400 C# Interview Questions with Answers 2026

Advanced Features: SOLID, learn dependency injection principles patterns architecture, Async/Await, and Generics.

Runtime & Performance: CLR, Garbage Collection, Span<T>, and Memory Management.

Web & APIs: ASP.NET Core, REST, EF Core, and Middleware.

Enterprise Practices: Unit Testing (xUnit/Moq), Security, and CI/CD.

B) ValueTask<T> should be preferred when the operation is expected to complete synchronously frequently.

C) ValueTask<T> allows for multiple awaits on the same instance without any risk.

D) ValueTask<T> is strictly faster than Task<T> in all asynchronous scenarios.

E) ValueTask<T> cannot be used with the async and await keywords.

F) ValueTask<T> is a class, whereas Task<T> is a struct.

Correct Answer: B

Overall Explanation: ValueTask<T> is a discriminating union of a T and a Task<T>, designed to reduce heap allocations in scenarios where an operation often completes synchronously.

Option Detail:

B - Correct: It prevents a heap allocation if the result is already available.

C - Incorrect: Awaiting a ValueTask<T> multiple times can lead to undefined behavior or errors.

D - Incorrect: If the operation is always asynchronous, the overhead of wrapping a Task in a struct can make it slightly slower.

E - Incorrect: It is fully compatible with async/await.

F - Incorrect: The types are the opposite; Task is the class, ValueTask is the struct.

Question 2: In the context of the .NET Garbage Collector (GC), what happens during a Generation 2 collection?

B) It is a "Full GC" that typically includes Gen 0, Gen 1, and the Large Object Heap (LOH).

C) It occurs every time a local variable goes out of scope.

D) It is faster and happens more frequently than Gen 0 collections.

E) It only clears objects that implement IDisposable.

F) It prevents the use of the Large Object Heap entirely.

Correct Answer: B

Overall Explanation: Gen 2 collections are the most expensive and comprehensive, often referred to as a "Full GC" because they sweep all generations to reclaim memory.

Option Detail:

B - Correct: Gen 2 includes all generations and the LOH.

C - Incorrect: GC is triggered by memory pressure, not immediately when variables go out of scope.

D - Incorrect: Gen 2 is the slowest and least frequent collection.

E - Incorrect: GC manages memory regardless of IDisposable; IDisposable is for unmanaged resources.

F - Incorrect: Gen 2 collections actually include the LOH in their sweep.

Question 3: Which SOLID principle is primarily violated if a "FileStore" class requires a change to its internal logic every time a new file format (e.g., XML, JSON, CSV) is added?

B) Open/Closed Principle

C) Liskov Substitution Principle

D) Interface Segregation Principle

E) Dependency Inversion Principle

F) Encapsulation Principle

Correct Answer: B

Overall Explanation: The Open/Closed Principle states that software entities should be open for extension but closed for modification.

Option Detail:

B - Correct: If you must modify the class code to add a new format, it is not "closed for modification."

C - Incorrect: LSP is about ensuring derived classes can stand in for base classes.

D - Incorrect: ISP deals with splitting large, bloated interfaces.

E - Incorrect: DIP is about depending on abstractions rather than concretions.

F - Incorrect: Encapsulation is a general OOP pillar, not a specific SOLID principle.

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