Fórum GroupBy ➡ Sum & Aggregate customers' names, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

// aljodav
// http://rextester.com/users/376

namespace Rextester
{
    using D=Rextester.CusCatData;
    public class CusCatData{
        public string Cus,Cat;
        public int S15,S16;
    }
    public class Program
    {
        public static void Main(string[] args)
        {
            List<D> datas= new List<D>{
                new D{Cus="kim",Cat="T3",S15=2,S16=5},
                    new D{Cus="kay",Cat="T2",S15=7,S16=6},
                    new D{Cus="ann",Cat="T1",S15=3,S16=3},
                    new D{Cus="jim",Cat="T2",S15=5,S16=6},
                    new D{Cus="tim",Cat="T1",S15=0,S16=5},
                    new D{Cus="bel",Cat="T2",S15=3,S16=6},
            };
            datas
                .GroupBy(x=>x.Cat,(cat,xs)=>new{
                    Cat=cat,
                    S15=xs.Sum(y=>y.S15),
                    S16=xs.Sum(y=>y.S16),
                    All=xs.Aggregate("",(a,y)=>(a==""?a:a+",")+y.Cus),
                })
                .ToList()
                .ForEach(grp=>Console.WriteLine("{0,3}  {1,3}  {2,3}  {3}",grp.Cat,grp.S15,grp.S16,grp.All))
                ;
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,19 sec, absolute running time: 0,14 sec, cpu time: 0,12 sec, average memory usage: 14 Mb, average nr of threads: 3