recursive talk - dictionary - make it compile - learn to check type using "is string", and type casting in the code writing process

RecursiveTalk_Dictionary_compiled.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace FlatDictionary_MockingExperience
{
    // Flat a ctionary 
    // key: string 
    // value is string, sometimes it is dictionary 
    // value '1' - string
    // 'a': '2'   key 'a' value string '2'
    // 'c':  key, value is Dictionary 
    // key1: 1
    // value, how can I determine that it is a string or dictionary 
    // Key2.a: '2'
    // key2.b: '3'
    // key is constructed by concatenated key2 -> DFS -> recursive function 
    // keystring prefix = empty string 
    // create new Dictionary <string, string> 
    // step 1: Dictionary -> keys -> iterate on the key
    // step 2: for each key, value object -> Determine Runtime type -> string , Dictionary
    // step 3: output the key value pair to new dictionary -> key, value is the same
    // step 4: if value is Dictinary, continue step 1 -> pass into key string prefix
    // return new dictionary <string, string>       

    class LearningExperience {
        static void Main(string[] args) {
          Console.WriteLine("Meet a talent and talk about algorithm");
          var data = new Dictionary<string, object>(); 
          data.Add("key1","1"); 

          var data2 = new Dictionary<string, object>(); 
          data.Add("key2","3");

          data.Add("key2", data2); 
         
          var flatten = new Dictionary<string, object>();
          FlattenDictionary(data, flatten, string.Empty);
       }
   
       /// key1: 1
       /// DFS -> overflow -> too deep -> iterative solution out-of-stack
       // 
       public static void FlattenDictionary(Dictionary<string, object> data, Dictionary<string, object> flatten, string prefix)
       {
          if(data == null || data.Count == 0) return; 
      
          foreach(string key in data.Keys)
          {
             var current = data[key]; 
             var newKey = (prefix == null)? key : (prefix +'.' +key) ; 
             if(current is string)
             {
                flatten.Add(newKey, current);
             }
             else
             {
                // it is a dictionary 
                prefix = newKey; // Key2
                FlattenDictionary((Dictionary<string, object>)current, flatten, prefix); 
             }
          }
      
          // do nothing 
          return; 
       }
    }
}