OrderedDictionary Class in C#

The OrderedDictionary class in C# represents a collection of key/value pairs that maintains insertion order and allows access by both key and index. It combines the functionality of a dictionary with the ordered nature of a list, making it useful when you need to preserve the order of items while still providing fast key-based lookup.

Syntax

Following is the syntax for creating and using an OrderedDictionary −

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

Properties

Following are the key properties of the OrderedDictionary class −

Key Methods

Using OrderedDictionary with Count Property

Example

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

public class Demo {
   public static void Main() {
      OrderedDictionary dict = new OrderedDictionary();
      dict.Add("A", "Home Appliances");
      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.Clear();
      Console.WriteLine("Count of elements in OrderedDictionary (Updated) = " + dict.Count);
   }
}

The output of the above code is −

OrderedDictionary elements...
A Home Appliances
B Electronics
C Smart Wearables
D Pet Supplies
E Clothing
F Footwear
Count of elements in OrderedDictionary = 6
Count of elements in OrderedDictionary (Updated) = 0

Using Index-Based Access

Example

using System;
using System.Collections.Specialized;

public class Demo {
   public static void Main() {
      OrderedDictionary dict = new OrderedDictionary();
      dict.Add("First", "Apple");
      dict.Add("Second", "Banana");
      dict.Add("Third", "Cherry");
      
      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]);
      
      Console.WriteLine("\nModifying value at index 1:");
      dict[1] = "Blueberry";
      Console.WriteLine("Index 1 now: " + dict[1]);
      Console.WriteLine("Key 'Second' now: " + dict["Second"]);
   }
}

The output of the above code is −

Access by key:
First: Apple

Access by index:
Index 0: Apple
Index 1: Banana
Index 2: Cherry

Modifying value at index 1:
Index 1 now: Blueberry
Key 'Second' now: Blueberry

Using Contains and Remove Methods

Example

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

public class Demo {
   public static void Main() {
      OrderedDictionary dict = new OrderedDictionary();
      dict.Add("X", "Mathematics");
      dict.Add("Y", "Physics");
      dict.Add("Z", "Chemistry");
      
      Console.WriteLine("Initial dictionary:");
      foreach(DictionaryEntry entry in dict) {
         Console.WriteLine(entry.Key + ": " + entry.Value);
      }
      
      Console.WriteLine("\nChecking if key 'Y' exists: " + dict.Contains("Y"));
      Console.WriteLine("Checking if key 'W' exists: " + dict.Contains("W"));
      
      dict.Remove("Y");
      Console.WriteLine("\nAfter removing key 'Y':");
      foreach(DictionaryEntry entry in dict) {
         Console.WriteLine(entry.Key + ": " + entry.Value);
      }
   }
}

The output of the above code is −

Initial dictionary:
X: Mathematics
Y: Physics
Z: Chemistry

Checking if key 'Y' exists: True
Checking if key 'W' exists: False

After removing key 'Y':
X: Mathematics
Z: Chemistry

Conclusion

The OrderedDictionary class in C# provides the combined benefits of a dictionary's key-based access and a list's ordered structure. It maintains insertion order while allowing access by both key and index, making it ideal for scenarios where order matters alongside fast lookups.