index-state.cs

var docs = new ConcurrentDictionary<long, DocRef>();
long index = 0;

var fields = new ConcurrentDictionary<string, ConcurrentQueue<long>>[]
{
    new ConcurrentDictionary<string, ConcurrentQueue<long>>(), // Body
    new ConcurrentDictionary<string, ConcurrentQueue<long>>(), // Subject
    new ConcurrentDictionary<string, ConcurrentQueue<long>>(), // Date
    new ConcurrentDictionary<string, ConcurrentQueue<long>>(), // From
    new ConcurrentDictionary<string, ConcurrentQueue<long>>(), // To 
};