public interface ILine
{
}

public class HeaderLine : ILine
{
	public byte HeaderLevel { get; set; }
	public string Text { get; set; }
}

public class ImageLine : ILine
{
	public byte[] Data { get; set; }
}

public class TextLine : ILine
{
	public string Text { get; set; }
}

private List<ILine> _lines = new List<ILine>();

public virtual async Task<IEnumerable<ILine>> ConvertAsync(FB2File file)
{
	return await Task.Factory.StartNew(() =>
	{
		_file = file;

		if (_file.MainBody != null)
		{
			PrepareBodies();
		}

		return _lines;
	});
}

protected virtual void PrepareBodies()
{
	foreach (var bodyItem in _file.Bodies)
	{
		AddTitle(bodyItem.Title);

		foreach (SectionItem sectionItem in bodyItem.Sections)
		{
			PrepareTextItem(sectionItem);
		}
	}
}

protected virtual void PrepareTextItems(IEnumerable<IFb2TextItem> textItems)
{
	foreach (var textItem in textItems)
	{
		if (textItem is IFb2TextItem)
		{
			PrepareTextItem(textItem);
		}
		else
		{
			AddTextLine(textItem.ToString());
		}
	}
}

protected virtual void PrepareTextItem(IFb2TextItem textItem)
{
	if (textItem is CiteItem)
	{
		PrepareTextItems(((CiteItem)textItem).CiteData);
		return;
	}

	if (textItem is PoemItem)
	{
		var item = (PoemItem)textItem;
		AddTitle(item.Title);
		PrepareTextItems(item.Content);
		return;
	}

	if (textItem is SectionItem)
	{
		var item = (SectionItem)textItem;
		AddTitle(item.Title);
		PrepareTextItems(item.Content);
		return;
	}

	if (textItem is StanzaItem)
	{
		var item = (StanzaItem)textItem;
		AddTitle(item.Title);
		PrepareTextItems(item.Lines);
		return;
	}

	if (textItem is ParagraphItem
		|| textItem is EmptyLineItem
		|| textItem is TitleItem
		|| textItem is SimpleText)
	{
		AddTextLine(textItem.ToString());
		return;
	}

	if (textItem is ImageItem)
	{
		var item = (ImageItem)textItem;
		var key = item.HRef.Replace("#", string.Empty);

		if (_file.Images.ContainsKey(key))
		{
			var data = _file.Images[key].BinaryData;
			_lines.Add(new ImageLine { Data = data });
		}
		return;
	}

	if (textItem is DateItem)
	{
		AddTextLine(((DateItem)textItem).DateValue.ToString());
		return;
	}

	if (textItem is EpigraphItem)
	{
		var item = (EpigraphItem) textItem;
		PrepareTextItems(item.EpigraphData);
		return;
	}

	throw new Exception(textItem.GetType().ToString());
}

protected virtual void AddTitle(TitleItem titleItem)
{
	if (titleItem != null)
	{
		foreach (var title in titleItem.TitleData)
		{
			_lines.Add(new HeaderLine { Text = title.ToString() });
		}
	}
}

protected virtual void AddTextLine(string text)
{
	_lines.Add(new TextLine { Text = text });
}