Two WPF converters used to get the item's index inside a ItemsControl

CollectionItemIndexConverter.cs

using System.Collections;
using System.Globalization;
using System.Windows.Data;

class ElementIndexConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        if (values.Length == 2 && values[0] is not null && values[1] is IList items)
            return (items.IndexOf(values[0]) + 1).ToString();
        return Binding.DoNothing;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

ItemIndexConverter.cs

using System.Globalization;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;

[ValueConversion(typeof(DependencyObject), typeof(string))]
class ItemIndexConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is DependencyObject item)
        {
            var box = ItemsControl.ItemsControlFromItemContainer(item);
            var index = box.ItemContainerGenerator.IndexFromContainer(item);
            return (index + 1).ToString();
        }
        return string.Empty;
    }

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