Top 50 C# Interview Questions & Answers (2026 Advanced Guide)
C# and the .NET ecosystem power everything from massive enterprise backend systems to cutting-edge Unity video games. In 2026, knowing basic C# syntax isn't enough; you need to understand the internals of the CLR.
Interviewers will test your depth on Memory Management, LINQ optimizations, and asynchronous programming. Here are the Top 50 Advanced C# Interview Questions you need to master to land your next role.
⚙️ Part 1: Core C# & Object-Oriented Principles
1. What is the difference between a Class and a Struct in C#?
A class is a Reference Type stored on the Heap, passed by reference, and supports inheritance. A struct is a Value Type stored on the Stack (usually), passed by value (copied), and does not support inheritance. Use structs only for small, immutable data structures.
2. What are Value Types vs Reference Types?
Value types (int, float, bool, struct) hold their data directly and are allocated on the Stack. Reference types (string, class, array, delegate) hold a pointer to their data, which is allocated dynamically on the Managed Heap.
3. Explain Boxing and Unboxing.
Boxing is the process of converting a Value Type to a Reference Type (object), which forces the CLR to allocate a new object on the heap and copy the value into it. Unboxing extracts the value type from the object. This carries a massive performance penalty and should be avoided by using Generics.
4. What is the difference between `const` and `readonly`?
const variables are evaluated at compile-time and must be initialized immediately. readonly variables are evaluated at runtime and can only be initialized at declaration or inside a class constructor. readonly is preferred for complex types.
5. What is the `ref` keyword?
The ref keyword is used to pass arguments by reference rather than by value. Any changes made to the parameter inside the method will be reflected in that variable when control returns to the calling method. The variable MUST be initialized before passing it as ref.
6. What is the `out` keyword?
Similar to ref, it passes by reference. However, an out variable does NOT need to be initialized before being passed. Instead, the method receiving the out parameter is *forced* to assign it a value before returning.
7. What is an Interface?
An interface is a contract that defines a set of methods, properties, and events. It has no implementation (prior to C# 8.0 default implementations). A class or struct that implements the interface must provide an implementation for all its members. It allows C# to achieve multiple inheritance.
8. Abstract Class vs Interface?
An abstract class can have access modifiers (public/private), fields, and fully implemented methods, but a class can only inherit from ONE abstract class. An interface (traditionally) only has method signatures, but a class can implement MULTIPLE interfaces.
9. What is method Overloading vs Overriding?
Overloading happens at compile-time when multiple methods share the same name but have different parameters. Overriding happens at runtime when a derived class provides a specific implementation for a method declared as virtual in the base class using the override keyword.
10. What does the `virtual` keyword do?
It marks a method or property in a base class as capable of being overridden in any derived class. This enables polymorphism, allowing the CLR to determine at runtime which version of the method to invoke based on the actual object type.
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💾 Part 2: Memory Management & GC
11. How does the Garbage Collector (GC) work in C#?
The GC operates on the Managed Heap. It periodically halts execution, builds a graph of all reachable objects starting from root references, and then reclaims the memory occupied by unreachable objects (sweeping). Finally, it compacts the heap to prevent fragmentation.
12. Explain GC Generations (0, 1, and 2).
To optimize performance, the GC categorizes objects by age. Generation 0 contains newly allocated short-lived objects. If an object survives a Gen 0 collection, it is promoted to Gen 1. If it survives Gen 1, it goes to Gen 2 (long-lived objects like static variables). The GC collects Gen 0 frequently and Gen 2 rarely.
13. What is the IDisposable interface?
The GC only manages managed memory (RAM). It does not know how to close file handles, database connections, or network sockets (Unmanaged Resources). IDisposable provides the Dispose() method, allowing developers to manually release these unmanaged resources.
14. What does the `using` statement do?
It provides a convenient syntax that ensures Dispose() is called on an object automatically when the block of code completes, even if an exception is thrown. It compiles down to a try-finally block.
15. What is a Finalizer (Destructor)?
A Finalizer (written as ~ClassName()) is a backup mechanism called by the GC right before it destroys the object. Because the GC runs non-deterministically on a separate thread, relying on Finalizers to close resources is a bad practice; use IDisposable instead.
16. Why are Strings immutable in C#?
Immutability makes strings thread-safe and allows the CLR to optimize memory via String Interning (storing only one instance of a specific string literal in a pool). However, this means concatenating strings in a loop creates a new object every time, causing memory bloat.
17. What is StringBuilder?
Because strings are immutable, StringBuilder represents a mutable string of characters. It allocates a buffer, and when you append to it, it modifies the existing buffer rather than creating new objects, making it massively faster for heavy string manipulation.
18. What is the LOH (Large Object Heap)?
Any object larger than 85,000 bytes (like large arrays or bitmaps) is placed directly on the Large Object Heap. The LOH is collected alongside Gen 2. Historically, it was never compacted, which could lead to severe memory fragmentation.
19. What is `GC.Collect()`?
It forces the Garbage Collector to run immediately. Calling it manually is almost always an anti-pattern because it disrupts the GC's self-tuning algorithms. It should only be used in very specific scenarios, like a massive application state transition.
20. How do memory leaks happen in C#?
Even with a GC, memory leaks occur through "unreachable" code that is still referenced. Common culprits include unsubscribed Event Handlers (where the publisher holds a strong reference to the subscriber) and static collections that grow indefinitely.
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📚 Part 3: LINQ, Collections & Generics
21. What are Generics?
Generics (List<T>) allow you to write a class or method that can work with any data type. They provide type safety at compile time and completely eliminate the need for Boxing/Unboxing, vastly improving performance.
22. IEnumerable vs IQueryable?
IEnumerable operates on in-memory collections (like Lists); it pulls all data into memory and then filters it. IQueryable is designed for out-of-memory databases (like Entity Framework); it translates the LINQ query into SQL and executes the filter on the database server itself.
23. What is LINQ?
Language Integrated Query (LINQ) provides a unified, declarative syntax to query data from any source (SQL databases, XML documents, in-memory collections) directly within C#.
24. What is Deferred Execution in LINQ?
When you write a LINQ query (like .Where(x => x > 5)), the query is NOT executed immediately. It is only executed when you actually iterate over the result (using a foreach loop, or by calling .ToList()).
25. Dictionary vs Hashtable?
Dictionary<TKey, TValue> is a generic collection, type-safe, and highly performant. Hashtable is a legacy, non-generic collection that stores everything as object, requiring boxing/unboxing. Always use Dictionary.
26. What is a Delegate?
A delegate is a type-safe function pointer. It holds a reference to a method with a specific signature. They are the foundation of Events, Callbacks, and LINQ queries in C#.
27. Difference between Action, Func, and Predicate?
They are built-in generic delegates. Action points to a method that returns void. Func points to a method that returns a specific value. Predicate is a special case of Func that always returns a bool.
28. What are Events?
Events are a special implementation of delegates using the Publisher/Subscriber model. The event keyword prevents subscribers from wiping out other subscribers (they can only use += and -=) and prevents them from invoking the event directly.
29. What is an Extension Method?
Extension methods allow you to add new methods to existing types (even sealed classes like string) without modifying the original type. They are defined as static methods inside static classes, using the this modifier on the first parameter.
30. Select vs SelectMany in LINQ?
Select performs a 1-to-1 projection (if you start with 5 elements, you end up with 5 elements). SelectMany performs a 1-to-many projection, flattening collections of collections into a single, flat sequence.
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⏱️ Part 4: Async/Await & Concurrency
31. Explain Async and Await.
They are keywords used to write asynchronous code that looks synchronous. await pauses the execution of the method until the awaited Task completes, freeing up the thread to do other work (like serving other web requests) instead of blocking while waiting for I/O operations.
32. What is a Task in C#?
A Task represents an asynchronous operation. Unlike a traditional Thread, Tasks are managed by the Thread Pool, which is significantly more efficient than spinning up and destroying heavy OS threads manually.
33. Task vs Thread?
A Thread is an actual OS-level construct; creating them is expensive and takes about 1MB of RAM. A Task is a higher-level abstraction (a promise to do work) that executes on a reused background thread from the ThreadPool.
34. Why should you avoid `async void`?
If an exception is thrown inside an async void method, it cannot be caught by a try-catch block surrounding the method call; it will crash the entire application process. Always return a Task instead, except for Event Handlers.
35. What is `Task.WhenAll` vs `Task.WaitAll`?
Task.WhenAll creates a new Task that completes when all provided tasks complete. It is non-blocking and used with await. Task.WaitAll synchronously blocks the current thread until all tasks complete (which can cause deadlocks).
36. Explain the `lock` statement.
The lock statement is used to ensure that only one thread can execute a piece of code at a time, preventing race conditions when multiple threads access shared resources. It compiles down to Monitor.Enter and Monitor.Exit.
37. What is ConfigureAwait(false)?
By default, an awaited task attempts to resume on the original "Synchronization Context" (like the UI thread). Using ConfigureAwait(false) tells the task it can resume on *any* background thread, improving performance and preventing deadlocks in library code.
38. What is a Mutex vs a Semaphore?
A Mutex allows only one thread to access a resource and can be shared across different processes. A Semaphore allows a specified *number* of threads (e.g., up to 5) to access a resource concurrently.
39. What is the `yield` keyword?
yield return is used inside an iterator method to return elements one at a time (custom state machine). It allows you to generate a sequence of items lazily, without having to build a massive list in memory first.
40. What is ThreadPool Starvation?
This happens when all available threads in the ThreadPool are busy (often blocked synchronously using `.Result` or `.Wait()`), preventing new tasks from executing and causing the application to hang.
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🚀 Part 5: Advanced C# & .NET Internals
41. What is Reflection?
Reflection allows C# code to inspect its own metadata at runtime. You can dynamically read the properties, methods, and attributes of a class, and even instantiate objects or invoke methods dynamically. It is powerful but slow.
42. Explain Dependency Injection (DI) in .NET Core.
DI is a design pattern used to achieve Inversion of Control. Instead of a class creating its own dependencies (via `new`), they are injected via the constructor. .NET Core has a built-in IoC container utilizing Scoped, Transient, and Singleton lifecycles.
43. Transient vs Scoped vs Singleton lifetimes?
Transient: A new instance is created every single time it is requested. Scoped: A new instance is created once per HTTP request. Singleton: One single instance is created and shared across the entire application's lifetime.
44. What are Attributes?
Attributes (like [Obsolete] or [ApiController]) are declarative tags used to convey metadata to the runtime or compiler. They don't do anything on their own; they are read via Reflection by frameworks like ASP.NET to dictate behavior.
45. What is the `dynamic` keyword?
It bypasses compile-time type checking. The type is resolved completely at runtime. It is heavily used when interacting with COM APIs, IronPython, or parsing complex, unknown JSON payloads.
46. What are Records in C# 9+?
A record is a reference type that provides built-in immutable semantics and value-based equality. If two records hold the exact same data, recordA == recordB returns true, unlike classes which compare memory addresses.
47. Explain the `is` and `as` operators.
is checks if an object is compatible with a given type, returning a boolean. as attempts to cast the object to a specific type; if it fails, it returns null rather than throwing a cast exception.
48. What is Entity Framework (EF) Core?
EF Core is the official Object-Relational Mapper (ORM) for .NET. It allows developers to work with databases using C# objects (DbSet) and LINQ queries, eliminating the need for most raw data-access SQL code.
49. What is a Memory or Span?
Introduced in modern C#, Span<T> is a struct that provides a type-safe, allocation-free window into a block of contiguous memory (like an array or string). It allows extremely fast slicing without creating copies.
50. What is Middleware in ASP.NET Core?
Middleware is software assembled into an application pipeline to handle requests and responses. Each component chooses whether to pass the request to the next component in the pipeline, allowing cross-cutting concerns like Authentication, Logging, and Error Handling.
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