i can't call ToIndexableCollection()

[hahabrobro]

https://staxmanade.com/2008/12/i4o-indexspecification-for/

in this example i see

// Turn the list of files into an Indexed collection of files
var indexedFileInfosFromDir = fileInfosFromDir.ToIndexableCollection(spec);

but in my environment i can't use object of List call the ToIndexableCollection methend

is any i missing ??