single Resx file – Part 3 – Form Validation Strings

• ASP.NET 8 – Multilingual Application with single Resx file – Part 4 – Resource Manager

By default, Asp.Net Core 8 MVC technology envisions separate resource file .resx for each controller and the view. But most people do not like it, since most multilanguage strings are the same in different places in the application, we would like it to be all in the same place. Literature [1] calls that approach the “Shared Resources” approach.

In order to implement it, we will create a marker class SharedResoureces.cs to group all the resources. Then in our application, we will invoke Dependency Injection (DI) for that particular class/type instead of a specific controller/view. That is a little trick mentioned in Microsoft documentation [1] that has been a source of confusion in StackOverflow articles [6]. We plan to demystify it here. While everything is explained in [1], what is needed are some practical examples, like the one we provide here.

4 Steps to Multilingual Application

4.1 Configuring Localization Services and Middleware

Localization services are configured in Program.cs:

private static void AddingMultiLanguageSupportServices(WebApplicationBuilder? builder)
{
    if (builder == null) { throw new Exception("builder==null"); };

    builder.Services.AddLocalization(options => options.ResourcesPath = "Resources");
    builder.Services.AddMvc()
            .AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix)
            .AddDataAnnotationsLocalization(options =>
            {
                options.DataAnnotationLocalizerProvider = (type, factory) =>
                    factory.Create(typeof(SharedResource));
            });

    builder.Services.Configure<RequestLocalizationOptions>(options =>
    {
        var supportedCultures = new[] { "en", "fr", "de", "it" };
        options.SetDefaultCulture(supportedCultures[0])
            .AddSupportedCultures(supportedCultures)
            .AddSupportedUICultures(supportedCultures);
    });
}

private static void AddingMultiLanguageSupport(WebApplication? app)
{
    app?.UseRequestLocalization();
}

4.2 Create marker class SharedResources.cs

This is just a dummy marker class to group shared resources. We need it for its name and type.It seems the namespace needs to be the same as the app root namespace, which needs to be the same as the assembly name. I had some problems when changing the namespace, it would not work. If it doesn't work for you, you can try to use the full class name in your DI instruction, like this one:

IStringLocalizer<SharedResources01.SharedResource> StringLocalizer

There is no magic in the name "SharedResource", you can name it "MyResources" and change all references in the code to "MyResources" and all will still work.

The location seems can be any folder, although some articles [6] claim it needs to be the root project folder I do not see such problems in this example. To me looks like it can be any folder, just keep your namespace tidy.

//SharedResource.cs===================================================
namespace SharedResources03
{
    /*
    * This is just a dummy marker class to group shared resources
    * We need it for its name and type
    * 
    * It seems the namespace needs to be the same as app root namespace
    * which needs to be the same as the assembly name.
    * I had some problems when changing the namespace, it would not work.
    * If it doesn't work for you, you can try to use full class name
    * in your DI instruction, like this one: SharedResources03.SharedResource
    * 
    * There is no magic in the name "SharedResource", you can
    * name it "MyResources" and change all references in the code
    * to "MyResources" and all will still work
    * 
    * Location seems can be any folder, although some
    * articles claim it needs to be the root project folder
    * I do not see such problems in this example. 
    * To me looks it can be any folder, just keep your
    * namespace tidy. 
    */

    public class SharedResource
    {
    }
}

4.3 Create language resources files

In the folder “Resources” create your language resources files, and make sure you name them SharedResources.xx.resx.

4.4 Selecting Language/Culture

Based on [5], the Localization service has three default providers:

1. QueryStringRequestCultureProvider
2. CookieRequestCultureProvider
3. AcceptLanguageHeaderRequestCultureProvider

Since most apps will often provide a mechanism to set the culture with the ASP.NET Core culture cookie, we will focus only on that approach in our example.This is the code to set .AspNetCore.Culture cookie:

private void ChangeLanguage_SetCookie(HttpContext myContext, string? culture)
{
    if (culture == null) { throw new Exception("culture == null"); };

    //this code sets .AspNetCore.Culture cookie
    CookieOptions cookieOptions=new CookieOptions();
    cookieOptions.Expires = DateTimeOffset.UtcNow.AddMonths(1);
    cookieOptions.IsEssential = true;

    myContext.Response.Cookies.Append(
        CookieRequestCultureProvider.DefaultCookieName,
        CookieRequestCultureProvider.MakeCookieValue(new RequestCulture(culture)),
        cookieOptions
    );
}

Cookie can be easily seen with Chrome DevTools:

I built a small application to demo it, and here is the screen where I can change the language:

Note that I added some debugging info into the footer, to show the value of the Request language cookie, to see if the app is working as desired.

4.5 Using Data Annotation – Field Validation

In your model class, you set up validation attributes with proper strings that need to be localized.

//LocalizationExampleViewModel.cs===============================================
namespace SharedResources03.Models.Home
{    public class LocalizationExampleViewModel
    {
        /* It is these field validation error messages
         * that are focus of this example. We want to
         * be able to present them in multiple languages
         */
        //model
        [Required(ErrorMessage = "The UserName field is required.")]
        [Display(Name = "UserName")]
        public string? UserName { get; set; }

        [EmailAddress(ErrorMessage = "The Email field is not a valid email address.")]
        [Display(Name = "Email")]
        public string? Email { get; set; }

        public bool IsSubmit { get; set; } = false;
    }
}

For model-level validation in the controller, we will use classical IStringLocalizer.

public class HomeController : Controller
{
    private readonly ILogger<HomeController> _logger;
    private readonly IStringLocalizer<SharedResource> _stringLocalizer;

    public HomeController(ILogger<HomeController> logger,
        IStringLocalizer<SharedResource> stringLocalizer)
    {
        _logger = logger;
        _stringLocalizer = stringLocalizer;
    }
	
	//--------------------------
	
	public IActionResult LocalizationExample(LocalizationExampleViewModel model)
{
    if(model.IsSubmit)
    {
        if (!ModelState.IsValid)
        {
            ModelState.AddModelError("", _stringLocalizer["Please correct all errors and submit again"]);
        }
    }
    else
    {
        ModelState.Clear();
    }

    return View(model);
}

4.6 Synchronization Asp.Net CSS error classes with Bootstrap CSS classes

Asp.Net will add CSS class .input-validation-error to form a field with an error. But, Bootstrap does not know what to do with that CSS class, so that class needs to be mapped to CSS class that Bootstrap understands, and that is CSS class .is-invalid.

That is the purpose of this JavaScript code that is here. Of course, we hook to the DOMContentLoaded event and do the mapping of CSS classes.

All this is to sync Asp.Net CSS error classes with Bootstrap CSS classes to mark error input elements border to red by Bootstrap.The final result is that the red line on the form control marks an invalid field.

@* _ValidationClassesSyncBetweenAspNetAndBootstrap.cshtml===================== *@
@*
All this is to sync Asp.Net CSS error classes with Bootstrap CSS classes to
mark error input elements border to red by Bootstrap.

Asp.Net will add CSS class .input-validation-error to form a field with an error.
But, Bootstrap does not know what to do with that CSS class, so that class
needs to be mapped to CSS class that Bootstrap understands, and that is
CSS class .is-invalid.

That is the purpose of this JavaScript code that is here. Of course, we hook
to DOMContentLoaded event and do the mapping of CSS classes.

The final result is that the red line on the form control marking an invalid field.
*@


<script type="text/javascript">
    window.addEventListener("DOMContentLoaded", () => {
        document.querySelectorAll("input.input-validation-error")
            .forEach((elem) => { elem.classList.add("is-invalid"); }
            );
    });
</script>

4.7 Sample view with field and model validation messages

Here is our sample view.

@* LocalizationExample.cshtml ====================================================*@
@using Microsoft.AspNetCore.Mvc.Localization
@using Microsoft.Extensions.Localization

@model LocalizationExampleViewModel

@{
    <partial name="_ValidationClassesSyncBetweenAspNetAndBootstrap" />

    <div style="width:500px ">
        <fieldset class="border rounded-3 p-3 bg-info">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <form id="formlogin" novalidate>
                <div class="form-group">
                    <label asp-for="UserName"></label>
                    <div>
                        <span asp-validation-for="UserName" class="text-danger"></span>
                    </div>
                    <input class="form-control" asp-for="UserName" />
                </div>
                <div class="form-group">
                    <label asp-for="Email"></label>
                    <div>
                        <span asp-validation-for="Email" class="text-danger"></span>
                    </div>
                    <input class="form-control" asp-for="Email" />
                </div>
                <input type="hidden" name="IsSubmit" value="true">
                <button type="submit" class="btn btn-primary mt-3 float-end"> Submit</button>
            </form>
        </fieldset>
    </div>    
}

Note that we used novalidate attribute in the form element to suppress browser-level integrated validation that is popping out and is not localized.So, there are 3 possible levels of validation:

1. Server-side validation – used in this example, and is localized (multilingual)
2. Client-side validation – in Asp.Net it can be enabled by using jquery.validate.unobtrusive.min.js, but we are not using it in this example.
3. Browser-integrated validation – disabled in this example by the usage of novalidate attribute, because it is not localized, and is always in English.

If you do not set novalidate attribute, the browser will pop up its validation dialog and you will see messages as on the following screen. It can be confusing to the user to multiple different messages.

4.8 Execution result

Here is what the execution result looks like:

Note that I added some debugging info into the footer, to show the value of the Request language cookie, to see if the app is working as desired.

5 Full Code

Since most people like code they can copy-paste, here is the full code of the application. Code can be downladed at GitHub [99].

//Program.cs===========================================================================
namespace SharedResources03
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //=====Middleware and Services=============================================
            var builder = WebApplication.CreateBuilder(args);

            //adding multi-language support
            AddingMultiLanguageSupportServices(builder);

            // Add services to the container.
            builder.Services.AddControllersWithViews();

            //====App===================================================================
            var app = builder.Build();

            //adding multi-language support
            AddingMultiLanguageSupport(app);

            // Configure the HTTP request pipeline.
            if (!app.Environment.IsDevelopment())
            {
                app.UseExceptionHandler("/Home/Error");
            }
            app.UseStaticFiles();

            app.UseRouting();

            app.UseAuthorization();

            app.MapControllerRoute(
                name: "default",
                pattern: "{controller=Home}/{action=ChangeLanguage}/{id?}");

            app.Run();
        }

        private static void AddingMultiLanguageSupportServices(WebApplicationBuilder? builder)
        {
            if (builder == null) { throw new Exception("builder==null"); };

            builder.Services.AddLocalization(options => options.ResourcesPath = "Resources");
            builder.Services.AddMvc()
                    .AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix)
                    .AddDataAnnotationsLocalization(options =>
                    {
                        options.DataAnnotationLocalizerProvider = (type, factory) =>
                            factory.Create(typeof(SharedResource));
                    });

            builder.Services.Configure<RequestLocalizationOptions>(options =>
            {
                var supportedCultures = new[] { "en", "fr", "de", "it" };
                options.SetDefaultCulture(supportedCultures[0])
                    .AddSupportedCultures(supportedCultures)
                    .AddSupportedUICultures(supportedCultures);
            });
        }

        private static void AddingMultiLanguageSupport(WebApplication? app)
        {
            app?.UseRequestLocalization();
        }
    }
}


//SharedResource.cs===================================================
namespace SharedResources03
{
    /*
    * This is just a dummy marker class to group shared resources
    * We need it for its name and type
    * 
    * It seems the namespace needs to be the same as app root namespace
    * which needs to be the same as the assembly name.
    * I had some problems when changing the namespace, it would not work.
    * If it doesn't work for you, you can try to use full class name
    * in your DI instruction, like this one: SharedResources03.SharedResource
    * 
    * There is no magic in the name "SharedResource", you can
    * name it "MyResources" and change all references in the code
    * to "MyResources" and all will still work
    * 
    * Location seems can be any folder, although some
    * articles claim it needs to be the root project folder
    * I do not see such problems in this example. 
    * To me looks it can be any folder, just keep your
    * namespace tidy. 
    */

    public class SharedResource
    {
    }
}

//ChangeLanguageViewModel.cs=====================================================
namespace SharedResources03.Models.Home
{
    public class ChangeLanguageViewModel
    {
        //model
        public string? SelectedLanguage { get; set; } = "en";

        public bool IsSubmit { get; set; } = false;

        //view model
        public List<SelectListItem>? ListOfLanguages { get; set; }
    }
}

//LocalizationExampleViewModel.cs===============================================
namespace SharedResources03.Models.Home
{    public class LocalizationExampleViewModel
    {
        /* It is these field validation error messages
         * that are focus of this example. We want to
         * be able to present them in multiple languages
         */
        //model
        [Required(ErrorMessage = "The UserName field is required.")]
        [Display(Name = "UserName")]
        public string? UserName { get; set; }

        [EmailAddress(ErrorMessage = "The Email field is not a valid email address.")]
        [Display(Name = "Email")]
        public string? Email { get; set; }

        public bool IsSubmit { get; set; } = false;
    }
}

//HomeController.cs================================================================
namespace SharedResources03.Controllers
{
    public class HomeController : Controller
    {
        private readonly ILogger<HomeController> _logger;
        private readonly IStringLocalizer<SharedResource> _stringLocalizer;

        public HomeController(ILogger<HomeController> logger,
            IStringLocalizer<SharedResource> stringLocalizer)
        {
            _logger = logger;
            _stringLocalizer = stringLocalizer;
        }

        public IActionResult ChangeLanguage(ChangeLanguageViewModel model)
        {
            if (model.IsSubmit)
            {
                HttpContext myContext = this.HttpContext;
                ChangeLanguage_SetCookie(myContext, model.SelectedLanguage);
                //doing funny redirect to get new Request Cookie
                //for presentation
                return LocalRedirect("/Home/ChangeLanguage");
            }

            //prepare presentation
            ChangeLanguage_PreparePresentation(model);
            return View(model);
        }

        private void ChangeLanguage_PreparePresentation(ChangeLanguageViewModel model)
        {
            model.ListOfLanguages = new List<SelectListItem>
                        {
                            new SelectListItem
                            {
                                Text = "English",
                                Value = "en"
                            },

                            new SelectListItem
                            {
                                Text = "German",
                                Value = "de",
                            },

                            new SelectListItem
                            {
                                Text = "French",
                                Value = "fr"
                            },

                            new SelectListItem
                            {
                                Text = "Italian",
                                Value = "it"
                            }
                        };
        }

        private void ChangeLanguage_SetCookie(HttpContext myContext, string? culture)
        {
            if (culture == null) { throw new Exception("culture == null"); };

            //this code sets .AspNetCore.Culture cookie
            CookieOptions cookieOptions=new CookieOptions();
            cookieOptions.Expires = DateTimeOffset.UtcNow.AddMonths(1);
            cookieOptions.IsEssential = true;

            myContext.Response.Cookies.Append(
                CookieRequestCultureProvider.DefaultCookieName,
                CookieRequestCultureProvider.MakeCookieValue(new RequestCulture(culture)),
                cookieOptions
            );
        }

        public IActionResult LocalizationExample(LocalizationExampleViewModel model)
        {
            if(model.IsSubmit)
            {
                if (!ModelState.IsValid)
                {
                    ModelState.AddModelError("", _stringLocalizer["Please correct all errors and submit again"]);
                }
            }
            else
            {
                ModelState.Clear();
            }

            return View(model);
        }


        public IActionResult Error()
        {
            return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
        }
    }
}

@* _ValidationClassesSyncBetweenAspNetAndBootstrap.cshtml===================== *@
@*
All this is to sync Asp.Net CSS error classes with Bootstrap CSS classes to
mark error input elements border to red by Bootstrap.

Asp.Net will add CSS class .input-validation-error to form a field with an error.
But, Bootstrap does not know what to do with that CSS class, so that class
needs to be mapped to CSS class that Bootstrap understands, and that is
CSS class .is-invalid.

That is the purpose of this JavaScript code that is here. Of course, we hook
to DOMContentLoaded event and do the mapping of CSS classes.

The final result is that the red line on the form control marking an invalid field.
*@


<script type="text/javascript">
    window.addEventListener("DOMContentLoaded", () => {
        document.querySelectorAll("input.input-validation-error")
            .forEach((elem) => { elem.classList.add("is-invalid"); }
            );
    });
</script>

@* ChangeLanguage.cshtml ===================================================*@
@model ChangeLanguageViewModel

@{
    <div style="width:500px">
        <p class="bg-info">
            <partial name="_Debug.AspNetCore.CultureCookie" /><br />
        </p>

        <form id="form1" >
            <fieldset class="border rounded-3 p-3">
                <legend class="float-none w-auto px-3">Change Language</legend>
                <div class="form-group">
                    <label asp-for="SelectedLanguage">Select Language</label>
                    <select class="form-select" asp-for="SelectedLanguage"
                            asp-items="@Model.ListOfLanguages">
                    </select>
                    <input type="hidden" name="IsSubmit" value="true">
                    <button type="submit" form="form1" class="btn btn-primary mt-3 float-end"
                            asp-area="" asp-controller="Home" asp-action="ChangeLanguage">
                        Submit
                    </button>
                </div>
            </fieldset>
        </form>
    </div>
}

@* LocalizationExample.cshtml ====================================================*@
@using Microsoft.AspNetCore.Mvc.Localization
@using Microsoft.Extensions.Localization

@model LocalizationExampleViewModel

@{
    <partial name="_ValidationClassesSyncBetweenAspNetAndBootstrap" />

    <div style="width:500px ">
        <fieldset class="border rounded-3 p-3 bg-info">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <form id="formlogin" novalidate>
                <div class="form-group">
                    <label asp-for="UserName"></label>
                    <div>
                        <span asp-validation-for="UserName" class="text-danger"></span>
                    </div>
                    <input class="form-control" asp-for="UserName" />
                </div>
                <div class="form-group">
                    <label asp-for="Email"></label>
                    <div>
                        <span asp-validation-for="Email" class="text-danger"></span>
                    </div>
                    <input class="form-control" asp-for="Email" />
                </div>
                <input type="hidden" name="IsSubmit" value="true">
                <button type="submit" class="btn btn-primary mt-3 float-end"> Submit</button>
            </form>
        </fieldset>
    </div>    
}

6 References

[1] Make an ASP.NET Core app's content localizable

[2] Provide localized resources for languages and cultures in an ASP.NET Core app

[3] Implement a strategy to select the language/culture for each request in a localized ASP.NET Core app

[4] Globalization and localization in ASP.NET Core

[5] Troubleshoot ASP.NET Core Localization

[6] ASP.NET Core Localization with help of SharedResources

[99] https://github.com/MarkPelf/AspNet8MultilingualApplicationWithSingleResxFile