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在Lucene.net实现自定义排序，需要实现两个Lucene.Net.Search的两个接口：
public interface SortComparatorSource
{
   ScoreDocComparator NewComparator(IndexReader reader , System.String fieldname) ;
}

public interface ScoreDocComparator
{
   int Compare(ScoreDoc i , ScoreDoc j) ;
   System.IComparable SortValue(ScoreDoc i) ;
   int SortType() ;
}

涉及到的一个类：
public class ScoreDoc
{
   public float score ;
   public int doc ;
   public ScoreDoc(int doc , float score)
   {
      this.doc = doc ;
      this.score = score ;
   }
}

Lucene.net 2.0包含的SortType有：
在Lucene.Net.Search.SortField里定义的：
public class SortField
{
   public const int SCORE = 0 ;  //相关度
   public const int DOC = 1 ;    //文挡号
   public const int AUTO = 2 ;   //自动识别
   public const int STRING = 3 ; //字符型
   public const int INT = 4 ;    //int
   public const int FLOAT = 5 ;  //float
   public const int CUSTOM = 9 ; //自定义
   .....
}

少了DateTime,那就实现DateTime类型的自定义排序来测试下：
Lucene.Net.Search.ScoreDocComparator接口的实现类:
    public class DateDocComparator : Lucene.Net.Search.ScoreDocComparator
    {
        private string fieldname = null;
        private System.IComparable[] cachedValues ;

        public DateDocComparator(System.IComparable[] cachedValues, string fieldname)
        {
            this.cachedValues = cachedValues;
            this.fieldname = string.Intern(fieldname) ;
        }

        public int Compare(ScoreDoc i, ScoreDoc j)
        {
            return this.cachedValues[i.doc].CompareTo(this.cachedValues[j.doc]) ;
        }

        public System.IComparable SortValue(ScoreDoc i)
        {
            return this.cachedValues[i.doc] ;
        }

        public int SortType()
        {
            return Lucene.Net.Search.SortField.CUSTOM ;
        }
    }

Lucene.Net.Search.SortComparatorSource接口的实现类:
    public class DateSortComparatorSource : Lucene.Net.Search.SortComparatorSource
    {
        public ScoreDocComparator NewComparator(Lucene.Net.Index.IndexReader reader, System.String field)
        {
            return new DateDocComparator(GetCustom(reader, field), field);
        }

        protected virtual System.IComparable[] GetCustom(Lucene.Net.Index.IndexReader reader, System.String field)
        {
                System.IComparable[] retArray = new System.IComparable[reader.MaxDoc()];
                Lucene.Net.Index.TermDocs termDocs = reader.TermDocs();
                Lucene.Net.Index.TermEnum termEnum = reader.Terms(new Lucene.Net.Index.Term(field, ""));
                try
                {
                    do
                    {
                        Lucene.Net.Index.Term term = termEnum.Term();
                        if (term == null || term.Field() != field)
                            break;
                        System.IComparable termval = Lucene.Net.Documents.DateTools.StringToDate(term.Text()) ;
                        termDocs.Seek(termEnum);
                        while (termDocs.Next())
                        {
                            retArray[termDocs.Doc()] = termval;
                        }
                    }
                    while (termEnum.Next());
                }
                finally
                {
                    termDocs.Close();
                    termEnum.Close();
                }
                return retArray;
        }
    }

使用：
Sort sort = new Sort(new SortField("datecreated",new DateSortComparatorSource(),true)) ;