Dynamic Collections: List<T> & Generics

In Plain English

Fixed-size arrays are rigid when user input or data size is unknown. The generic List<T> class (from System.Collections.Generic) is a dynamic resizable array that automatically expands and contracts as you add or remove items.

The Mental Model

"If an array is a fixed 6-pack egg carton, a List<T> is a magical accordion shopping cart that expands every time you drop another item into it."

Deep Dive: How It Works

using System.Collections.Generic;: Required namespace.

Declaration: List<string> tasks = new List<string>(); or List<int> scores = new();.

.Add(item): Appends an element to the end of the list.

.Remove(item): Removes the first occurrence of an item.

.RemoveAt(index): Removes item at specific index.

.Count: Returns current number of items (equivalent to array .Length).

.Contains(item): Returns true if item exists in the list.

Syntax Blueprint

using System.Collections.Generic;

List<string> list = new List<string>();
list.Add("First");
list.Add("Second");
Console.WriteLine($"Count: {list.Count}");

Dynamic resizable collection with generic type parameter <T>.

Core Rules to Remember

Automatic Resizing: Expands underlying memory buffer automatically as items are added.

Type-Safe Generics (<T>): List<int> only accepts integers; prevents accidental type corruption.

The Modern Standard: Used far more frequently than raw fixed-size arrays in modern .NET.

Common Beginner Traps & How to Fix Them

Mistake:Using .Length instead of .Count on a List<T>

Why it happens: Confusion with arrays.

How to fix: Arrays use .Length; Lists and Collections use .Count.

Pro Tip: Initialize lists with collection initializers: var fruits = new List<string> { "Apple", "Banana", "Orange" };!

Live Interactive Example

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Managing a Dynamic Inventory with List<T>

C# 12 / .NET

Output Console

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How it works: List<T> dynamically managed additions and removals.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Build a numbers list

Create a List<int> nums = new List<int>();
Add 50 and 100 with .Add().
Print "List size: 2".

Sandbox Output

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