List generic collection in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a List<T> in C#?

A List<T> is a generic collection in C# that stores elements of the same type in a dynamic array. It can grow or shrink as needed.

beginner

How do you add an item to a List<T>?

Use the Add() method. For example: myList.Add(item); adds item to the end of the list.

beginner

How do you access an element at a specific position in a List<T>?

Use the indexer with square brackets. For example: myList[0] accesses the first element.

intermediate

What happens if you try to access an index outside the List<T> range?

An ArgumentOutOfRangeException is thrown because the index is invalid.

beginner

How can you find the number of elements in a List<T>?

Use the Count property. For example: int size = myList.Count;

Which method adds an element to a List<T>?

A Remove()

B Add()

C Insert()

D Clear()

The Add() method appends an element to the end of the List<T>.

How do you get the number of elements in a List<T>?

A Length

B Size

C Capacity

D Count

The Count property returns the number of elements currently in the List<T>.

What type of collection is List<T>?

A Linked list

B Fixed-size array

C Generic dynamic array

D Dictionary

List<T> is a generic collection that uses a dynamic array internally.

What happens if you access myList[10] when myList has 5 elements?

A Throws ArgumentOutOfRangeException

B Returns default value

C Returns null

D Returns last element

Accessing an index outside the valid range throws an ArgumentOutOfRangeException.

Which of these methods removes all elements from a List<T>?

A Clear()

B Remove()

C Delete()

D Drop()

The Clear() method removes all elements from the List<T>.

Explain what a List<T> is and how it differs from a regular array.

Think about flexibility and type safety.

Describe how to add, access, and count elements in a List<T>.

Focus on common operations with List&lt;T&gt;.

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