How to localize Data Annotation validation error messages in a WinUI3 MVVM application?

I'm building a WinUI3 application using the MVVM Toolkit with [ObservableProperty] and ObservableValidator. I'm using Data Annotations like [Required], [MinLength], [MaxLength] to validate my properties.

I want to localize the validation error messages using .resw resource files. so that when the application language changes, the validation messages are automatically shown in the correct language.

Here is a simplified example from my ViewModel:

public partial class MyViewModel: ObservableValidator
{
    [ObservableProperty]
    [Required(ErrorMessage = "First name is required!")]
    [MinLength(3, ErrorMessage = "First name should be minimum 3 characters!")]
    [MaxLength(100, ErrorMessage = "First name should be maximum 100 characters!")]
    public partial string? FirstName { get; set; }
}

❓ What I want to achieve:

When I change the application language, the ErrorMessage for each validation attribute should be localized using .resw strings or any other method.

❓ My questions:

1. How can I use .resw localization with ErrorMessage, or is there a workaround for WinUI3?
2. Can ErrorMessageResourceType and ErrorMessageResourceName work with .resw files in WinUI3?
3. What's the best practice to handle validation localization in WinUI3 apps using MVVM Toolkit?

🔧 Environment:

• WinUI3 (.NET 9)
• MVVM Toolkit (ObservableValidator, [ObservableProperty])
• Localization using .resw resource files. and localize language flow direction is RightToLeft.
• Manual validation using ValidateAllProperties()

Any guidance or examples would be appreciated. Thanks!

To implement localization, check:

• This answer
• Localize strings in your UI and the app package manifest

Now let me show you an example with the ObservableValidator:

Resources

SomePage.xaml

<StackPanel Orientation="Horizontal">
    <TextBox Text="{x:Bind ViewModel.Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
    <Button Command="{x:Bind ViewModel.SubmitCommand}" Content="Submit" />
</StackPanel>
<TextBlock Text="{x:Bind ViewModel.ErrorMessage, Mode=OneWay}" />

SomePageViewModel.cs

public partial class SomePageViewModel : ObservableValidator
{
    [ObservableProperty]
    [Required(ErrorMessage = "NameIsRequiredErrorMessage")]
    public partial string Name { get; set; }

    [ObservableProperty]
    public partial string ErrorMessage { get; protected set; } = string.Empty;

    [RelayCommand]
    private void Submit()
    {
        ValidateAllProperties();

        if (HasErrors)
        {
            string errorMessage = GetErrors()?
                .FirstOrDefault()?
                .ErrorMessage ?? "Unknown error";
            var resourceLoader = new Microsoft.Windows.ApplicationModel.Resources.ResourceLoader();
            string localizedErrorMessage = resourceLoader.GetString(errorMessage);
            ErrorMessage = !string.IsNullOrEmpty(localizedErrorMessage)
                ? localizedErrorMessage
                : errorMessage;
        }
    }
}