Learn Dictionaries vs. Lists: Choosing the Right Tool | Dictionaries and Key-Value Collections

When working with collections in C#, you often need to choose between using a List<T> or a Dictionary<TKey, TValue>. Each type offers distinct advantages and is suited to different scenarios. The following table presents a direct comparison to help you decide which is best for your needs:

Feature	List	Dictionary<TKey, TValue>
Lookup Speed	O(n) — must check each item	O(1) — direct access by key
Memory Usage	Lower (stores only values)	Higher (stores keys and values, plus hashing overhead)
Duplicate Values	Allowed	Values allowed, but keys must be unique
Key-based Access	Not supported	Supported (fast lookup by key)
Index-based Access	Supported (by position)	Not supported (no order guarantee)
Typical Use Cases	Ordered lists, simple collections	Fast lookups, mapping unique keys to values

Program.cs


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using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            // List of users (each user is a tuple of ID and Name)
            var usersList = new List<(int Id, string Name)>
            {
                (1, "Alice"),
                (2, "Bob"),
                (3, "Charlie"),
            };

            // Search for user with ID 2 in the list
            string foundName = null;
            foreach (var user in usersList)
            {
                if (user.Id == 2)
                {
                    foundName = user.Name;
                    break;
                }
            }
            Console.WriteLine("List search found: " + foundName);

            // Dictionary of users (ID as key, Name as value)
            var usersDict = new Dictionary<int, string>
            {
                {1, "Alice"},
                {2, "Bob"},
                {3, "Charlie"},
            };

            // Search for user with ID 2 in the dictionary
            string dictName = null;
            if (usersDict.TryGetValue(2, out dictName))
            {
                Console.WriteLine("Dictionary search found: " + dictName);
            }
        }
    }
}

When you switch from a list to a dictionary for lookups, you gain significant speed improvements for large collections. However, dictionaries do use more memory because they must store keys, values, and maintain a hash table to enable fast access. If your data is small or you need to preserve order or allow duplicates, a list might be simpler and more memory-efficient. For large datasets where you need to find items by a unique identifier quickly, a dictionary is almost always the better choice.

Program.cs


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using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            // Original approach: searching a list for a product by ID
            var productsList = new List<(int Id, string Name)>
            {
                (101, "Laptop"),
                (102, "Phone"),
                (103, "Tablet"),
            };

            string productName = null;
            foreach (var product in productsList)
            {
                if (product.Id == 102)
                {
                    productName = product.Name;
                    break;
                }
            }
            Console.WriteLine("List search: " + productName);

            // Refactored approach: using a dictionary for fast lookup
            var productsDict = new Dictionary<int, string>
            {
                {101, "Laptop"},
                {102, "Phone"},
                {103, "Tablet"},
            };

            string dictProductName = null;
            if (productsDict.TryGetValue(102, out dictProductName))
            {
                Console.WriteLine("Dictionary lookup: " + dictProductName);
            }
        }
    }
}

1. When is a dictionary more efficient than a list?

2. What is a drawback of using dictionaries?

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Section 2. Chapter 6

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Feature	List	Dictionary<TKey, TValue>
Lookup Speed	O(n) — must check each item	O(1) — direct access by key
Memory Usage	Lower (stores only values)	Higher (stores keys and values, plus hashing overhead)
Duplicate Values	Allowed	Values allowed, but keys must be unique
Key-based Access	Not supported	Supported (fast lookup by key)
Index-based Access	Supported (by position)	Not supported (no order guarantee)
Typical Use Cases	Ordered lists, simple collections	Fast lookups, mapping unique keys to values

Program.cs


              1234567891011121314151617181920212223242526272829303132333435363738394041424344454647
            
using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            // List of users (each user is a tuple of ID and Name)
            var usersList = new List<(int Id, string Name)>
            {
                (1, "Alice"),
                (2, "Bob"),
                (3, "Charlie"),
            };

            // Search for user with ID 2 in the list
            string foundName = null;
            foreach (var user in usersList)
            {
                if (user.Id == 2)
                {
                    foundName = user.Name;
                    break;
                }
            }
            Console.WriteLine("List search found: " + foundName);

            // Dictionary of users (ID as key, Name as value)
            var usersDict = new Dictionary<int, string>
            {
                {1, "Alice"},
                {2, "Bob"},
                {3, "Charlie"},
            };

            // Search for user with ID 2 in the dictionary
            string dictName = null;
            if (usersDict.TryGetValue(2, out dictName))
            {
                Console.WriteLine("Dictionary search found: " + dictName);
            }
        }
    }
}

Program.cs


              123456789101112131415161718192021222324252627282930313233343536373839404142434445
            
using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            // Original approach: searching a list for a product by ID
            var productsList = new List<(int Id, string Name)>
            {
                (101, "Laptop"),
                (102, "Phone"),
                (103, "Tablet"),
            };

            string productName = null;
            foreach (var product in productsList)
            {
                if (product.Id == 102)
                {
                    productName = product.Name;
                    break;
                }
            }
            Console.WriteLine("List search: " + productName);

            // Refactored approach: using a dictionary for fast lookup
            var productsDict = new Dictionary<int, string>
            {
                {101, "Laptop"},
                {102, "Phone"},
                {103, "Tablet"},
            };

            string dictProductName = null;
            if (productsDict.TryGetValue(102, out dictProductName))
            {
                Console.WriteLine("Dictionary lookup: " + dictProductName);
            }
        }
    }
}

1. When is a dictionary more efficient than a list?

2. What is a drawback of using dictionaries?

Everything was clear?

Thanks for your feedback!

Section 2. Chapter 6