String combinations, 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;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
           
            var sln = new solution();
            Dictionary<int, string> dctnry= new Dictionary<int, string>();
            
            dctnry.Add(0, "abc");
            dctnry.Add(1, "def");
            dctnry.Add(2, "ghi");
            
            List<string> strList = sln.GetStrCombinations(dctnry, new int[] {0,1,2});
            
            foreach(var str in strList)
            {
                Console.WriteLine(str);
            }
            
            Console.WriteLine("-----------------");
            
            strList = sln.GetStrCombinations2(dctnry, new int[] {0,1,2});
            
            foreach(var str in strList)
            {
                Console.WriteLine(str);
            }

}
    }
    
    public class solution {
              
        
        public List<string> GetStrCombinations(Dictionary<int, string> strDictionary, int[] keys){
            
            List<string> strCombinations = new List<string> ();
            
            GetStrRecursive(strDictionary, ref strCombinations, keys, string.Empty);
            return strCombinations;
            
        }
        
        private void GetStrRecursive(Dictionary<int, string> strdctnry, ref List<string> strlist, int[] keys, string str){
            
            if( str.Length == keys.Length)
            {
                strlist.Add(str);
                return;
            }
            
            string temp = strdctnry[keys[str.Length]];
                
            for(int j = 0 ; j < temp.Length; j++){
                GetStrRecursive(strdctnry, ref strlist, keys, str + temp[j]);
            } 
                
            
        }
        
        public List<string> GetStrCombinations2(Dictionary<int, string> dctnry , int [] keys ){
            
           List<string> strLists= new List<string>();
            if ( keys.Length == 0 ) {
                return strLists;
            }
            
            var str = dctnry[keys[0]];
            
            for (int i = 0 ; i < str.Length ; i++){
                strLists.Add( "" + str[i]);
            }
            
            for(int i = 1 ; i < keys.Length; i++){
                strLists = GetStrIterative(strLists, dctnry[keys[i]]);
            }
            
          return strLists;  
                 
        }
        
        
        private List<string> GetStrIterative(List<string>strList, string str){
            
            var newStrList  = new List<string> ();
            
            foreach( var s in strList){
                
                for(int i =0; i < str.Length ; i ++){
                    
                    newStrList.Add(s + str[i]);
                }
            }
           return newStrList; 
        }
        
    }

}

Show compiler warnings [ + ] Show input
Compilation time: 0,16 sec, absolute running time: 0,08 sec, cpu time: 0,08 sec, average memory usage: 12 Mb, average nr of threads: 3