Sorting a HashMap according to keys in C#

In C#, the equivalent of Java's HashMap is the Dictionary class, which stores key-value pairs. Sorting a Dictionary by keys requires extracting the keys, sorting them, and then accessing the values in the sorted order.

Syntax

Following is the syntax for creating and sorting a Dictionary by keys −

Dictionary<TKey, TValue> dict = new Dictionary<TKey, TValue>();

// Sort keys using LINQ
var sortedByKeys = dict.OrderBy(x => x.Key);

// Or extract and sort keys manually
var keys = dict.Keys.ToList();
keys.Sort();

Using LINQ OrderBy Method

The most efficient way to sort a Dictionary by keys is using LINQ's OrderBy method, which returns the key-value pairs in sorted order −

using System;
using System.Collections.Generic;
using System.Linq;

class Program {
   static void Main() {
      Dictionary<string, int> sports = new Dictionary<string, int>();
      sports.Add("soccer", 1);
      sports.Add("cricket", 2);
      sports.Add("tennis", 3);
      sports.Add("rugby", 4);
      sports.Add("basketball", 5);

      Console.WriteLine("Original Dictionary:");
      foreach (var item in sports) {
         Console.WriteLine($"{item.Key}: {item.Value}");
      }

      Console.WriteLine("\nSorted by Keys:");
      var sortedDict = sports.OrderBy(x => x.Key);
      foreach (var item in sortedDict) {
         Console.WriteLine($"{item.Key}: {item.Value}");
      }
   }
}

The output of the above code is −

Original Dictionary:
soccer: 1
cricket: 2
tennis: 3
rugby: 4
basketball: 5

Sorted by Keys:
basketball: 5
cricket: 2
rugby: 4
soccer: 1
tennis: 3

Using Manual Key Sorting

You can also extract the keys, sort them manually, and then iterate through the sorted keys to access Dictionary values −

using System;
using System.Collections.Generic;
using System.Linq;

class Program {
   static void Main() {
      Dictionary<string, int> sports = new Dictionary<string, int>();
      sports.Add("soccer", 1);
      sports.Add("cricket", 2);
      sports.Add("tennis", 3);
      sports.Add("rugby", 4);

      // Get keys and sort them
      var keys = sports.Keys.ToList();
      keys.Sort();

      Console.WriteLine("Dictionary sorted by keys:");
      foreach (var key in keys) {
         Console.WriteLine($"{key}: {sports[key]}");
      }
   }
}

The output of the above code is −

Dictionary sorted by keys:
cricket: 2
rugby: 4
soccer: 1
tennis: 3

Sorting by Keys in Descending Order

To sort keys in descending order, use OrderByDescending method −

using System;
using System.Collections.Generic;
using System.Linq;

class Program {
   static void Main() {
      Dictionary<int, string> numbers = new Dictionary<int, string>();
      numbers.Add(3, "Three");
      numbers.Add(1, "One");
      numbers.Add(4, "Four");
      numbers.Add(2, "Two");

      Console.WriteLine("Sorted by Keys (Descending):");
      var sortedDesc = numbers.OrderByDescending(x => x.Key);
      foreach (var item in sortedDesc) {
         Console.WriteLine($"{item.Key}: {item.Value}");
      }
   }
}

The output of the above code is −

Sorted by Keys (Descending):
4: Four
3: Three
2: Two
1: One

Comparison of Sorting Methods

Conclusion

Sorting a Dictionary by keys in C# can be accomplished using LINQ's OrderBy method for a clean approach, or by manually extracting and sorting keys. The LINQ approach is preferred as it maintains the key-value relationship and provides cleaner, more readable code.