Your Updated Guide to Building a Multi-language Asp.Net 8 MVC Application: Forms Validation Strings

This part 3 of a 4-part series. Read part 1 here and part 2 here. 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 3 Shared Resources approach 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 (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 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: QueryStringRequestCultureProvider CookieRequestCultureProvider 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 _logger; private readonly IStringLocalizer _stringLocalizer;

public HomeController(ILogger logger, IStringLocalizer 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. *@

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

@{

Submit } 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: Server-side validation – used in this example, and is localized (multilingual) 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. 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 (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 ? 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 _logger; private readonly IStringLocalizer _stringLocalizer;

public HomeController(ILogger logger, IStringLocalizer 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 { 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. *@

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

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

@{

Change Language Select Language Submit }

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

@model LocalizationExampleViewModel

@{

Submit } 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 This part 3 of a 4-part series. Read part 1 here and part 2 here. This part 3 of a 4-part series. Read part 1 here and part 2 here. This part 3 of a 4-part series. Read part 1 here and part 2 here . part 1 here part 2 here 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. Asp.Net 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 1 • ASP.NET 8 – Multilingual Application with single Resx file – Part 2 – Alternative Approach 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 3 – Form Validation Strings • ASP.NET 8 – Multilingual Application with single Resx file – Part 4 – Resource Manager ASP.NET 8 – Multilingual Application with single Resx file – Part 4 – Resource Manager 3 Shared Resources approach 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. Asp.Net .resx “Shared Resources” 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. SharedResoureces.cs 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 (options => { var supportedCultures = new[] { "en", "fr", "de", "it" }; options.SetDefaultCulture(supportedCultures[0]) .AddSupportedCultures(supportedCultures) .AddSupportedUICultures(supportedCultures); }); }

private static void AddingMultiLanguageSupport(WebApplication? app) { app?.UseRequestLocalization(); } 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 (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 StringLocalizer IStringLocalizer 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 { } } //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 . SharedResources.xx.resx 4.4 Selecting Language/Culture Based on [5], the Localization service has three default providers: QueryStringRequestCultureProvider CookieRequestCultureProvider AcceptLanguageHeaderRequestCultureProvider QueryStringRequestCultureProvider QueryStringRequestCultureProvider CookieRequestCultureProvider CookieRequestCultureProvider AcceptLanguageHeaderRequestCultureProvider 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: .AspNetCore.Culture 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 ); } 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; } } //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 _logger; private readonly IStringLocalizer _stringLocalizer;

public HomeController(ILogger logger, IStringLocalizer 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); } public class HomeController : Controller { private readonly ILogger _logger; private readonly IStringLocalizer _stringLocalizer;

public HomeController(ILogger logger, IStringLocalizer 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 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. Asp.Net .input-validation-error 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. *@

window.addEventListener("DOMContentLoaded", () => { document.querySelectorAll("input.input-validation-error") .forEach((elem) => { elem.classList.add("is-invalid"); } ); }); @* _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. *@

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

@{

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

@model LocalizationExampleViewModel

@{

Submit } 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: Server-side validation – used in this example, and is localized (multilingual) 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. Browser-integrated validation – disabled in this example by the usage of novalidate attribute, because it is not localized, and is always in English. Server-side validation – used in this example, and is localized (multilingual) Server-side validation 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. Client-side validation Asp.Net Browser-integrated validation – disabled in this example by the usage of novalidate attribute, because it is not localized, and is always in English. Browser-integrated validation 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. novalidate 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 (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 ? 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 _logger; private readonly IStringLocalizer _stringLocalizer;

public HomeController(ILogger logger, IStringLocalizer 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 { 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 }); } } } //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 (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 ? 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 _logger; private readonly IStringLocalizer _stringLocalizer;

public HomeController(ILogger logger, IStringLocalizer 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 { 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. *@

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

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

@{

Change Language Select Language Submit }

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

@model LocalizationExampleViewModel

@{

Submit } @* _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. *@

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

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

@{

Change Language Select Language Submit }

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

@model LocalizationExampleViewModel

@{

Submit } 6 References [1] Make an ASP.NET Core app's content localizable Make an ASP.NET Core app's content localizable [2] Provide localized resources for languages and cultures in an ASP.NET Core app 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 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 Globalization and localization in ASP.NET Core [5] Troubleshoot ASP.NET Core Localization Troubleshoot ASP.NET Core Localization [6] ASP.NET Core Localization with help of SharedResources ASP.NET Core Localization with help of SharedResources [99] https://github.com/MarkPelf/AspNet8MultilingualApplicationWithSingleResxFile https://github.com/MarkPelf/AspNet8MultilingualApplicationWithSingleResxFile