Localizing dialogs in MVVM: is it good to reference the resources from the ViewModel?

Question

Resources is the data that uses a View, and my opinion is that is not advisable from the ViewModel refer to resources. On the other hand, if it is a class (may be static) that stores a specific strings, and knows nothing of the View it will be some abstraction that can be in the ViewModel. In any case, you should try to work with the resources on the side View using techniques that I will give, or any other.

Using x:Static Member

In WPF, it is possible to bind static data from a class like this:

<x:Static Member="prefix : typeName . staticMemberName" .../>

Below is an example where the format string is in a class, the format used to display the date and time.

XAML

xmlns:local="clr-namespace:YourNameSpace"
xmlns:sys="clr-namespace:System;assembly=mscorlib"

<Grid>
    <TextBlock Text="{Binding Source={x:Static sys:DateTime.Now}, StringFormat={x:Static Member=local:StringFormats.DateFormat}}" 
               HorizontalAlignment="Right" />

    <TextBlock Text="{Binding Source={x:Static sys:DateTime.Now}, StringFormat={x:Static Member=local:StringFormats.Time}}" />
</Grid>

Code behind

public class StringFormats 
{
    public static string DateFormat = "Date: {0:dddd}";

    public static string Time = "Time: {0:HH:mm}";
}

In this case, the StringFormats class be regarded as a resource, although actually it is a normal class. For more information, please see x:Static Markup Extension on MSDN.

Using Converter

If you have the resources stored in Application.Current.Resources and need to add some logic, in this case, you can use the converter. This example is taken from here:

XAML

<Button Content="{Binding ResourceKey, Converter={StaticResource resourceConverter}}" />

Code behind

public class StaticResourceConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var resourceKey = (string)value;

        // Here you can add logic

        return Application.Current.Resources[resourceKey];
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new Exception("The method or operation is not implemented.");
    }
}

Note: In the converter, it is better not to use heavy logic, because it can affect the performance. For more complex logic, see below.

Attached Behavior

Attached behavior should be used for complex actions with visual elements when no x:Static Member and converter is not helped. Attached behavior is very powerful and convenient solution that fully satisfies the MVVM pattern, which can also be used in the Blend (with a pre-defined interface). You can define an attached property in which property handler to access elements and to its resources.

Examples of implementation attached behaviors, see below:

Set focus to a usercontrol when it is made visible

Animated (Smooth) scrolling on ScrollViewer

Setting WindowStartupLocation from ResourceDictionary throws XamlParseException

Example with converter

App.xaml

Here I store strings for each culture.

<Application x:Class="MultiLangConverterHelp.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:sys="clr-namespace:System;assembly=mscorlib"
             StartupUri="MainWindow.xaml">

    <Application.Resources>
        <sys:String x:Key="HelloStringEN">Hello in english!</sys:String>
        <sys:String x:Key="HelloStringRU">Привет на русском!</sys:String>
    </Application.Resources>
</Application>

MainWindow.xaml

The input is the current culture, which can be obtained within the converter, for simplicity of an example I did so.

<Window x:Class="MultiLangConverterHelp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        
        xmlns:local="clr-namespace:MultiLangConverterHelp"
        WindowStartupLocation="CenterScreen"
        Title="MainWindow" Height="350" Width="525">

    <Window.Resources>
        <local:StaticResourceConverter x:Key="converter" />
        <local:TestViewModel x:Key="viewModel" />
    </Window.Resources>

    <Grid DataContext="{StaticResource viewModel}">
        <TextBlock Text="{Binding Path=CurrentCulture, Converter={StaticResource converter}}" />
    </Grid>
</Window>

MainWindow.xaml.cs

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }
}

public class StaticResourceConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var currentCulture = (string)value;

        if (currentCulture.Equals("EN-en")) 
        {
            return Application.Current.Resources["HelloStringEN"];
        }
        else if (currentCulture.Equals("RU-ru"))
        {
            return Application.Current.Resources["HelloStringRU"];
        }

        return null;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return DependencyProperty.UnsetValue;
    }
}

public class TestViewModel : NotificationObject
{
    private string _currentCulture = "EN-en";

    public string CurrentCulture
    {
        get
        {
            return _currentCulture;
        }

        set
        {
            _currentCulture = value;
            NotifyPropertyChanged("CurrentCulture");
        }
    }
}

Also, I advise you to learn more simple ways, which is already in the WPF technology:

WPF Localization for Dummies

WPF Globalization and Localization Overview