Part 8: Collections in C#

Collections in C# provide powerful tools for managing and organising data efficiently. They enable developers to handle groups of objects with ease, whether you need fixed-size arrays or dynamic and flexible data structures like lists and dictionaries. In this article, we'll explore the primary types of collections in C#, their methods, and practical use cases.

1. Types of Collections in C#

1.1 Arrays

An array is a fixed-size collection of elements of the same type. It’s useful for scenarios where the number of items is known and does not change.

Example:

int[] numbers = {1, 2, 3, 4, 5};

foreach (int number in numbers)
{
    Console.WriteLine(number);
}

Key Methods and Properties:

• Length: Returns the number of elements.
• Array.Sort(array): Sorts the elements.
• Array.Reverse(array): Reverses the order of elements.

1.2 Lists

A List is a dynamic collection that can grow or shrink in size. Lists are part of the System.Collections.Generic namespace and are more versatile than arrays.

Example:

List<string> names = new List<string> {"John", "Adrian", "Charlie"};

names.Add("Adrian");

foreach (string name in names)
{
    Console.WriteLine(name);
}

Key Methods:

• Add(item): Adds an item to the list.
• Remove(item): Removes the first occurrence of an item.
• Contains(item): Checks if an item exists in the list.

1.3 Dictionaries

A Dictionary is a collection of key-value pairs, where each key is unique. Dictionaries are ideal for scenarios where quick lookups are needed.

Example:

Dictionary<int, string> students = new Dictionary<int, string>
{
    {1, "Adrian"},
    {2, "John"},
    {3, "Charlie"}
};

if (students.ContainsKey(2))
{
    Console.WriteLine($"Student ID 2: {students[2]}");
}

Key Methods:

• Add(key, value): Adds a new key-value pair.
• Remove(key): Removes an item by key.
• ContainsKey(key): Checks if a specific key exists.

Performance Note: Dictionaries provide fast lookups due to their hashing mechanism, making them suitable for scenarios where keys are frequently searched.

1.4 Sets

A HashSet is a collection that contains unique elements and does not allow duplicates. It’s optimised for fast lookups.

Example:

HashSet<int> uniqueNumbers = new HashSet<int> {1, 2, 3, 3, 4};

foreach (int number in uniqueNumbers)
{
    Console.WriteLine(number);
}

Key Methods:

• Add(item): Adds an item if it doesn’t already exist.
• Remove(item): Removes an item.
• Contains(item): Checks if an item exists.

Performance Note: Use HashSet when you need to ensure uniqueness in your collection.

2. Methods of Collections and Their Uses

Common Methods Across Collections:

(LINQ will be discussed in the following articles)

• Iteration: Use foreach to iterate through items in a collection.
• Sorting: Use Sort() in lists or OrderBy() in LINQ for advanced sorting.
• Filtering: LINQ methods like Where() allow filtering based on conditions.
• Conversion: Convert between collections using ToArray(), ToList(), etc.

Example of LINQ Filtering:

List<int> numbers = new List<int> {1, 2, 3, 4, 5};

var evenNumbers = numbers.Where(n => n % 2 == 0);

foreach (int number in evenNumbers)
{
    Console.WriteLine(number);
}

3. Practical Example: Data Management with Collections

Scenario: Managing a Student Roster

This example demonstrates how to use multiple types of collections to manage student data.

Example:

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Store student data
        Dictionary<int, string> students = new Dictionary<int, string>
        {
            {1, "Adrian"},
            {2, "John"},
            {3, "Charlie"}
        };

        // Add new students
        students.Add(4, "Diana");

        // All students
        Console.WriteLine("Student Roster:");
        foreach (var student in students)
        {
            Console.WriteLine($"ID: {student.Key}, Name: {student.Value}");
        }

        // Check if a student exists
        if (students.ContainsKey(2))
        {
            Console.WriteLine("Student ID 2 is in the roster.");
        }

        // Remove a student
        students.Remove(3);
        Console.WriteLine("Updated Roster:");
        foreach (var student in students)
        {
            Console.WriteLine($"ID: {student.Key}, Name: {student.Value}");
        }
    }
}

/*
Student Roster:
ID: 1, Name: Adrian
ID: 2, Name: John
ID: 3, Name: Charlie
ID: 4, Name: Diana
Student ID 2 is in the roster.
Updated Roster:
ID: 1, Name: Adrian
ID: 2, Name: John
ID: 4, Name: Diana
*/

4. Advanced Topics

Thread-Safe Collections

For applications requiring thread-safe operations, consider using ConcurrentDictionary or other thread-safe collections from the System.Collections.Concurrent namespace.

Example:

using System;
using System.Collections.Concurrent;

class Program
{
    static void Main()
    {
        ConcurrentDictionary<int, string> safeDict = new ConcurrentDictionary<int, string>();

        safeDict.TryAdd(1, "Adrian");
        safeDict.TryAdd(2, "John");

        foreach (var item in safeDict)
        {
            Console.WriteLine($"Key: {item.Key}, Value: {item.Value}");
        }
    }
}

Immutable Collections

To ensure the collection cannot be modified after creation, use immutable collections from System.Collections.Immutable.

Example:

using System.Collections.Immutable;

ImmutableList<string> immutableNames = ImmutableList.Create("Adrian", "John", "Charlie");

foreach (var name in immutableNames)
{
    Console.WriteLine(name);
}