List generic collection in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this C# List operation?

Consider the following C# code that uses a List of integers. What will be printed to the console?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        List<int> numbers = new List<int>() {1, 2, 3, 4, 5};
        numbers.RemoveAt(2);
        Console.WriteLine(numbers.Count);
    }
}

A 4

B 5

C 3

D Throws an exception

💡 Hint

Removing an item reduces the count by one.

The list starts with 5 elements. Removing the element at index 2 removes one item, so the count becomes 4.

📝 Syntax

intermediate

Which option correctly adds an element to a List in C#?

You have a List called fruits. Which line of code correctly adds "Apple" to the list?

A fruits.Insert("Apple");

B fruits.Append("Apple");

C fruits.Add("Apple");

D fruits.Push("Apple");

💡 Hint

The method to add an item at the end of a List is Add.

The Add method adds an element to the end of the List. Append is not a List method, Insert requires an index, and Push is not a List method.

❓ optimization

advanced

Which option is the most efficient way to check if a List contains the number 10?

Given a List numbers, which code snippet is the best choice to check if 10 is in the list?

A if (numbers.Find(x => x == 10) != 0) { /* found */ }

B if (numbers.IndexOf(10) != -1) { /* found */ }

C foreach (int n in numbers) { if (n == 10) { /* found */ break; } }

D if (numbers.Contains(10)) { /* found */ }

💡 Hint

Look for the method designed to check existence directly.

Contains is optimized to check if an item exists and returns a boolean. IndexOf returns an index which is less direct. The foreach loop is less concise and slower. Find returns the element or default, which can be confusing if 0 is a valid element.

🔧 Debug

advanced

What error does this code produce?

Examine the following code snippet. What error will it cause when run?

C Sharp (C#)

List<string> names = null;
names.Add("John");

A NullReferenceException

B No error, adds "John"

C InvalidOperationException

D ArgumentNullException

💡 Hint

Think about what happens when you call a method on a null object.

The list 'names' is null, so calling Add on it causes a NullReferenceException.

🧠 Conceptual

expert

What is the time complexity of accessing an element by index in a List?

In C#, what is the time complexity to access an element at a specific index in a List?

A O(n) - Linear time

B O(1) - Constant time

C O(log n) - Logarithmic time

D O(n^2) - Quadratic time

💡 Hint

Think about how List stores elements internally.

List uses an array internally, so accessing by index is constant time O(1).

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