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

The correct answer is C: List. List is not thread-safe. All Concurrent* collections (ConcurrentDictionary, ConcurrentBag, ConcurrentQueue) are thread-safe.

What does the Peek() method do in a Stack?

The correct answer is B: Returns the top element without removing it. Peek() returns the top element without modifying the stack. Pop() removes and returns it. Peek() throws InvalidOperationException if the stack is empty.

Which interface must be implemented to use a collection in a foreach loop?

The correct answer is A: IEnumerable. IEnumerable interface provides GetEnumerator() method, enabling foreach iteration. IEnumerable is the generic version.

In an inventory management system, a Product class has a Price property. Which approach best ensures price cannot be negative?

The correct answer is B: Use private backing field with property validation in setter. Properties with private backing fields allow validation in setters, ensuring data integrity by preventing invalid values like negative prices.

In a banking application, you need to create a base class that cannot be instantiated but defines common properties for SavingsAccount and CheckingAccount. Which should you use?

The correct answer is C: abstract class. An abstract class is ideal when you want to provide common functionality and properties to derived classes while preventing direct instantiation. It can have both abstract and concrete members.

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C# and .NET for campus placement

15 Q 4 Topics Take Mock Test

Difficulty: All Easy Medium Hard 11–15 of 15

Topics in C# Programming

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

Q.11 Hard Collections

In a concurrent environment, which collection should be used instead of Dictionary in C# 2024-25?

A Hashtable

B ConcurrentDictionary

C SortedDictionary

D List

Correct Answer: B. ConcurrentDictionary

EXPLANATION

ConcurrentDictionary<K,V> from System.Collections.Concurrent is thread-safe for multi-threaded applications, unlike Dictionary<K,V>.

Test

Q.12 Hard Collections

What is the difference between ICollection and IEnumerable in terms of capabilities?

A ICollection adds Count and Add/Remove/Clear methods

B Both are identical

C IEnumerable is a collection, ICollection is not

D ICollection cannot iterate, IEnumerable can

Correct Answer: A. ICollection adds Count and Add/Remove/Clear methods

EXPLANATION

IEnumerable<T> only provides iteration. ICollection<T> extends it with Count property and methods like Add(), Remove(), Clear() for modification.

Test

Q.13 Hard Collections

Consider: var result = list.OrderByDescending(x => x.Age).ThenBy(x => x.Name); What does this do?

A Sorts by Age ascending then Name descending

B Sorts by Age descending then Name ascending

C Sorts by Name then Age

D Throws a compilation error

Correct Answer: B. Sorts by Age descending then Name ascending

EXPLANATION

OrderByDescending sorts by Age in descending order. ThenBy applies secondary sort by Name in ascending order. This is a multi-level LINQ sort.

Test

Q.14 Hard Collections

What is the space complexity of a HashSet with n elements storing unique values?

A O(n)

B O(log n)

C O(1)

D O(n²)

Correct Answer: A. O(n)

EXPLANATION

HashSet<T> requires O(n) space to store n unique elements. Hash tables require linear space proportional to the number of stored elements.

Test

Q.15 Hard Collections

In C# 2024-25, which LINQ method would you use to find all elements matching a condition in a List?

A FindAll()

B Where()

C Both A and B work

D Filter()

Correct Answer: C. Both A and B work

EXPLANATION

Both FindAll() and Where() (LINQ) can filter collections. Where() returns IEnumerable and is more modern. FindAll() returns a List directly.

Test

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