If the element is present in the collection, this method will return the
zero-based index of the first occurrence of it; otherwise this method will
return -1.
Remarks
This method implements a standard collection operation. It searches
the collection for a reference to the given element. If it finds a
reference to it, it will return its index. If it cannot find a reference
to it, this method will return -1. If the element is present more than
once in the collection, this method will return the lowest index of a
match.