using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        List<GroupResult<string>> intput = new List<GroupResult<string>>(){new GroupResult<string>()

        {Key = "1", Items = new List<string>(){"A", "B"}}, new GroupResult<string>()

        {Key = "1", Items = new List<string>(){"C", "D"}}, new GroupResult<string>()

        {Key = "2", Items = new List<string>(){"E", "F"}}

        };

        List<GroupResult<string>> result = intput.GroupBy(x => x.Key).Select(x => new GroupResult<string>(){Key = x.Key, Items = x.SelectMany(y => y.Items)}).ToList();

        foreach (GroupResult<string> item in result)

        {

            Console.WriteLine("Key:{0}, Values:{1}", item.Key, string.Join(",", item.Items));

        }

    }

​

    public class GroupResult<T>

    {

        public string Key{get;set;}

        public IEnumerable<T> Items{get;set;}

    }

}