400 C# Interview Questions with Answers 2026

C# & .NET Core Interview Practice Questions are designed to bridge the gap between basic syntax and the high-level engineering expectations of modern tech recruiters and senior architects. I have meticulously crafted these exams to simulate the pressure of a real technical interview, moving beyond simple definitions to test your ability to solve complex architectural puzzles and optimize performance-critical code. Whether you are navigating the nuances of memory management and Garbage Collection, implementing thread-safe singleton patterns, or configuring middleware pipelines in ASP.NET Core, these questions force you to think like a professional developer. By providing exhaustive explanations for every single option—not just the correct one—I ensure you understand the "why" behind every design choice, helping you eliminate guesswork and build the confidence needed to tackle senior-level roles in the .NET ecosystem.Exam Domains & Sample TopicsC# Fundamentals: OOP, Data Types, Delegates, and LINQ Basics.Advanced Features: SOLID, Dependency Injection, 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.Sample Practice QuestionsQuestion 1: Which of the following best describes the behavior of ValueTask<T> compared to Task<T> in a high-performance C# application?A) ValueTask<T> is a reference type and always allocates on the heap.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: BOverall 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:A - Incorrect: ValueTask<T> is a value type (struct), not a reference type.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?A) Only objects in Gen 0 are inspected and cleared.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: BOverall 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:A - Incorrect: That describes a Gen 0 collection.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?A) Single Responsibility PrincipleB) Open/Closed PrincipleC) Liskov Substitution PrincipleD) Interface Segregation PrincipleE) Dependency Inversion PrincipleF) Encapsulation PrincipleCorrect Answer: BOverall Explanation: The Open/Closed Principle states that software entities should be open for extension but closed for modification.Option Detail:A - Incorrect: While related, SRP focuses on having only one reason to change, not the mechanism of adding new types.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.Welcome to the best practice exams to help you prepare for your C# & .NET Core Interview Practice Questions.You can retake the exams as many times as you wantThis is a huge original question bankYou get support from instructors if you have questionsEach question has a detailed explanationMobile-compatible with the Udemy app30-day money-back guarantee if you're not satisfiedI hope that by now you're convinced! And there are a lot more questions inside the course. Enroll today and take the final step toward getting certified!