Recursive practice - dictionary - how to flatten it?

RecursivePractice_Dictionary.cs

using System;
using System.Collections.Generic;

class Solution
{
  public static Dictionary<string, string> FlattenDictionary(Dictionary<string, object> dict)
  {
    // your code goes here
    var flatten = new Dictionary<string, string>(); 
    
    FlattenDictionaryHelper(dict, "", flatten); 
    return flatten;
  }
  
   public static void FlattenDictionaryHelper(
     Dictionary<string, object> dict, 
     string prefix, 
     Dictionary<string, string> flatten)
   {               
     if(dict == null || dict.Count == 0)
     {
       return ; 
     }
     
     foreach(var pair in dict)
     {
       var key = pair.Key; 
       var value = pair.Value; 
       
       // https://stackoverflow.com/questions/16956903/determine-if-type-is-dictionary
       var t = value.GetType();
       bool isDict = t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Dictionary<,>);
       
       if(isDict)
       {
         var newPrefix = key ; 
         newPrefix = (prefix.Length == 0)?  newPrefix : (prefix + "." + newPrefix); 
         FlattenDictionaryHelper((Dictionary<string, object>)value, newPrefix, flatten); 
       }
       else 
       {
         string newKey = (prefix.Length == 0 )? key : ((key.Length == 0)? prefix : (prefix + "." + key)); // i think you need to check either prefix is "" or key is "", if key is also "", then no ".key"
         // case 5 is last key ""
         
         flatten.Add(newKey, (string)value); 
       }       
     }       
   }

  static void Main(string[] args)
  {

  }
}

// Key1 : 1 
// Key2:  value object is a dictionary 
// prefix of key Key2. = key + '.'
// prefix + key - handle key