using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    internal class EventWithDescription

    {

        public string EventKey { get; set; }

        public override string ToString()

        {

            return EventKey;

        }

    }

​

    internal class EventPropertiesRepository

    {

        public bool TryGet(string eventKey, out TEvetProperties evetProperties)

        {

            evetProperties = null;

            return true;

        }

    }

​

    internal class TEvetProperties

    {

    }

    private static EventWithDescription Map(TEvetProperties eventProperties, EventWithDescription mappedEvent)

    {

        return new EventWithDescription {EventKey = string.Format("new key with {0}", mappedEvent.EventKey)};

    }

​

    private static IEnumerable<EventWithDescription> FillEventsProperties(EventWithDescription[] mappedEvents)

    {

            var eventPropertiesRepository = new EventPropertiesRepository();

            var result = new List<EventWithDescription>();

​

            foreach (var grouping in mappedEvents.GroupBy(x => x.EventKey))

            {

                TEvetProperties eventProperties = null;

                if (eventPropertiesRepository.TryGet(grouping.Key, out eventProperties))

                {

                    result.AddRange(grouping.Select(x => Map(eventProperties, x)));

                }

            }

​

            return result;

    }

    public static void Main()

    {

        var eventWithDescriptions = new[]

            {

                new EventWithDescription {EventKey = "1"},

                new EventWithDescription {EventKey = "1"},

                new EventWithDescription {EventKey = "2"}

            };

​

        var newEvents = FillEventsProperties(eventWithDescriptions);

        foreach(var newEvent in newEvents)

        {

            Console.WriteLine(newEvent);

        }

    }

}