Alternative for SetNextReader to return all strings about lucenenet HOT 1 OPEN

The Migration Guide covers this very issue with an example:

LUCENE-2380: FieldCache.GetStrings/Index --> FieldCache.GetDocTerms/Index

• The field values returned when sorting by SortField.STRING are now
  BytesRef. You can call value.Utf8ToString() to convert back to
  string, if necessary.

• In FieldCache, GetStrings (returning string[]) has been replaced
  with GetTerms (returning a BinaryDocValues instance).
  BinaryDocValues provides a Get method, taking a docID and a BytesRef
  to fill (which must not be null), and it fills it in with the
  reference to the bytes for that term.
  
  If you had code like this before:

  string[] values = FieldCache.DEFAULT.GetStrings(reader, field);
  ...
  string aValue = values[docID];

  you can do this instead:

  BinaryDocValues values = FieldCache.DEFAULT.GetTerms(reader, field);
  ...
  BytesRef term = new BytesRef();
  values.Get(docID, term);
  string aValue = term.Utf8ToString();

  Note however that it can be costly to convert to String, so it's better to work directly with the BytesRef.

• Similarly, in FieldCache, GetStringIndex (returning a StringIndex
  instance, with direct arrays int[] order and String[] lookup) has
  been replaced with GetTermsIndex (returning a
  SortedDocValues instance). SortedDocValues provides the
  GetOrd(int docID) method to lookup the int order for a document,
  LookupOrd(int ord, BytesRef result) to lookup the term from a given
  order, and the sugar method Get(int docID, BytesRef result)
  which internally calls GetOrd and then LookupOrd.
  
  If you had code like this before:

  StringIndex idx = FieldCache.DEFAULT.GetStringIndex(reader, field);
  ...
  int ord = idx.order[docID];
  String aValue = idx.lookup[ord];

  you can do this instead:

  DocTermsIndex idx = FieldCache.DEFAULT.GetTermsIndex(reader, field);
  ...
  int ord = idx.GetOrd(docID);
  BytesRef term = new BytesRef();
  idx.LookupOrd(ord, term);
  string aValue = term.Utf8ToString();

  Note however that it can be costly to convert to String, so it's better to work directly with the BytesRef.
  
  DocTermsIndex also has a GetTermsEnum() method, which returns an iterator (TermsEnum) over the term values in the index (ie, iterates ord = 0..NumOrd-1).

Furthermore, if you drill down into the issue LUCENE-2380, there is an explanation for the change: primarily, this was done for performance reasons. There is no longer a string[] stored in the field cache, the underlying data is now a byte[] so extra steps are required to get a UTF8 string.

Do note that you are meant to reuse the BytesRef instance that is passed in to get better performance.

from lucenenet.