How to create an OrderedDictionary in C#?

An OrderedDictionary in C# is a specialized collection that maintains the insertion order of key-value pairs. Unlike a regular Dictionary, it preserves the sequence in which elements were added, making it useful when order matters.

The OrderedDictionary class is part of the System.Collections.Specialized namespace and provides both indexed access and key-based access to elements.

Syntax

Following is the syntax for creating an OrderedDictionary −

OrderedDictionary dict = new OrderedDictionary();
dict.Add(key, value);
dict[index] = value;  // Access by index
dict[key] = value;    // Access by key

Creating and Populating an OrderedDictionary

Example

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

public class Demo {
   public static void Main() {
      OrderedDictionary dict = new OrderedDictionary();
      dict.Add("A", "Books");
      dict.Add("B", "Electronics");
      dict.Add("C", "Smart Wearables");
      dict.Add("D", "Pet Supplies");
      dict.Add("E", "Clothing");
      dict.Add("F", "Footwear");
      Console.WriteLine("OrderedDictionary elements...");
      foreach(DictionaryEntry d in dict) {
         Console.WriteLine(d.Key + " " + d.Value);
      }
      Console.WriteLine("Count of elements in OrderedDictionary = " + dict.Count);
      dict.Remove("E");
      Console.WriteLine("Count of elements in OrderedDictionary (Updated)= " + dict.Count);
   }
}

The output of the above code is −

OrderedDictionary elements...
A Books
B Electronics
C Smart Wearables
D Pet Supplies
E Clothing
F Footwear
Count of elements in OrderedDictionary = 6
Count of elements in OrderedDictionary (Updated)= 5

Accessing Keys Collection

You can extract and work with the keys collection from an OrderedDictionary −

Example

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

public class Demo {
   public static void Main() {
      OrderedDictionary dict = new OrderedDictionary();
      dict.Add("1", "One");
      dict.Add("2", "Two");
      dict.Add("3", "Three");
      dict.Add("4", "Four");
      dict.Add("5", "Five");
      dict.Add("6", "Six");
      dict.Add("7", "Seven");
      dict.Add("8", "Eight");
      ICollection col = dict.Keys;
      String[] strKeys = new String[dict.Count];
      col.CopyTo(strKeys, 0);
      Console.WriteLine("Displaying the keys...");
      for (int i = 0; i < dict.Count; i++) {
         Console.WriteLine(strKeys[i]);
      }
   }
}

The output of the above code is −

Displaying the keys...
1
2
3
4
5
6
7
8

Index-based Access

OrderedDictionary provides unique functionality to access elements by both key and index −

Example

using System;
using System.Collections.Specialized;

public class Demo {
   public static void Main() {
      OrderedDictionary dict = new OrderedDictionary();
      dict.Add("first", 100);
      dict.Add("second", 200);
      dict.Add("third", 300);
      
      Console.WriteLine("Access by key:");
      Console.WriteLine("first = " + dict["first"]);
      
      Console.WriteLine("\nAccess by index:");
      Console.WriteLine("Index 0 = " + dict[0]);
      Console.WriteLine("Index 1 = " + dict[1]);
      Console.WriteLine("Index 2 = " + dict[2]);
      
      dict[1] = 250; // Modify by index
      Console.WriteLine("\nAfter modifying index 1:");
      Console.WriteLine("second = " + dict["second"]);
   }
}

The output of the above code is −

Access by key:
first = 100

Access by index:
Index 0 = 100
Index 1 = 200
Index 2 = 300

After modifying index 1:
second = 250

Common Methods

Conclusion

OrderedDictionary in C# provides a hybrid collection that maintains insertion order while offering both key-based and index-based access. It's ideal when you need dictionary functionality with predictable element ordering, combining the benefits of both dictionaries and lists.