https://www.codewars.com/kata/554e4a2f232cdd87d9000038/train/csharp

ComplementaryDNA.cs

using System;
using System.Collections.Generic;

public class DnaStrand 
    {
        public static string MakeComplement(string dna)
        {
            List<char> compliment = new List<char>();
            
            foreach (var nucleo in dna)
            {
              switch (nucleo)
              {
                case 'A':
                  compliment.Add('T');
                  break;
                case 'T':
                  compliment.Add('A');
                  break;
                case 'C':
                  compliment.Add('G');
                  break;
                case 'G':
                  compliment.Add('C');
                  break;
              }
            }
            return string.Join("", compliment);
        }