List generic collection in C Sharp (C#)

Introduction

A List is like a flexible row of boxes where you can store items in order. You can add, remove, or find items easily.

When you want to keep a collection of items that can grow or shrink.

When you need to access items by their position quickly.

When you want to add items at the end or remove items from anywhere.

When you want to loop through items in order.

When you want a simple way to store and manage a group of similar things.

Syntax

C Sharp (C#)

using System;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        List<int> listName = new List<int>();
    }
}

Replace with the type of items you want to store, like int, string, or a custom class.

List is part of System.Collections.Generic namespace, so include it with using.

Examples

Create a list of integers and add two numbers.

C Sharp (C#)

List<int> numbers = new List<int>();
numbers.Add(10);
numbers.Add(20);

Create an empty list of strings.

C Sharp (C#)

List<string> names = new List<string>();
// Empty list with no items yet

Create a list with one item already inside.

C Sharp (C#)

List<int> singleItemList = new List<int> { 5 };

An empty list has zero items.

C Sharp (C#)

List<int> emptyList = new List<int>();
// List is empty, Count is 0

Sample Program

This program creates an empty list of strings, adds three fruit names, and prints the count and items before and after adding.

C Sharp (C#)

using System;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        List<string> fruits = new List<string>();
        Console.WriteLine("Before adding items:");
        Console.WriteLine($"Count: {fruits.Count}");

        fruits.Add("Apple");
        fruits.Add("Banana");
        fruits.Add("Cherry");

        Console.WriteLine("After adding items:");
        Console.WriteLine($"Count: {fruits.Count}");

        Console.WriteLine("Items in the list:");
        foreach (string fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

Important Notes

Adding an item to a List is usually very fast (amortized O(1) time).

Accessing an item by index is very fast (O(1) time).

Removing items can be slower because the list shifts items to fill gaps (O(n) time).

Common mistake: Forgetting to include System.Collections.Generic namespace.

Use List when you need a flexible, ordered collection. Use arrays if size is fixed and performance is critical.

Summary

List<T> stores items in order and can grow or shrink.

You can add, remove, and access items by position easily.

List<T> is simple and useful for many common collection tasks.

Practice

1. What is the main feature of a List<T> in C#?

easy

A. It can only hold a fixed number of items.

B. It stores only unique items and does not allow duplicates.

C. It automatically sorts items when added.

D. It stores items in order and allows easy access by position.

Solution

1. Step 1: Understand List<T> behavior

   A List<T> stores items in the order they are added and allows access by index.

2. Step 2: Compare options with List<T> features

   Only It stores items in order and allows easy access by position. correctly describes this behavior; others describe different collection types or incorrect features.

3. Final Answer:

   It stores items in order and allows easy access by position. -> Option D

4. Quick Check:

   List<T> = ordered, indexed collection [OK]

Hint: Remember List<T> keeps order and supports indexing [OK]

Common Mistakes:

• Thinking List<T> enforces uniqueness
• Assuming List<T> auto-sorts items
• Believing List<T> has fixed size

2. Which of the following is the correct way to declare a List of integers in C#?

easy

A. List<int> numbers = new List<int>();

B. List numbers = new List<int>();

C. List<int> numbers = List<int>();

D. List<int> numbers = new List();

Solution

1. Step 1: Recall correct List<T> syntax

   In C#, to declare a generic List, you must specify the type and use the new keyword with constructor.

2. Step 2: Check each option for syntax correctness

   List<int> numbers = new List<int>(); correctly declares and initializes a List of int. Others miss type, constructor, or use wrong syntax.

3. Final Answer:

   List<int> numbers = new List<int>(); -> Option A

4. Quick Check:

   Generic List declaration = new List<T>() [OK]

Hint: Use new List<T>() with type specified [OK]

Common Mistakes:

• Omitting new keyword
• Not specifying generic type in constructor
• Using non-generic List without type

3. What will be the output of this C# code?

var fruits = new List<string> { "apple", "banana", "cherry" };
fruits.RemoveAt(1);
Console.WriteLine(fruits[1]);

medium

A. banana

B. IndexOutOfRangeException

C. cherry

D. apple

Solution

1. Step 1: Understand RemoveAt effect on list

   RemoveAt(1) removes the item at index 1, which is "banana". The list becomes ["apple", "cherry"].

2. Step 2: Access the item at index 1 after removal

   After removal, fruits[1] is "cherry" because the list shifted left.

3. Final Answer:

   cherry -> Option C

4. Quick Check:

   RemoveAt shifts items left, fruits[1] = cherry [OK]

Hint: RemoveAt shifts list left; index 1 now points to next item [OK]

Common Mistakes:

• Assuming removed item still exists
• Expecting original index items unchanged
• Confusing RemoveAt with Remove

4. Identify the error in this C# code snippet using List<string>:

List<string> colors = new List<string>();
colors.Add("red");
colors[1] = "blue";
Console.WriteLine(colors[1]);

medium

A. IndexOutOfRangeException because index 1 does not exist yet.

B. Syntax error in Add method usage.

C. Cannot assign string to List<string> element.

D. No error; code runs and prints 'blue'.

Solution

1. Step 1: Analyze list content after Add

   After colors.Add("red"), list has one element at index 0 only.

2. Step 2: Check assignment to colors[1]

   colors[1] does not exist yet, so assigning to it causes IndexOutOfRangeException.

3. Final Answer:

   IndexOutOfRangeException because index 1 does not exist yet. -> Option A

4. Quick Check:

   Assigning to non-existing index throws exception [OK]

Hint: List index must exist before assignment; use Add to add items [OK]

Common Mistakes:

• Trying to assign to index without adding
• Confusing Add and index assignment
• Expecting automatic list expansion

5. Given a List<int> named numbers containing {1, 2, 3, 4, 5}, which code snippet correctly doubles each number in the list?

hard

A. numbers = numbers.Select(n => n * 2).ToList();

B. for (int i = 0; i < numbers.Count; i++) { numbers[i] = numbers[i] * 2; }

C. foreach (int n in numbers) { n = n * 2; }

D. numbers.ForEach(n => n = n * 2);

Solution

1. Step 1: Understand how to modify List elements

   Using a for loop with index allows modifying elements directly by assignment.

2. Step 2: Evaluate each option's effect

   for (int i = 0; i < numbers.Count; i++) { numbers[i] = numbers[i] * 2; } modifies elements in place. foreach (int n in numbers) { n = n * 2; } modifies copy of elements (no effect). numbers = numbers.Select(n => n * 2).ToList(); creates a new list but requires LINQ and ToList(). numbers.ForEach(n => n = n * 2); modifies copies in ForEach (no effect).

3. Final Answer:

   for (int i = 0; i < numbers.Count; i++) { numbers[i] = numbers[i] * 2; } -> Option B

4. Quick Check:

   Use for loop with index to update List elements [OK]

Hint: Use for loop with index to update List items directly [OK]

Common Mistakes:

• Using foreach expecting to modify list items
• Using ForEach with lambda that doesn't assign back
• Not creating new list when using LINQ Select