public static int FindIndex <TSource, TTemporary, TElement>(this IEnumerable <TSource> source, Func <TSource, TTemporary> sourceSelector, TElement element, Func <TElement, TTemporary> elementSelector) => EnumerableEx.FindIndex(source, sourceSelector, element, elementSelector, EqualityComparer <TTemporary> .Default);
public static int FindIndex <TSource>(this IEnumerable <TSource> source, TSource element) => EnumerableEx.FindIndex(source, element, EqualityComparer <TSource> .Default);