What is the difference between list and dictionary in C#?

A List and Dictionary are both generic collections in C#, but they serve different purposes. A List<T> stores elements in a sequential order with index-based access, while a Dictionary<TKey, TValue> stores key-value pairs for fast lookups.

Understanding when to use each collection type is crucial for writing efficient C# applications. Lists are ideal for ordered data where you need to access elements by position, while dictionaries are perfect for mapping relationships and quick key-based retrieval.

Syntax

Following is the syntax for creating a List −

List<T> listName = new List<T>();
// or with initialization
List<T> listName = new List<T>() { item1, item2, item3 };

Following is the syntax for creating a Dictionary −

Dictionary<TKey, TValue> dictName = new Dictionary<TKey, TValue>();
// or with initialization
Dictionary<TKey, TValue> dictName = new Dictionary<TKey, TValue>() {
   { key1, value1 },
   { key2, value2 }
};

Using List Collection

A List stores elements in sequential order and provides index-based access. Elements can be duplicated and are accessed using zero-based indexing −

using System;
using System.Collections.Generic;

class Program {
   public static void Main() {
      List<string> subjects = new List<string>() {
         "Maths",
         "English", 
         "Science"
      };
      
      // Access by index
      Console.WriteLine("First subject: " + subjects[0]);
      
      // Add new element
      subjects.Add("History");
      
      // Loop through all elements
      Console.WriteLine("All subjects:");
      for(int i = 0; i < subjects.Count; i++) {
         Console.WriteLine((i + 1) + ". " + subjects[i]);
      }
   }
}

The output of the above code is −

First subject: Maths
All subjects:
1. Maths
2. English
3. Science
4. History

Using Dictionary Collection

A Dictionary stores key-value pairs where each key must be unique. It provides fast lookups based on the key −

using System;
using System.Collections.Generic;

class Program {
   public static void Main() {
      Dictionary<string, int> sports = new Dictionary<string, int>();
      
      // Add key-value pairs
      sports.Add("football", 11);
      sports.Add("basketball", 5);
      sports.Add("tennis", 2);
      
      // Access by key
      Console.WriteLine("Football players: " + sports["football"]);
      
      // Check if key exists
      if(sports.ContainsKey("rugby")) {
         Console.WriteLine("Rugby players: " + sports["rugby"]);
      } else {
         Console.WriteLine("Rugby not found in dictionary");
      }
      
      // Loop through all key-value pairs
      Console.WriteLine("All sports:");
      foreach(KeyValuePair<string, int> sport in sports) {
         Console.WriteLine(sport.Key + ": " + sport.Value + " players");
      }
   }
}

The output of the above code is −

Football players: 11
Rugby not found in dictionary
All sports:
football: 11 players
basketball: 5 players
tennis: 2 players

Comparison

Common Use Cases

Use List when:

• You need to store items in a specific order

• You want to access elements by index position

• You need to allow duplicate values

• You want simple iteration through all elements

Use Dictionary when:

• You need fast lookups based on a unique identifier (key)

• You want to associate values with specific keys

• You need to check if a specific key exists

• You're implementing mappings or relationships between data

Conclusion

Lists provide sequential, index-based storage ideal for ordered collections, while Dictionaries offer fast key-value pair lookups perfect for mapping relationships. Choose Lists for ordered data access and Dictionaries when you need efficient key-based retrieval and unique key constraints.