Why the use of ReadOnlyCollection in this Convertion method

Hi there:

I'm studying Data Validation in Bindings, and I ran accross this Validation method:

public class ErrorsToMessageConverter : IValueConverter {
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
        var sb = new StringBuilder();
        var errors = value as ReadOnlyCollection<ValidationError>;
        if (errors != null) {
            foreach (var e in errors.Where(e => e.ErrorContent != null)) {
                sb.AppendLine(e.ErrorContent.ToString());
            }
        }

        return sb.ToString();
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
        throw new NotImplementedException();
    }
}

This is the xaml that uses it:

<TextBlock Margin="2" Foreground="Red" FontWeight="Bold" 
        Text="{Binding ElementName=AddressBox, 
                        Path=(Validation.Errors),
                        Converter={StaticResource ErrorsToMessageConverter}}" />

It's clear that we are obtaining a collection of errors (Validation.Errors), but why do we use
ReadOnlyCollection<ValidationError> instead of just Collection or Array?, I tried changing it, and the app ceased to work!

Thanks in advance,

Rafael

Believe you can do it, and you will!!