Using IFilter in C#

using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
using System.Runtime.InteropServices;

namespace EPocalipse.IFilter
{
  /// <summary>
  /// Implements a TextReader that reads from an IFilter. 
  /// </summary>
  public class FilterReader : TextReader
  {
    IFilter _filter;
    private bool _done;
    private STAT_CHUNK _currentChunk;
    private bool _currentChunkValid;
    private char[] _charsLeftFromLastRead;

    public override void Close()
    {
      Dispose(true);
      GC.SuppressFinalize(this);
    }

    ~FilterReader()
    {
      Dispose(false);
    }

    protected override void Dispose(bool disposing)
    {
      if (_filter!=null)
        Marshal.ReleaseComObject(_filter);
    }

    public override int Read(char[] array, int offset, int count)
    {
      int endOfChunksCount=0;
      int charsRead=0;

      while (!_done && charsRead<count)
      {
        if (_charsLeftFromLastRead!=null)
        {
          int charsToCopy=(_charsLeftFromLastRead.Length<count-charsRead)?_charsLeftFromLastRead.Length:count-charsRead;
          Array.Copy(_charsLeftFromLastRead,0, array, offset+charsRead, charsToCopy);
          charsRead+=charsToCopy;
          if (charsToCopy<_charsLeftFromLastRead.Length)
          {
            char[] tmp=new char[_charsLeftFromLastRead.Length-charsToCopy];
            Array.Copy(_charsLeftFromLastRead, charsToCopy, tmp, 0, tmp.Length);
            _charsLeftFromLastRead=tmp;
          }
          else
            _charsLeftFromLastRead=null;
          continue;
        };

        if (!_currentChunkValid)
        {
          IFilterReturnCode res=_filter.GetChunk(out _currentChunk);
          _currentChunkValid=(res==IFilterReturnCode.S_OK) && ((_currentChunk.flags & CHUNKSTATE.CHUNK_TEXT)!=0);

          if (res==IFilterReturnCode.FILTER_E_END_OF_CHUNKS)
            endOfChunksCount++;

          if (endOfChunksCount>1)
            _done=true; //That's it. no more chuncks available
        }

        if (_currentChunkValid)
        {
          uint bufLength=(uint)(count-charsRead);
          if (bufLength<8192)
            bufLength=8192; //Read ahead

          char[] buffer=new char[bufLength];
          IFilterReturnCode res=_filter.GetText(ref bufLength, buffer);
          if (res==IFilterReturnCode.S_OK || res==IFilterReturnCode.FILTER_S_LAST_TEXT)
          {
            int cRead=(int)bufLength;
            if (cRead+charsRead>count)
            {
              int charsLeft=(cRead+charsRead-count);
              _charsLeftFromLastRead=new char[charsLeft];
              Array.Copy(buffer, cRead-charsLeft, _charsLeftFromLastRead, 0, charsLeft);
              cRead-=charsLeft;
            }
            else
              _charsLeftFromLastRead=null;

            Array.Copy(buffer, 0, array, offset+charsRead, cRead);
            charsRead+=cRead;
          }

          if (res==IFilterReturnCode.FILTER_S_LAST_TEXT || res==IFilterReturnCode.FILTER_E_NO_MORE_TEXT)
            _currentChunkValid=false;
        }
      }
      return charsRead;
    }

    public FilterReader(string fileName)
    {
      _filter=FilterLoader.LoadAndInitIFilter(fileName);
      if (_filter==null)
        throw new ArgumentException("no filter defined for "+fileName);
    }
  }
}