50 C# Interview Questions Every Senior .NET Developer Should Know

Preparing for a .NET interview? This guide covers the most commonly asked C# interview questions from junior to senior level developers.

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1. What is C#?

C# is a modern, object-oriented programming language developed by Microsoft as part of the .NET ecosystem. It is designed to be simple, powerful, and productive while supporting multiple programming paradigms such as object-oriented, functional, and asynchronous programming. C# is widely used for building web applications, desktop applications, cloud services, mobile applications, games, and enterprise systems. Over the years, the language has evolved significantly with features such as LINQ, async/await, records, pattern matching, and nullable reference types. Because of its versatility and strong ecosystem, C# remains one of the most popular programming languages for professional software development.

2. What are the four pillars of OOP?

The four pillars of Object-Oriented Programming are Encapsulation, Inheritance, Polymorphism, and Abstraction. Encapsulation protects data by controlling access through methods and properties. Inheritance allows a class to reuse functionality from another class. Polymorphism enables the same interface to represent different implementations. Abstraction hides unnecessary implementation details and exposes only essential functionality. These principles help developers build software that is easier to maintain, extend, and test. Understanding the four pillars is one of the most fundamental topics in C# interviews because they influence how applications are designed and structured.

3. What is the difference between class and struct?

Classes and structs are both used to define custom types in C#, but they have important differences. A class is a reference type, meaning instances are allocated on the managed heap and variables store references to objects. A struct is a value type, meaning the data itself is copied when assigned to another variable. Structs are generally used for small, lightweight objects such as coordinates, dates, or numeric values. Classes are better suited for complex business entities and objects that require inheritance. Choosing between a class and a struct requires understanding memory usage, performance characteristics, and application design requirements.

4. What is boxing and unboxing?

Boxing is the process of converting a value type into a reference type. During boxing, the CLR allocates an object on the heap and copies the value into that object. Unboxing is the reverse process, where the value is extracted from the boxed object and converted back into its original value type. Although boxing and unboxing are sometimes necessary, they can negatively impact performance because they involve memory allocations and type conversions. In high-performance applications, developers should minimize unnecessary boxing operations. Interviewers often ask about boxing and unboxing to evaluate a candidate's understanding of memory management and runtime behavior.

5. What is a delegate?

A delegate is a type-safe function pointer that allows methods to be referenced, stored, and invoked dynamically. Delegates enable developers to pass behavior as parameters, making code more flexible and reusable. They are heavily used throughout the .NET ecosystem, particularly in event handling, callbacks, asynchronous programming, and LINQ. Built-in delegate types such as Action, Func, and Predicate simplify many common programming scenarios. Understanding delegates is important because they form the foundation for several advanced language features and design patterns that appear frequently in enterprise .NET applications.

6. What is an event?

An event is a mechanism that allows one object to notify other objects when a particular action occurs. Events are built on top of delegates and follow the publisher-subscriber pattern. The object raising the event is known as the publisher, while the objects responding to the event are subscribers. Events are commonly used in user interface programming, messaging systems, domain-driven design, and many enterprise applications. They help reduce coupling between components because subscribers do not need direct knowledge of the publisher's implementation. This makes systems easier to extend and maintain over time.

7. What is a lambda expression?

A lambda expression is a concise syntax for defining anonymous functions in C#. Lambda expressions allow developers to write shorter and more readable code compared to traditional delegate implementations. They are extensively used in LINQ queries, asynchronous programming, event handlers, and functional programming scenarios. For example, a lambda expression can be used to filter a collection, sort data, or define custom business logic directly within a method call. Because lambda expressions appear throughout modern C# codebases, understanding how they work is essential for both technical interviews and day-to-day development.

8. What is an interface?

An interface defines a contract that implementing classes must follow. Unlike classes, interfaces do not typically contain implementation details. Instead, they define a set of members such as methods, properties, and events that implementing types must provide. Interfaces promote loose coupling and make applications easier to test, maintain, and extend. They are widely used in dependency injection, clean architecture, and enterprise application development. Understanding interfaces is critical because they play a central role in modern software design and are frequently discussed in .NET interviews.

9. What is abstraction?

Abstraction is the process of hiding implementation details while exposing only the functionality that users of a component need to know. The goal of abstraction is to reduce complexity and improve maintainability by separating what an object does from how it does it. In C#, abstraction is commonly achieved through interfaces and abstract classes. For example, a payment service may expose a ProcessPayment method without revealing the underlying integration logic. Effective use of abstraction helps developers build systems that are easier to modify and extend as requirements evolve.

10. What is polymorphism?

Polymorphism is the ability of objects to take multiple forms and exhibit different behavior through a common interface. In C#, polymorphism is commonly achieved through method overriding, interface implementation, and inheritance. This allows developers to write flexible code that works with abstractions rather than concrete implementations. For example, a collection of shapes may contain circles, rectangles, and triangles, all of which implement a common Draw method differently. Polymorphism improves extensibility, reduces coupling, and enables cleaner application architectures. It is considered one of the most important principles of object-oriented programming.

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11. What is the difference between abstract class and interface?

Both abstract classes and interfaces are used to define contracts and promote abstraction, but they serve different purposes. An abstract class can contain both implemented and abstract members, allowing developers to share common functionality across derived classes. Interfaces, on the other hand, define capabilities that implementing classes must provide. Since C# supports multiple interface implementations but only single inheritance, interfaces are often preferred when defining behaviors that can be shared across unrelated classes. Understanding when to use an interface versus an abstract class is a common senior-level interview topic.

12. What is garbage collection in C#?

Garbage Collection (GC) is an automatic memory management system provided by the .NET runtime. Its primary responsibility is to reclaim memory occupied by objects that are no longer reachable by the application. This reduces memory leaks and simplifies development because programmers do not need to manually free memory. The GC operates in generations and periodically scans memory to determine which objects can be removed. Understanding how garbage collection works is important when building high-performance applications and troubleshooting memory-related issues.

13. What is the difference between stack and heap?

The stack and heap are two different memory regions used by .NET applications. The stack is typically used for local variables, method calls, and value types. It is very fast because memory allocation and deallocation follow a simple last-in-first-out pattern. The heap stores reference type objects and has a longer lifetime. Memory on the heap is managed by the garbage collector. Understanding the distinction between stack and heap helps developers write more efficient code and avoid unnecessary allocations.

14. What is a value type?

A value type stores its actual data directly within the variable. Examples include int, double, bool, DateTime, and structs. When a value type is assigned to another variable, the data itself is copied. Because value types are copied by value, modifying one variable does not affect the other. This behavior differs significantly from reference types and is an important concept in C# interviews.

15. What is a reference type?

A reference type stores a reference to an object rather than the object itself. Examples include classes, arrays, delegates, and strings. Multiple variables can reference the same object in memory. When one variable modifies the object, other references observe the same changes because they point to the same instance. Understanding reference semantics is critical when working with object-oriented applications.

16. What is boxing?

Boxing is the process of converting a value type into a reference type. When boxing occurs, the runtime allocates an object on the heap and copies the value into that object. Although boxing provides flexibility, it introduces memory allocations and can negatively impact performance when used excessively. Developers should be aware of situations that cause implicit boxing, especially when optimizing high-performance applications.

17. What is unboxing?

Unboxing is the process of extracting a value type from a boxed object. The runtime verifies the object's type before copying the value back into the target variable. If the object contains an incompatible type, an InvalidCastException is thrown. Because boxing and unboxing involve allocations and type checks, they should be minimized in performance-critical code paths.

18. What is IDisposable?

IDisposable is an interface used to release unmanaged resources explicitly. Examples include database connections, file handles, network sockets, and other external resources that are not managed by the garbage collector. The Dispose method allows developers to clean up resources deterministically rather than waiting for garbage collection. Proper implementation of IDisposable is an important best practice in enterprise .NET applications.

19. What is the using statement?

The using statement ensures that objects implementing IDisposable are properly disposed when they are no longer needed. It automatically invokes the Dispose method even if an exception occurs within the block. This helps prevent resource leaks and improves application stability. Using statements are commonly used when working with database connections, streams, file operations, and HTTP clients.

20. What is a memory leak in .NET?

Although .NET provides automatic garbage collection, memory leaks can still occur. A memory leak happens when objects remain referenced and therefore cannot be collected by the garbage collector. Common causes include static references, event subscriptions, cached objects, and improperly managed collections. Over time, memory leaks can increase memory consumption and reduce application performance. Identifying and resolving memory leaks is a valuable skill for senior .NET developers.

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21. What is a delegate in C#?

A delegate is a type-safe function pointer that allows methods to be passed as parameters, stored in variables, and invoked dynamically at runtime. Delegates are commonly used in event-driven programming, callbacks, LINQ, and asynchronous programming. They enable loose coupling between components because code can execute methods without knowing their exact implementation details. The .NET Framework relies heavily on delegates in many APIs, including Task Parallel Library, event handling, and LINQ. Understanding delegates is essential because they form the foundation for events, lambda expressions, Action, Func, and many advanced programming techniques used in modern C# applications.

22. What is the difference between delegates and interfaces?

Delegates and interfaces both enable abstraction, but they solve different problems. A delegate represents a method signature and is typically used when a class needs to execute behavior supplied by another component. An interface defines a contract containing one or more members that implementing classes must provide. Delegates are often more lightweight when only a single operation is required. Interfaces are generally preferred when multiple related behaviors need to be grouped together. Choosing between delegates and interfaces depends on the complexity and flexibility requirements of the application.

23. What is an event in C#?

An event is a mechanism that allows one object to notify other objects when something important happens. Events are built on top of delegates and follow the publisher-subscriber pattern. The publisher raises the event, while subscribers register handlers that respond when the event occurs. Examples include button clicks, file system changes, and business process notifications. Events help reduce coupling between components and make applications easier to maintain and extend. They are one of the most common concepts tested in intermediate and senior C# interviews.

24. What is the difference between delegates and events?

Events are based on delegates but provide additional encapsulation and safety. A delegate can be invoked directly by any code that has access to it. An event restricts invocation to the class that owns the event, preventing external components from triggering it accidentally. This restriction helps maintain control over application behavior and ensures that only the publisher can raise notifications. Events are generally preferred whenever implementing publisher-subscriber communication patterns.

25. What is Func in C#?

Func is a built-in generic delegate that represents a method returning a value. The last generic parameter specifies the return type, while the preceding parameters represent input arguments. Func delegates are commonly used in LINQ, dependency injection, asynchronous programming, and callback scenarios. Because Func eliminates the need to define custom delegates for many situations, it helps reduce boilerplate code and improves readability. Senior developers frequently use Func when designing flexible and reusable APIs.

26. What is Action in C#?

Action is a built-in generic delegate that represents a method that does not return a value. It can accept zero or more input parameters and is widely used for callbacks, event handlers, and configurable behaviors. Action simplifies code by eliminating the need for custom delegate definitions in many scenarios. Developers often use Action when passing behavior into methods or configuring services. Together, Action and Func form the foundation of many modern C# programming patterns.

27. What is Predicate<T>?

Predicate<T> is a built-in delegate that represents a method returning a boolean value for a given input. It is commonly used for filtering operations, validation logic, and searching collections. Methods such as List<T>.Find and List<T>.Exists frequently use Predicate<T>. Although Func<T, bool> can achieve the same result, Predicate<T> remains widely used throughout the .NET ecosystem and is commonly encountered in legacy and modern codebases.

28. What are lambda expressions?

Lambda expressions provide a concise syntax for creating anonymous functions. They are heavily used in LINQ queries, event handlers, asynchronous operations, and functional programming patterns. Lambda expressions improve readability by reducing boilerplate code associated with delegate definitions. For example, instead of creating a separate method to filter data, developers can express the logic inline. Understanding lambda expressions is critical because they appear throughout modern C# applications and are frequently discussed during technical interviews.

29. What is a generic type?

Generics allow developers to create reusable classes, methods, and interfaces that work with different data types while maintaining compile-time type safety. Without generics, developers often relied on object references, which required casting and introduced runtime errors. Generics eliminate these problems while improving performance by reducing boxing and unboxing operations. Examples include List<T>, Dictionary<TKey, TValue>, and Task<T>. Generics are one of the most important language features in C# because they enable highly reusable and efficient code.

30. What are the advantages of generics?

Generics provide type safety, improved performance, and greater code reuse. By specifying types at compile time, generics reduce runtime casting errors and make code easier to understand. They also avoid unnecessary boxing and unboxing operations when working with value types. Modern .NET libraries rely extensively on generics because they allow developers to write flexible and reusable components without sacrificing performance. A strong understanding of generics is expected from mid-level and senior C# developers.

31. What is IEnumerable?

IEnumerable is the most fundamental collection interface in .NET and represents a sequence of elements that can be iterated over. It exposes a single method called GetEnumerator, which allows developers to traverse a collection using a foreach loop. Many built-in collection types such as List<T>, Array, and HashSet<T> implement IEnumerable. IEnumerable supports deferred execution and forms the foundation of LINQ. Because LINQ queries often operate on IEnumerable sequences, understanding how enumeration works is important for writing efficient and maintainable code.

32. What is ICollection?

ICollection extends IEnumerable and provides additional functionality for working with collections. It includes members such as Count, Add, Remove, and Clear, allowing developers to manage collection contents more effectively. ICollection is useful when a component needs both enumeration capabilities and collection management features. Many collection types implement ICollection because it provides a common contract for interacting with groups of objects while remaining independent of the underlying implementation.

33. What is IList?

IList represents an ordered collection of elements that can be accessed by index. Unlike ICollection, IList provides functionality such as inserting items at specific positions and retrieving elements using index-based access. This makes it suitable for scenarios where order matters and random access is required. Common implementations include List<T> and arrays. Understanding IList helps developers choose the most appropriate collection type based on application requirements.

34. What is List<T>?

List<T> is one of the most commonly used generic collection types in .NET. It provides dynamic sizing, efficient index-based access, and a rich set of methods for adding, removing, searching, and sorting elements. Internally, List<T> is backed by an array that automatically grows as more elements are added. Because of its flexibility and ease of use, List<T> is often the default choice when developers need an ordered collection of items.

35. What is Dictionary<TKey, TValue>?

Dictionary<TKey, TValue> stores key-value pairs and provides extremely fast lookups using a hash table. Instead of searching through an entire collection, developers can retrieve values directly using their associated keys. This makes Dictionary ideal for caching, configuration storage, lookup tables, and indexing scenarios. The average lookup complexity is O(1), making Dictionary significantly faster than linear searches in large collections.

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36. What is HashSet<T>?

HashSet<T> is a collection designed to store unique values. Unlike List<T>, HashSet automatically prevents duplicate entries and provides efficient lookup operations using hash-based storage. It is particularly useful when checking whether an item already exists within a collection. HashSet is commonly used in validation logic, duplicate detection, and set operations such as unions, intersections, and differences.

37. What is Queue<T>?

Queue<T> is a collection that follows the First-In-First-Out (FIFO) principle. The first element added to the queue is the first element removed. Queue is commonly used in task scheduling systems, message processing pipelines, background job execution, and breadth-first search algorithms. Methods such as Enqueue and Dequeue make Queue easy to use when processing items in the order they arrive.

38. What is Stack<T>?

Stack<T> is a collection that follows the Last-In-First-Out (LIFO) principle. The most recently added element is removed first. Common use cases include undo functionality, expression evaluation, recursive algorithm support, and navigation history management. Stack provides methods such as Push, Pop, and Peek, making it a simple yet powerful data structure for many programming scenarios.

39. When should you use Dictionary instead of List?

Dictionary should be used when fast lookups by key are required. A List typically requires iterating through elements to find a match, resulting in O(n) lookup complexity. Dictionary uses hashing to provide average O(1) lookups, making it significantly faster for large datasets. However, Lists are often more memory-efficient and simpler when ordered data or sequential processing is the primary requirement. Choosing the right collection depends on access patterns and performance needs.

40. What is the difference between Array and List<T>?

Arrays have a fixed size that must be defined when they are created, while List<T> can grow and shrink dynamically. Arrays generally provide slightly better performance because their size is fixed and memory layout is simpler. However, List<T> offers greater flexibility through methods such as Add, Remove, Insert, and Sort. In modern C# applications, List<T> is often preferred unless there is a specific performance requirement or a fixed-size collection is sufficient.

41. What is a record in C#?

A record is a reference type introduced in C# 9 that is designed primarily for storing immutable data. Unlike traditional classes, records provide built-in value-based equality. This means two record instances are considered equal if their properties contain the same values rather than if they reference the same object in memory. Records reduce boilerplate code by automatically generating methods such as Equals, GetHashCode, and ToString. They are commonly used for DTOs, API contracts, event messages, and domain models where immutability is important.

42. What is a record struct?

A record struct combines the benefits of records and structs. Like records, record structs provide value-based equality and concise syntax. Like structs, they are value types rather than reference types. Record structs are useful when representing small immutable data structures that should be copied by value. They can help reduce heap allocations and improve performance in scenarios involving large numbers of short-lived objects.

43. What is Span<T>?

Span<T> is a high-performance type introduced to enable efficient access to contiguous regions of memory without additional allocations. It can reference data stored in arrays, stack memory, or unmanaged memory. Unlike traditional collections, Span<T> avoids copying data and allows developers to work directly with memory segments. Span<T> is heavily used in performance-critical applications, parsers, networking libraries, and high-throughput systems where minimizing memory allocations is important.

44. What problem does Span<T> solve?

Span<T> solves the problem of excessive memory allocations and unnecessary data copying. Before Span<T>, operations such as substring extraction or array slicing often required creating new objects and allocating additional memory. These allocations could increase garbage collection pressure and reduce application performance. By allowing developers to work with views over existing memory, Span<T> significantly improves performance while reducing memory consumption in high-throughput applications.

45. What is the ref keyword in C#?

The ref keyword allows arguments to be passed by reference instead of by value. When using ref, both the caller and the called method operate on the same memory location. Any modifications made within the method are reflected in the original variable. Ref is useful when working with large value types or when a method needs to modify the caller's variable directly. However, it should be used carefully because it can make code harder to understand and maintain.

46. What is the out keyword?

The out keyword is used to pass arguments by reference when a method needs to return multiple values. Unlike ref parameters, out parameters do not need to be initialized before being passed into a method. However, the called method must assign a value before returning. Out parameters are commonly used in methods such as int.TryParse, where the method returns both a success indicator and the parsed result.

47. What is the in keyword?

The in keyword passes an argument by reference while preventing modification of the value. This allows large value types to be passed efficiently without creating copies while still maintaining immutability. Using in can improve performance in scenarios involving large structs because it reduces copying overhead while preserving safety and readability.

48. What is pattern matching in C#?

Pattern matching is a language feature that simplifies type checking, casting, and conditional logic. It allows developers to express complex conditions in a more readable and maintainable way. Examples include type patterns, property patterns, relational patterns, and switch expressions. Pattern matching reduces boilerplate code and improves clarity, making it one of the most valuable additions to modern versions of C#.

49. What are nullable reference types?

Nullable reference types were introduced to help developers identify and prevent NullReferenceException errors. When enabled, the compiler distinguishes between nullable and non-nullable references and generates warnings when potentially unsafe code is detected. This feature improves code quality by encouraging developers to explicitly define whether a variable can contain null values. Many modern .NET applications enable nullable reference types as part of their coding standards.

50. What are the most common C# mistakes developers make?

Common C# mistakes include overusing static state, creating unnecessary allocations, misunderstanding value versus reference types, ignoring asynchronous programming best practices, and failing to dispose unmanaged resources properly. Developers also frequently misuse LINQ, create performance bottlenecks through excessive object creation, and write code that is difficult to test due to tight coupling. Senior developers are expected not only to understand language features but also to recognize common pitfalls and apply best practices that improve maintainability, scalability, and performance. Interviewers often focus on these practical issues because they reveal real-world experience rather than theoretical knowledge alone.

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