Which of the following correctly demonstrates an abstract class with an abstract method in C#?

The correct answer is A: public abstract class Base { public abstract void Method(); }. An abstract class uses the 'abstract' keyword, and abstract methods must be declared without implementation inside abstract classes.

In a real-time data processing application, which concurrent collection would be best for producer-consumer pattern?

The correct answer is B: BlockingCollection. BlockingCollection supports blocking operations for producer-consumer patterns, allowing threads to wait for items or space.

Which access modifier allows a member to be accessed only within the same assembly?

The correct answer is B: internal. 'internal' access modifier restricts member access to the same assembly only. This is useful for creating assembly-wide internal APIs.

What is the correct way to declare a constant variable in C#?

The correct answer is A: const int MAX = 100;. The 'const' keyword is used to declare constants in C#. 'final' is Java syntax. While 'readonly' can be used, 'const' is the proper keyword for compile-time constants.

Which LINQ method should you use to transform each element of a collection into a new form?

The correct answer is B: Select(). Select() is the LINQ method that projects each element to a new form, similar to map in functional programming languages.

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C# Programming
Collections

C# and .NET for campus placement

38 Q 4 Topics Take Mock Test

Difficulty: All Easy Medium Hard 21–30 of 38

Topics in C# Programming

All Basics & Syntax 100 OOP in C# 100 Collections 71 ASP.NET Basics 19

Q.21 Medium Collections

What is the output of the following code?
var dict = new Dictionary();
dict[1] = "A";
dict[1] = "B";
Console.WriteLine(dict[1]);

A A

B B

C ArgumentException

D Null

Correct Answer: B. B

EXPLANATION

Using the indexer on Dictionary with an existing key updates the value, not throws. Output is 'B'.

Test

Q.22 Medium Collections

In C# 2024-25, which collection should be used for fast membership testing with no duplicates?

A List

B HashSet

C SortedSet

D Queue

Correct Answer: B. HashSet

EXPLANATION

HashSet<T> provides O(1) average lookup and prevents duplicates. SortedSet maintains order but slower. List<T> requires O(n) lookup.

Test

Q.23 Medium Collections

What is the time complexity for removing an element from the middle of a LinkedList?

A O(1)

B O(n)

C O(log n)

D O(n log n)

Correct Answer: A. O(1)

EXPLANATION

If you have a LinkedListNode<T>, removal is O(1). However, finding the node is O(n), so overall search + remove is O(n).

Test

Q.24 Medium Collections

Which of the following collections preserves insertion order and is thread-safe?

A SortedDictionary

B ConcurrentDictionary

C Dictionary

D HashSet

Correct Answer: B. ConcurrentDictionary

EXPLANATION

ConcurrentDictionary is thread-safe. While it doesn't strictly preserve insertion order like some collections, it's the thread-safe option among these choices.

Test

Q.25 Medium Collections

What does the Contains() method use in HashSet to determine membership?

A Sequential search

B Binary search

C Hash code and equality comparison

D Index-based lookup

Correct Answer: C. Hash code and equality comparison

EXPLANATION

HashSet<T> uses hash codes for O(1) average lookup. It computes hash code first, then uses equality comparison.

Test

Q.26 Medium Collections

Which collection in C# is NOT thread-safe by default?

A ConcurrentDictionary

B ConcurrentBag

C List

D ConcurrentQueue

Correct Answer: C. List

EXPLANATION

List<T> is not thread-safe. All Concurrent* collections (ConcurrentDictionary, ConcurrentBag, ConcurrentQueue) are thread-safe.

Test

Q.27 Medium Collections

What is the primary difference between SortedList and SortedDictionary?

A SortedList uses arrays internally; SortedDictionary uses a tree

B SortedDictionary allows faster insertion

C SortedList supports duplicate keys

D They have identical performance characteristics

Correct Answer: A. SortedList uses arrays internally; SortedDictionary uses a tree

EXPLANATION

SortedList uses arrays (faster retrieval, slower insertion). SortedDictionary uses red-black tree (balanced insertion/deletion). SortedDictionary has better insertion time O(log n) vs O(n) for SortedList.

Test

Q.28 Medium Collections

Which LINQ method returns the first element matching a condition, or throws if none exists?

A First()

B FirstOrDefault()

C Find()

D Single()

Correct Answer: A. First()

EXPLANATION

First() throws InvalidOperationException if no match. FirstOrDefault() returns null/default. Find() is a List<T> method.

Test

Q.29 Medium Collections

What happens when you add a duplicate key to a Dictionary?

A It adds both entries

B It throws ArgumentException

C It updates the existing key's value

D It ignores the new entry

Correct Answer: B. It throws ArgumentException

EXPLANATION

Dictionary<TKey, TValue> throws ArgumentException when adding duplicate keys. Use indexer notation to update values.

Test

Q.30 Medium Collections

Which collection type is best suited for implementing a priority queue in C#?

A Stack

B PriorityQueue

C SortedDictionary

D LinkedList

Correct Answer: B. PriorityQueue

EXPLANATION

PriorityQueue<T, TPriority> was introduced in .NET 6 specifically for priority queue implementation.

Test

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