Validation

Plugin.Avalonia.MVVMExpress.Validation ships DataAnnotations plus IValidator and MustMatchAttribute. The package includes ILLink.Descriptors.xml that roots Required, MinLength, MaxLength, StringLength, Range, RegularExpression, EmailAddress, Compare, and MustMatch. Custom attributes need an app-level descriptor. FluentValidation is an adapter the app may add. Avalonia bindings use AvaloniaFormViewModel (INotifyDataErrorInfo).

public sealed class ProductDraft
{
    [Required, StringLength(80)]
    public string Name { get; set; } = "";

    [Range(0.01, 1_000_000)]
    public decimal Price { get; set; }
}

var summary = await validator.ValidateAsync(draft, cancellationToken);
if (!summary.IsValid)
{
    return Outcome.Failure("validation", summary.Messages[0].Message);
}