Dictionary - recursive function - depth first search

Dictionary_recursivePractice.cs

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

namespace flattenDictionary
{
    /// <summary>
    /// Flatten dictionary - recursive function practice
    /// </summary>
    class Program
    {       
        static Dictionary<string, object> flattenDictionary(Dictionary<string, object> dict)
        {
            // your code goes here
            flatten.Clear();
            flattenDictionaryHelper(dict, "");
            return flatten;
        }

        public static Dictionary<string, object> flatten = new Dictionary<string, object>();

        /// <summary>
        /// 
        /// </summary>
        /// <param name="dict"></param>
        /// <param name="prefix"></param>
        public static void flattenDictionaryHelper(Dictionary<string, object> dict, string prefix)
        {
            if (dict == null)
            {
                return;
            }
                        
            foreach (var pair in dict)
            {
                var key   = pair.Key;
                var value = pair.Value;

                bool isDictionaryTrue = isDictionary(value);

                // added after mocking, reset for every pair. Bug found in mocking test
                var nextPrefix = prefix; 

                nextPrefix += (prefix.Length > 0) ? "." : "";
                nextPrefix += key;

                if (isDictionaryTrue)
                {
                    flattenDictionaryHelper((Dictionary<string, object>)value, nextPrefix);
                }
                else
                {
                    flatten.Add(nextPrefix, value); // prefix includes key 
                }
            }
        }

        // Check Object type in C# 
        // https://stackoverflow.com/questions/3561202/check-if-instance-of-a-type
        private static bool isDictionary(Object value)
        {
            return value.GetType() == typeof(Dictionary<string, object>);
        }

        static void Main(string[] args)
        {
            var dictionary =
                    new Dictionary<string, Object>();

            dictionary.Add("Key1", "1");

            var subDict1 =
                    new Dictionary<string, Object>();

            subDict1.Add("a", "2");
            subDict1.Add("b", "3");

            var subDict2 =
                    new Dictionary<string, Object>();
            subDict2.Add("d", "2");
            subDict2.Add("e", "3");

            subDict1.Add("c", subDict2);

            dictionary.Add("Key2", subDict1);

            var flatten = flattenDictionary(dictionary);

            foreach (var kvp in flatten)
            {
                Console.WriteLine("Key = {0}, Value = {1}", kvp.Key, kvp.Value);
            }
        }
    }
}