6.2 Lists, Dictionaries, Sets

Let’s go through Lists, Dictionaries, and Sets in C# — key collection types in .NET that are more flexible than arrays.

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📌 1. List<T>

A List is a dynamic array — it can grow or shrink in size.

Key Points

• Generic (T specifies the type of elements)

• Supports adding, removing, sorting, searching

• Part of System.Collections.Generic

Example

using System;
using System.Collections.Generic;
 
List<int> numbers = new List<int>();
numbers.Add(10);
numbers.Add(20);
numbers.Add(30);
 
Console.WriteLine(numbers[1]);  // 20
numbers.Remove(20);             // Remove element
Console.WriteLine(numbers.Count); // 2
 
// Iterating
foreach (int num in numbers)
    Console.WriteLine(num);

Other Useful Methods

• Insert(index, value) → insert at specific position

• Sort() → sort the list

• Contains(value) → check if value exists

• Clear() → remove all elements

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📌 2. Dictionary<K,V>

A Dictionary stores key-value pairs.

• Generic types: K = key, V = value

• Keys must be unique

• Part of System.Collections.Generic

Example

Dictionary<string, int> ages = new Dictionary<string, int>();
ages.Add("Alice", 25);
ages.Add("Bob", 30);
 
// Access by key
Console.WriteLine(ages["Alice"]); // 25
 
// Update value
ages["Alice"] = 26;
 
// Iterating
foreach (var kvp in ages)
    Console.WriteLine($"{kvp.Key}: {kvp.Value}");

Other Useful Methods

• ContainsKey(key) → check if key exists

• Remove(key) → remove a key-value pair

• Count → number of entries

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📌 3. HashSet

A HashSet is an unordered collection of unique elements.

• No duplicates allowed

• Efficient for lookups

• Part of System.Collections.Generic

Example

HashSet<int> numbers = new HashSet<int>();
numbers.Add(1);
numbers.Add(2);
numbers.Add(2); // Ignored, already exists
 
Console.WriteLine(numbers.Count); // 2
 
// Check if exists
if (numbers.Contains(1))
    Console.WriteLine("1 exists");
 
// Iterating
foreach (int num in numbers)
    Console.WriteLine(num);

Other Useful Methods

• Remove(value) → remove element

• UnionWith(otherSet) → combine sets

• IntersectWith(otherSet) → common elements

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📌 4. Comparison Table

Collection	Type of Data	Key Feature	Example Use Case
List	Ordered list	Dynamic size, allows duplicates	Storing a list of names
Dictionary<K,V>	Key-value pairs	Fast lookup by key	Storing user IDs and names
HashSet	Unique elements	No duplicates, fast lookups	Unique tags, IDs, or items

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✅ Tips

1. List → use for ordered data where duplicates are allowed

2. Dictionary<K,V> → use for lookup by key

3. HashSet → use for unique elements and efficient membership checks