Use recursive function to apply depth first search to visit a dictionary, and then flatten the dictionary's key with concatenating "."

DictionaryFlatten_practice.cs

using System;
using System.Collections.Generic;

class Solution
{
  public static Dictionary<string, string> FlattenDictionary(Dictionary<string, object> dict) // "Key1"
  {
    if(dict == null || dict.Count == 0) // false 
    {
      return new Dictionary<string, string>(); 
    }
    
    var flattenDictionary = new Dictionary<string, string>(); 
    
    FlattenDictionaryRecursive(dict, flattenDictionary, ""); 
    
    return flattenDictionary; 
  }
  
  private static void FlattenDictionaryRecursive(Dictionary<string, object> original, Dictionary<string, string> flatten, string prefix)
  {
    foreach(var pair in original)  // two pairs, first pair, key = "Key1", key2
    {
      var key   = pair.Key;  // "Key1", "Key2"
      var value = pair.Value; // object, "1"
      
      var valueIsDictionary = value.GetType() == typeof(Dictionary<string, object>); // false , true 
      
      var newKey = prefix == null || prefix.Length == 0? key : (key.Length == 0)? prefix : (prefix + "." + key); // "Key1", "Key2"
      
      if(!valueIsDictionary)
      {        
        var newValue = (string) value; // "1"
        
        flatten.Add(newKey, newValue); // ("Key1", "1")
      }
      else 
      {
         var newPrefix = newKey; 
         
         FlattenDictionaryRecursive((Dictionary<string, object>)value, flatten, newPrefix); 
      }
    }
  }

  static void Main(string[] args)
  {

  }
}

// prefix -> build key-> prefix.isNullorEmpty -> 
// type not dictionary -> newPrefix = prefix + "." + key -> new entry 
// C# GetType -> stackoverflow 

// From StackOverflow:
// if (obj1.GetType() == typeof(Dictionary))