using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Product[] ProArr = new Product[3];

        ProArr[0] = new Product()

        {Id = 1, Name = "Name1", Color = "Green", Size = new string[]{"XL"}};

        ProArr[1] = new Product()

        {Id = 1, Name = "Name1", Color = "Blue", Size = new string[]{"XL"}};

        ProArr[2] = new Product()

        {Id = 3, Name = "Name2", Color = "Red", Size = new string[]{"S"}};

        var result2 = ProArr.GroupBy(p => new

        {

        p.Id, p.Name, p.Size

        }

​

        ).Select(x => new

        {

        Id = x.Key.Id, Name = x.Key.Name, Color = String.Join(",", x.Select(c => c.Color)), Size = x.Key.Size.First().ToString()}

​

        );

        foreach (var result in result2)

        {

            Console.WriteLine(result);

        }

    }

}

​

public class Product

{

    public int Id;

    public string Name;

    public string Color;

    public string[] Size;

}