Which collection in C# maintains insertion order and allows duplicate elements?

The correct answer is A: List. List maintains insertion order and allows duplicates. HashSet doesn't allow duplicates, SortedSet sorts elements, and Queue follows FIFO.

Which of the following is a correct way to declare a property with auto-implementation in C#?

The correct answer is A: public int Age { get; set; }. Auto-implemented properties in C# use the syntax: public Type PropertyName { get; set; } without explicitly defining the backing field.

What is covariance in C# generics and interfaces?

The correct answer is B: Allowing derived types where base types are expected in return types. Covariance (using 'out' keyword) allows a more derived type to be returned where a base type is expected, improving type safety and flexibility.

Which of the following correctly declares a nullable integer in C#?

The correct answer is A: int? x = null;. In C#, 'int?' is used to declare a nullable integer. A regular 'int' cannot be assigned null. int* is a pointer (unsafe code), not a nullable type.

In ASP.NET Core, which method is used to register services in the dependency injection container?

The correct answer is A: services.AddScoped(), services.AddSingleton(), services.AddTransient(). AddScoped, AddSingleton, and AddTransient are methods to register services with different lifetimes in ASP.NET Core DI.

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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 1–10 of 38

Topics in C# Programming

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

Q.1 Medium Collections

In a scenario with frequent additions and removals at both ends, which collection is most efficient?

A List

B LinkedList

C Queue

D Stack

Correct Answer: B. LinkedList

EXPLANATION

LinkedList<T> provides O(1) AddFirst(), AddLast(), RemoveFirst(), and RemoveLast() operations. List<T> requires O(n) for head removals due to reindexing.

Test

Q.2 Medium Collections

What is the time complexity of Contains() method in a HashSet?

A O(n)

B O(log n)

C O(1) average case

D O(n²)

Correct Answer: C. O(1) average case

EXPLANATION

HashSet<T> uses hash-based lookup for Contains(), achieving O(1) average-case time complexity, with O(n) worst-case in case of hash collisions.

Test

Q.3 Medium Collections

Which collection maintains elements in sorted order and is backed by a binary search tree?

A SortedList

B SortedDictionary

C SortedSet

D TreeMap

Correct Answer: B. SortedDictionary

EXPLANATION

SortedDictionary<K,V> uses a red-black tree (binary search tree) for O(log n) operations. SortedSet<T> also uses BST but for single values.

Test

Q.4 Medium Collections

In a data processing pipeline, if you need FIFO (First-In-First-Out) semantics with O(1) enqueue/dequeue, which collection is best?

A Stack

B LinkedList

C Queue

D SortedList

Correct Answer: C. Queue

EXPLANATION

Queue<T> provides FIFO semantics with O(1) Enqueue() and Dequeue() operations. Stack<T> is LIFO, and LinkedList operations vary.

Test

Q.5 Medium Collections

Which LINQ method is used to convert a Dictionary to a List of key-value pairs?

A ToArray()

B ToList()

C AsEnumerable()

D Cast()

Correct Answer: B. ToList()

EXPLANATION

ToList() converts the Dictionary's KeyValuePair<K,V> enumeration into a List<KeyValuePair<K,V>>. ToArray() would create an array instead.

Test

Q.6 Medium Collections

What happens when you modify a List while iterating over it using a foreach loop?

A It works without issues

B It throws InvalidOperationException

C It silently skips elements

D It doubles the iteration time

Correct Answer: B. It throws InvalidOperationException

EXPLANATION

Modifying a collection during enumeration invalidates the enumerator and throws InvalidOperationException. Use a for loop or ToList() to avoid this.

Test

Q.7 Medium Collections

In a multiprocessing environment, which thread-safe collection should be used for concurrent operations?

A List

B ConcurrentBag

C HashSet

D Dictionary

Correct Answer: B. ConcurrentBag

EXPLANATION

ConcurrentBag<T> and other Concurrent* collections from System.Collections.Concurrent are thread-safe for multi-threaded scenarios without explicit locking.

Test

Q.8 Medium Collections

What is the primary difference between ArrayList and List?

A ArrayList is faster

B List is type-safe and more performant

C ArrayList uses generics

D No difference in modern C#

Correct Answer: B. List is type-safe and more performant

EXPLANATION

List<T> is strongly-typed (type-safe) and eliminates boxing/unboxing overhead. ArrayList is non-generic and slower due to boxing of value types.

Test

Q.9 Medium Collections

Which method should be used to safely check and retrieve a value from a Dictionary without throwing an exception?

A Get()

B TryGetValue()

C ContainsKey() followed by indexer

D Retrieve()

Correct Answer: B. TryGetValue()

EXPLANATION

TryGetValue() is the safe method that returns a boolean indicating success and outputs the value without throwing KeyNotFoundException.

Test

Q.10 Medium Collections

In a scenario where you need to store key-value pairs with fast lookup times, which collection is optimal?

A List

B Dictionary

C Queue

D LinkedList

Correct Answer: B. Dictionary

EXPLANATION

Dictionary<K,V> provides O(1) average-case lookup time using hash tables, making it optimal for key-value pair storage with fast retrieval.

Test

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