Localizing ASP.NET Applications

Working with Cultures, Resources, & ASP.NET.

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Localization has been around since .NET 1.1. Looking at the interfaces of ResourceManager and CultureInfo, along with the static nature of resource access, it's easy to see the ties to legacy client technology WinForms. With the release of ASP.NET Core, developers have new tools to work with localization in the realm of server-based web apps.

We'll explore these new mechanisms and how the ASP.NET team designed them to work in a web-centric world. You'll need a new Razor Pages project to follow along, or you can clone the complete solution from the GitHub repository.

In Startup

The configuration found in our Startup is essential to localizing an ASP.NET Core web application. We'll be altering both the ConfigureServices method and the Configure method to set up our localized app.

Configure Services

To opt in to localization, we need to modify our Startup file. We'll be registering a few services, configuring options, and registering middleware. All steps that are common-place for additions in an ASP.NET application.

Starting in our ConfigureServices method, we need to make a call to AddLocalization. It's essential to set the ResourcesPath as it tells the ASP.NET localization feature where to find our .resx files.

 services.AddLocalization(options =>
 {
    options.ResourcesPath = "Resources";
 });

We follow up on the call to add localization by configuring the RequestLocalizationOptions object. We registered this object when we called AddLocalization.

 services.Configure<RequestLocalizationOptions>(options =>
 {
     options.SetDefaultCulture("en-Us");
     options.AddSupportedUICultures("en-US", "de-DE", "ja-JP");
     options.FallBackToParentUICultures = true;

     options
         .RequestCultureProviders
         .Remove(typeof(AcceptLanguageHeaderRequestCultureProvider));
 });

We should take note of a few critical elements:

  1. We set the default culture to en-US.
  2. We are choosing to support en-US (US English), de-DE (German), and ja-JP (Japanese) cultures.
  3. We remove the AcceptLanguageHeaderRequestCultureProvider. This provider sets the culture based on the client's HTTP locale via a header value. In this project, we'll be letting the user choose their culture.

Finally, we add view localization to the services collection.

 services
  .AddRazorPages()
  .AddViewLocalization();

The complete ConfigureServices method is below.

 public void ConfigureServices(IServiceCollection services)
 {
     services.AddLocalization(options =>
     {
        options.ResourcesPath = "Resources";
     });

     services.Configure<RequestLocalizationOptions>(options =>
     {
         options.SetDefaultCulture("en-Us");
         options.AddSupportedUICultures("en-US", "de-DE", "ja-JP");
         options.FallBackToParentUICultures = true;

         options
         .RequestCultureProviders
         .Remove(typeof(AcceptLanguageHeaderRequestCultureProvider));
     });

     services
         .AddRazorPages()
         .AddViewLocalization();

     services.AddScoped<RequestLocalizationCookiesMiddleware>();
 }

Note that we have an optional RequestLocalizationCookieMiddleware which retains the user-selected culture between requests. We can see the code for that on the GitHub repository.

Configure

The Configure method is the simpler of the two setups. Here, we need to register the RequestLocalizationMiddleware utilizing the registration method provided by ASP.NET Core.

app.UseRequestLocalization();

We need to ensure that we register the middleware before any middleware that requires access to the current culture. Other than that critical factor, we can register it anywhere in our pipeline. Optionally, we register our custom RequestLocalizationCookiesMiddleware into the ASP.NET pipeline as well. Here is the final Configure method.

 public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
 {
     if (env.IsDevelopment())
     {
        app.UseDeveloperExceptionPage();
     }
     else
     {
         app.UseExceptionHandler("/Error");
         // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
         app.UseHsts();
     }

     app.UseHttpsRedirection();
     app.UseStaticFiles();
     app.UseRequestLocalization();

     // will remember to write the cookie
     app.UseRequestLocalizationCookies();

     app.UseRouting();
     app.UseAuthorization();

     app.UseEndpoints(endpoints => { endpoints.MapRazorPages(); });
 }

Resource Files

We may remember setting the ResourcesPath property in our ConfigureServices method. This property requires a matching physical directory in our project. This Resources directory will hold all of our resx files. Before we start creating files, we'll need to understand some additional naming conventions, along with cultural suffixes we learned from the previous tutorial.

  1. We should name files after the class names, which will be utilizing the resource values. For example, Program.cs and Resources/Program.resx. We can access any resource file, but this organization helps keep us focused.
  2. Resource files for ASP.NET views should have the same directory structure as the views they represent. For example, Pages/Index.cshtml and Resources/Pages/Index.resx.
  3. In some cases, naming isn't as important, as we can select which resource file we want ASP.NET to look for a resource value.

We recommend looking at the Resources directory in our sample project to see some of the different naming conventions in practice.

Accessing Resource Values

There are three interfaces ASP.NET developers will be working with when localizing a web application:

  1. IStringLocalizer
  2. IHtmlLocalizer
  3. IViewLocalizer

IStringLocalizer

The IStringLocalizer interface reads values from our resource files using a ResourceManager internally. It is a no-frills interface, compared to the other two interfaces. We can specify where the instance of IStringLocalizer looks for resource values by specifying a generic type argument. Here is an example of us injecting an instance into a Razor page.

@inject IStringLocalizer<IndexModel> Localizer

We can access a value using the index operator.

@{
 ViewData["Title"] = @Localizer["Home page"];
}

IHtmlLocalizer

Similar to IStringLocalizer, the IHtmlLocalizer has all the same features, but we can expect the resource values to contain HTML elements. Any argument we pass to the localizer will be encoded, but the resource value will not be encoded.

@inject IHtmlLocalizer<IndexModel> Localizer
@{
   var name = "Khalid";
   var message = Localizer["<div>Hello, {0}</div>", /* encoded */ name];
}

IViewLocalizer

The IViewLocalizer implementation is shorthand for accessing the resources matching the current view. It implements the same behaviors as IHtmlLocalizer. We can retrieve an instance by injecting it into our Razor Page or ASP.NET MVC view.

@* ViewLocalizer path is Resources.Pages.Index *@
@inject IViewLocalizer ViewLocalizer

The IViewLocalizer is the recommended approach for anyone localizing an ASP.NET application. It has all the same features as the previous interfaces and handles the resource file location for us.

Example Usage

Here is an example of a Razor page utilizing all three interfaces. We can see that once injected, they all use an index operator to handle

@page
@using Microsoft.Extensions.Localization
@using Microsoft.AspNetCore.Mvc.Localization
@model IndexModel

@inject IStringLocalizer<IndexModel> Localizer
@inject IHtmlLocalizer<IndexModel> HtmlLocalizer

@* ViewLocalizer path is Resources.Pages.Index *@
@inject IViewLocalizer ViewLocalizer

@{
 ViewData["Title"] = @Localizer["Home page"];
}

<div class="text-center">
 <h1 class="display-4">@ViewLocalizer["Home page"]</h1>
 <p>
 @HtmlLocalizer["Learn"]
 </p>
</div>

<form method="post" asp-page="Index">
 <div class="form-group">
 <label asp-for="Superhero" class="control-label"></label>
 <input asp-for="Superhero" class="form-control"/>
 <span asp-validation-for="Superhero" class="text-danger"></span>
 </div>
 <button type="submit">@Localizer["Submit"]</button>
</form>

Data Annotations and HTML Forms

Localizing static content using the localization interfaces is straight-forward. Localizing C# objects take a bit more effort, but not much more. Our sample project has a SuperHero property bound to an ASP.NET Razor text input element. We can define the display and error messages by using Data Annotation attributes.

[Display(Name = "Superhero", ResourceType = typeof(Resources.Pages.IndexModel))]
[Required(
    ErrorMessageResourceName = nameof(Resources.Pages.IndexModel.SuperHeroFieldIsRequired),
    ErrorMessageResourceType = typeof(Resources.Pages.IndexModel)
)]
public string Superhero { get; set; }

The Razor view utilizes the DisplayAttribute to determine a <label> tag's value. ASP.NET uses the RequiredAttribute for validation, and determines the error messages using the attribute properties of ErrorMessageResourceName and ErrorMessageResourceType.

Conclusion

With a few steps, we can understand the ideas behind localizing an ASP.NET application. We can transfer many of our existing skills regarding resource files to the new paradigm created for web applications. Once configured, the localization framework stays out of our way, allowing us to develop our app while also leaving room to expand our audience.

I hope this short tutorial has inspired you to localize your ASP.NET applications. Remember, you can access a working sample of the project seen in the video at this GitHub repository.

More Resources

Some of the resources utilized to write this tutorial are listed below:

Localization Basics