Localization Extensibility

By Hisham Bin Ateya

This article:

  • Lists the extensibility points on the localization APIs.
  • Provides instructions on how to extend ASP.NET Core app localization.

Extensible Points in Localization APIs

ASP.NET Core localization APIs are built to be extensible. Extensibility allows developers to customize the localization according to their needs. For instance, OrchardCore has a POStringLocalizer. POStringLocalizer describes in detail using Portable Object localization to use PO files to store localization resources.

This article lists the two main extensibility points that localization APIs provide:

Localization Culture Providers

ASP.NET Core localization APIs have four default providers that can determine the current culture of an executing request:

The preceding providers are described in detail in the Localization Middleware documentation. If the default providers don't meet your needs, build a custom provider using one of the following approaches:

Use CustomRequestCultureProvider

CustomRequestCultureProvider provides a custom RequestCultureProvider that uses a simple delegate to determine the current localization culture:

options.RequestCultureProviders.Insert(0, new CustomRequestCultureProvider(async context =>
{
    var currentCulture = "en";
    var segments = context.Request.Path.Value.Split(new char[] { '/' }, 
        StringSplitOptions.RemoveEmptyEntries);

    if (segments.Length > 1 && segments[0].Length == 2)
    {
        currentCulture = segments[0];
    }

    var requestCulture = new ProviderCultureResult(culture);

    return Task.FromResult(requestCulture);
}));
options.AddInitialRequestCultureProvider(new CustomRequestCultureProvider(async context =>
{
    var currentCulture = "en";
    var segments = context.Request.Path.Value.Split(new char[] { '/' }, 
        StringSplitOptions.RemoveEmptyEntries);

    if (segments.Length > 1 && segments[0].Length == 2)
    {
        currentCulture = segments[0];
    }

    var requestCulture = new ProviderCultureResult(culture);

    return Task.FromResult(requestCulture);
}));

Use a new implemetation of RequestCultureProvider

A new implementation of RequestCultureProvider can be created that determines the request culture information from a custom source. For example, the custom source can be a configuration file or database.

The following example shows AppSettingsRequestCultureProvider, which extends the RequestCultureProvider to determine the request culture information from appsettings.json:

public class AppSettingsRequestCultureProvider : RequestCultureProvider
{
    public string CultureKey { get; set; } = "culture";

    public string UICultureKey { get; set; } = "ui-culture";

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

        var configuration = httpContext.RequestServices.GetService<IConfigurationRoot>();
        var culture = configuration[CultureKey];
        var uiCulture = configuration[UICultureKey];

        if (culture == null && uiCulture == null)
        {
            return Task.FromResult((ProviderCultureResult)null);
        }

        if (culture != null && uiCulture == null)
        {
            uiCulture = culture;
        }

        if (culture == null && uiCulture != null)
        {
            culture = uiCulture;
        }
        
        var providerResultCulture = new ProviderCultureResult(culture, uiCulture);

        return Task.FromResult(providerResultCulture);
    }
}

Localization resources

ASP.NET Core localization provides ResourceManagerStringLocalizer. ResourceManagerStringLocalizer is an implementation of IStringLocalizer that is uses resx to store localization resources.

You aren't limited to using resx files. By implementing IStringLocalized, any data source can be used.

The following example projects implement IStringLocalizer: