using System;

using System.Collections.Generic;

using System.Linq;

​

public static class Extensions

{

    public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)

    {

        HashSet<TKey> seenKeys = new HashSet<TKey>();

        foreach (TSource element in source)

        {

            if (seenKeys.Add(keySelector(element)))

            {

                yield return element;

            }

        }

    }        

}

public class Program

{

    public class TypeValue

    {

        public string Type;

        public string Value;

    }   

    public static void Main()

    {

        List<TypeValue> objs = new List<TypeValue>(){

            new TypeValue() { Type = "a", Value = "a1" },

            new TypeValue() { Type = "a", Value = "a2" },

            new TypeValue() { Type = "b", Value = "b" },

            new TypeValue() { Type = "c", Value = "c" },

        };

        var o2 = objs

            .DistinctBy(x => x.Type)

            .Select(x => new TypeValue() {

                Type = x.Type,

                Value = String.Join(", ", objs.Where(o => o.Type == x.Type).Select(o => o.Value).ToList())

            })          

            .ToList();

        foreach (var o in o2)

        {

            Console.WriteLine(o.Value);

        }

    }

}