A simple C# script to demo MongoDB aggregation performing group by count.

groupby_count_aggregation.cs

using System;
using MongoDB.Bson;
using MongoDB.Driver;
using System.Threading.Tasks;
using System.Linq;

/* Where document structure stored in localhost MongoDB : {token:"word"} 
*/

namespace Aggregation_01
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Start Async ");
            var client = new MongoClient("mongodb://localhost:27017");
            var database = client.GetDatabase("test");
            var collection = database.GetCollection<BsonDocument>("tokens");

            Task t = queryDatabase(collection);
            t.ContinueWith((str) =>
            {
                Console.WriteLine(str.Status.ToString());
                Console.WriteLine("Query Ends.");
            });
            t.Wait();
            Console.ReadKey();
        }

        public async static Task<string> queryDatabase(IMongoCollection<BsonDocument> collection)
        {
            Console.WriteLine("Query Starts...");

            var aggregate = collection.Aggregate()
                                       .Group(new BsonDocument { { "_id", "$token" }, { "count", new BsonDocument("$sum", 1) } })
                                       .Sort(new BsonDocument { { "count", -1 } })
                                       .Limit(10);
            var results = await aggregate.ToListAsync();
            foreach (var obj in results)
            {
                Console.WriteLine(obj.ToString());
            }
            return "finished";
        }
    }
}

Hi @sindbach how to use this group pipeline with in Generic Types ? Is it possible ?