Writing a custom request culture provider in ASP.NET Core 2.1

ASP.NET Core already delivers a handful of request culture providers. But what if you do not want to use one of the default request culture provider like: CookieRequestCultureProvider. With just a little bit of code it is possible to implement your custom way of setting the request culture. We had the requirement that a user can store his desired culture in his profile and so we had to create a custom culture provider.

But first we have to set up localization which is really simple in ASP.NET Core.

Step 1

Localized strings can be partitioned by controller, area or you can have one single container. Out of simplicity we will use one container. Create a dummy class in the root of your project called SharedResource.cs.

namespace MLSoftware.Localization
{
    public class SharedResource
    {
    }
}

Step 2

Next we will have to configure localization inside of our Startup.cs class. Add the following lines to your ConfigureServices method.

services.AddLocalization(options => options.ResourcesPath = "Resources"); // Adds the localization services and sets the resources path to "Resources"

var supportedCultures = new[]
{
    new CultureInfo("en-US"),
    new CultureInfo("de-CH"),
};

services.Configure<RequestLocalizationOptions>(options =>
{
    options.DefaultRequestCulture = new RequestCulture(culture: "de-CH", uiCulture: "de-CH");
    options.SupportedCultures = supportedCultures;
    options.SupportedUICultures = supportedCultures;
});

services.AddMvc()
    .AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix)
    .AddDataAnnotationsLocalization();

Step 3

We have to configure the ASP.NET Core middleware to use the request localization. Add the following lines inside the Configure method.

app.UseRequestLocalization();

Done. Request localization has now been set up. Resource (.resx) files have to be added to the project under /Resources/SharedResource.[language].resx. From now on, on every request the list of culture providers that are configured will be enumerated and the first provider that can successfully resolve the culture will be used.

Creating the custom request culture provider

Microsoft provides three default culture providers that are automatically configured.

QueryStringRequestCultureProvider
CookieRequestCultureProvider
AcceptLanguageHeaderRequestCultureProvider

If you want to create a custom culture provider it is best to clear the list of culture providers and then add your custom culture provider. If required add the default ones back to the list. Update your RequestLocalizationOptions to the following:

services.Configure<RequestLocalizationOptions>(options =>
{
    options.DefaultRequestCulture = new RequestCulture(culture: "de-CH", uiCulture: "de-CH");
    options.SupportedCultures = supportedCultures;
    options.SupportedUICultures = supportedCultures;
		options.RequestCultureProviders.Clear(); // Clears all the default culture providers from the list
    options.RequestCultureProviders.Add(new UserProfileRequestCultureProvider()); // Add your custom culture provider back to the list
});

Now it is time for the last piece of code. The custom culture provider. This is a simple class that inherits from RequestCultureProvider and needs to implement one single method called DetermineProviderCultureResult. Inside this method you can do everything you need to resolve the culture for the request. Remember: This will get called on every request to your server, so don't put any heavy compute calls in here.

public class UserProfileRequestCultureProvider : RequestCultureProvider
{
    public override Task<ProviderCultureResult> DetermineProviderCultureResult(HttpContext httpContext)
    {
        if (httpContext == null)
        {
            throw new ArgumentNullException(nameof(httpContext));
        }

        if (!httpContext.User.Identity.IsAuthenticated)
        {
            return Task.FromResult((ProviderCultureResult)null);
        }

        var culture = httpContext.User.GetCulture();
        if (culture == null)
        {
            return Task.FromResult((ProviderCultureResult)null);
        }

        return Task.FromResult(new ProviderCultureResult(culture));
    }
}