Beginners' guide to using MongoDB 4.0.2 and the official C# driver

using System;
using System.Collections.Generic;
using System.Linq;

namespace TestMongo
{
    using MongoDB.Bson;
    using MongoDB.Driver;
    using MongoDB.Driver.Builders;

    public partial class Program
    {

        public static void DemoMapReduce(MongoCollection<ClubMember> collection)
        {
            //The match function has to be written in JavaScript 
            //the map method outputs a key value pair for each document
            //in this case the key is the Lastname property value and the value is the Age property value
            var map =
                new BsonJavaScript(
                    @"function() 
                                            {
                                             //Associate each LastName property with the Age value
                                               emit(this.Lastname,this.Age);
                                             }");
            //The MapReduce method  uses the output from the Map method
            //   to produce a list of ages for each unique LastName value.
            //  Then each key and its associated list of values is presented to the Reduce method in turn.
            var reduce =
                new BsonJavaScript(
                    @"function(lastName,ages) 
                                             {
                                                 return Array.sum(ages);
                                              }");
            //The Reduce method returns the Lastname as the key and the sum of the ages as the value
            //The beauty of this technique is that data can be processed in batches
            //The output of one batch is combined with that of another and fed back through the Reducer
            //This is repeated until the output is reduced to the number of unique keys.
            // The results are output to a new collection named ResultsCollection on the server. 
            // This saves on the use of computer memory and enables the results to be queried effectively by using indexes. 
            IEnumerable<BsonDocument> resultAsBsonDocumentCollection =
                collection.MapReduce(map, reduce, MapReduceOptions.SetOutput(MapReduceOutput.Replace("Reduce"))).
                    GetResults();
            Console.WriteLine("The total age for every member of each family  is ....");
            var reduction =
                resultAsBsonDocumentCollection.Select(
                    doc => new { family = doc["_id"].AsString, age = (int)doc["value"].AsDouble });
            foreach (var anon in reduction)
            {
                Console.WriteLine("{0} Family Total Age {1}", anon.family, anon.age);
            }
        }
    }
}