Making an application multilingual is a common feature request nowadays. Therefore Catel provides the resources in several languages and provides the LanguageService to give the developers full control over the translation process in their applications.

Setting up the LanguageService

Setting cultures

By default the LanguageService will use the current UI culture to retrieve the right language values. Inject ILanguageService via the constructor and customize:

private readonly ILanguageService _languageService;

public MyViewModel(IServiceProvider serviceProvider, ILanguageService languageService)
    : base(serviceProvider)
{
    _languageService = languageService;
}

_languageService.PreferredCulture = new CultureInfo("nl-NL");
_languageService.FallbackCulture = new CultureInfo("en-US");

Registering custom language sources

In order to customize the language sources, custom language sources can be registered via the RegisterLanguageSource method.

The code below shows how to add a new LanguageResourceSource which represents a resource file in a specific assembly:

// Create source for assembly MyApplication where the Resources.resx is located in the Properties folder
var resourcesSource = new LanguageResourceSource("MyApplication", "MyApplication.Properties", "Resources");
_languageService.RegisterLanguageSource(resourcesSource);

// Create source for assembly MyApplication where the Exceptions.resx is located in the Properties folder
var exceptionsSource = new LanguageResourceSource("MyApplication", "MyApplication.Properties", "Exceptions");
_languageService.RegisterLanguageSource(exceptionsSource);

The LanguageService will now automatically query these sources for the translations.

Using the LanguageService

To use the LanguageService, inject it via the constructor and use the provided methods. The example below retrieves the WarningTitle resource string in the PreferredCulture. If the resource cannot be found in the PreferredCulture, it will be retrieved for the FallbackCulture. If that cannot be found, it will return null.

var warningTitle = _languageService.GetString("WarningTitle");

Using the LanguageService in XAML

To use the LanguageService in XAML, Catel provides the markup extensions.

Using the LanguageBinding

To use the LanguageBinding markup extension, use the following code:

<TextBlock Text="{markup:LanguageBinding WarningTitle}" />

Implementing custom LanguageService (from database)

Implementing a custom LanguageService consists of several steps which are described below.

Note that this implementation queries the database for each translation. It is best to read all translations into memory at once to improve performance

Creating a custom ILanguageSource implementation

First of all, we need to implement a customized language source to allow the custom service to know what source to read for translations:

public class DbLanguageSource : ILanguageSource
{
    public DbLanguageSource(string connectionString)
    {
        ConnectionString = connectionString;
    }

    public string ConnectionString { get; private set; }

    public string GetSource()
    {
        return ConnectionString;
    }
}

Creating a custom DbLanguageService

Below is a custom implementation of the LanguageService. Note that we only have to derive a single method to fully customize the implementation:

public class DbLanguageService : LanguageService
{
    protected override string GetString(ILanguageSource languageSource, string resourceName, CultureInfo cultureInfo)
    {
        var connectionString = languageSource.GetSource();
        using (var dbConnection = new SqlConnection(connectionString))
        {
            dbConnection.Open();

            var sqlCommand = dbConnection.CreateCommand();
            sqlCommand.CommandType = CommandType.Text;
            sqlCommand.CommandText = @"SELECT [Name] FROM [Translations] WHERE [ResourceName] = @ResourceName AND [CultureName] = @CultureName";
            sqlCommand.Parameters.Add(new SqlParameter("ResourceName", resourceName));
            sqlCommand.Parameters.Add(new SqlParameter("CultureName", cultureInfo.ThreeLetterISOLanguageName));

            var translation = sqlCommand.ExecuteScalar() as string;
            if (!string.IsNullOrWhiteSpace(translation))
            {
                return translation;
            }
        }

        // Resource not found, fall back to base if you like, or simply return null
        return base.GetString(languageSource, resourceName, cultureInfo);
    }
}

Enabling the custom DbLanguageService

To enable the custom DbLanguageService, register it in the service collection:

services.AddSingleton<ILanguageService>(sp =>
{
    var dbLanguageService = new DbLanguageService();
    var dbLanguageSource = new DbLanguageSource("myConnectionString");
    dbLanguageService.RegisterLanguageSource(dbLanguageSource);
    return dbLanguageService;
});