How does Dictionary in C# add Key/Value?

Question

Hello, I removed a key/value from the middle of the Dictionary and then added the same key/value to this. The key/value pair is added in the previous(middle) position. (I expected new key/value pair added to begin or last) How does Dictionary in C# add Key/Value?

// Example file for C# Applied Data Structures by Joe Marini
// Programming Challenge

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;

namespace Challenge
{
     class Program
     {
          static void PrintDic(Dictionary<string, string> dic)
          {
               Console.WriteLine("Dic\t");
               foreach (var item in dic)
               {
                    Console.WriteLine($"{item.Key}\t{item.Value}");
               }
               Console.WriteLine();
          }
          static void PrintOrderedDic(OrderedDictionary dic)
          {
               Console.WriteLine("Ordered Dic\n");
               foreach (DictionaryEntry item in dic)
               {
                    Console.WriteLine($"{item.Key}\t{item.Value}");
               }
               Console.WriteLine();
          }
          static void Main(string[] args)
          {
               Dictionary<string, string> Dic = new();

               Dic.Add("Key1", "Value1");
               Dic.Add("Key2", "Value2");
               Dic.Add("Key5", "Value5");
               Dic.Add("Key4", "Value4");
               Dic.Add("Key3", "Value3");

               PrintDic(Dic);
               Dic.Remove("Key5");
               PrintDic(Dic);
               Dic.Add("Key5", "Value5");
               PrintDic(Dic);

               //OrderedDictionary orderedDic = new();

               //orderedDic.Add("Key1", "Value1");
               //orderedDic.Add("Key2", "Value2");
               //orderedDic.Add("Key5", "Value5");
               //orderedDic.Add("Key4", "Value4");
               //orderedDic.Add("Key3", "Value3");
               
               //PrintOrderedDic(orderedDic);
               //orderedDic.Remove("Key5");
               //PrintOrderedDic(orderedDic);
               //orderedDic.Add("Key5", "Value5");
               //PrintOrderedDic(orderedDic);
          }

     }
}

Answer 1

Hi @Shervan360 ,

Dictionary<TKey, TValue> class does not guarantee any specific order of elements. The order in which elements are stored in the dictionary is not necessarily the order in which they were added.

The Dictionary<TKey, TValue> class uses a hash table to store key/value pairs. The hash table organizes the elements based on the hash code of the keys. The hash code determines the position of the element in the internal data structure, providing a fast way to look up values based on their keys.

When you add or remove items, the internal organization may change, and the new key/value pair may end up in a different position. This is why you observe that the new key/value pair is added in a different position than expected.

If you need to maintain the order of elements based on their insertion order, you can use the OrderedDictionary Class. Best Regards.

Jiachen Li

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