C# Interview Questions and Answers

1. What is C#?

   • Answer: C# is a modern, object-oriented programming language developed by Microsoft that runs on the .NET framework. It's known for its versatility, being used for building various applications, from web and desktop apps to mobile and game development.

2. What are the key features of C#?

   • Answer: Key features include strong typing, automatic garbage collection, component-oriented programming, and a rich standard library. It supports various programming paradigms like object-oriented, imperative, and declarative programming.

3. Explain the difference between `ref` and `out` parameters.

   • Answer: Both `ref` and `out` pass variables by reference, modifying the original variable. However, `ref` requires the variable to be initialized before passing it to the method, while `out` does not. `out` indicates the method will assign a value to the parameter.

4. What is the difference between a class and a struct in C#?

   • Answer: Classes are reference types, meaning variables hold references to objects. Structs are value types, meaning variables hold the data directly. Structs are generally smaller and faster, suitable for simple data structures, while classes are better for complex objects.

5. Explain the concept of inheritance in C#.

   • Answer: Inheritance allows a class (derived class) to inherit properties and methods from another class (base class). It promotes code reusability and establishes an "is-a" relationship between classes. C# supports single inheritance (a class can inherit from only one base class) but allows implementing multiple interfaces.

6. What are interfaces in C#?

   • Answer: Interfaces define a contract that classes must implement. They specify a set of methods, properties, and events that a class must provide. Interfaces support polymorphism, enabling different classes to implement the same interface in their own way.

7. What is polymorphism?

   • Answer: Polymorphism allows objects of different classes to be treated as objects of a common type. This is achieved through inheritance and interfaces, enabling flexibility and extensibility in code.

8. Explain the concept of encapsulation in C#.

   • Answer: Encapsulation bundles data (fields) and methods that operate on that data within a class, protecting the internal state from direct access. Access modifiers like `public`, `private`, `protected`, and `internal` control the visibility and accessibility of members.

9. What is abstraction in C#?

   • Answer: Abstraction hides complex implementation details and exposes only essential information to the user. Abstract classes and interfaces are key tools for achieving abstraction. Abstract classes can contain both abstract and non-abstract methods.

10. What is a delegate in C#?

    • Answer: A delegate is a type that represents a reference to a method. It allows passing methods as parameters to other methods, enabling flexible and event-driven programming.

11. What is an event in C#?

    • Answer: An event is a mechanism that allows a class to notify other classes when something significant happens. It uses delegates to subscribe and unsubscribe event handlers.

12. Explain the difference between `string` and `StringBuilder` in C#.

    • Answer: `string` is immutable; each modification creates a new string object. `StringBuilder` is mutable, making it more efficient for frequent string manipulations, especially with large strings.

13. What are generics in C#?

    • Answer: Generics allow writing type-safe code that can work with different data types without losing type information at compile time. This improves code reusability and performance.

14. What is LINQ (Language Integrated Query)?

    • Answer: LINQ is a set of technologies that allows querying various data sources (databases, XML, collections) using a consistent syntax. It integrates seamlessly with C# and improves data manipulation.

15. Explain different types of LINQ operators.

    • Answer: LINQ operators are categorized into filtering (Where), projection (Select), ordering (OrderBy, ThenBy), grouping (GroupBy), joining (Join), set (Union, Intersect), and quantifiers (Any, All).

16. What is exception handling in C#?

    • Answer: Exception handling uses `try`, `catch`, and `finally` blocks to manage runtime errors. `try` encloses code that might throw an exception, `catch` handles specific exceptions, and `finally` executes regardless of whether an exception occurred.

17. What are different types of exceptions in C#?

    • Answer: C# has various built-in exceptions like `SystemException`, `IOException`, `ArgumentException`, `NullReferenceException`, etc., categorized broadly into system exceptions, application exceptions, and custom exceptions.

18. How to create a custom exception in C#?

    • Answer: Create a new class that inherits from the `Exception` class or one of its subclasses. Override the constructor to provide specific exception information.

19. What is the purpose of the `using` statement in C#?

    • Answer: The `using` statement ensures that disposable objects (implementing `IDisposable`) are properly released, preventing resource leaks. It automatically calls the `Dispose()` method.

20. What is asynchronous programming in C#?

    • Answer: Asynchronous programming allows performing long-running operations without blocking the main thread, improving responsiveness. Keywords like `async` and `await` simplify asynchronous code.

21. Explain the difference between `Task` and `Thread` in C#.

    • Answer: `Thread` represents a system-level thread, managed directly by the operating system. `Task` is a higher-level abstraction built on top of threads, managed by the .NET runtime, simplifying parallel programming.

22. What is a property in C#?

    • Answer: A property provides controlled access to a private field using getter and setter methods, encapsulating data and providing flexibility in how data is accessed and modified.

23. What is an indexer in C#?

    • Answer: An indexer allows accessing elements of a class or struct using array-like syntax, providing custom indexing mechanisms.

24. What is a constructor in C#?

    • Answer: A constructor is a special method used to initialize objects of a class. It is automatically called when a new object is created.

25. What is a destructor in C#?

    • Answer: A destructor (finalizer) is a special method that is automatically called before an object is garbage collected. It's used to release unmanaged resources.

26. What is boxing and unboxing in C#?

    • Answer: Boxing converts a value type to a reference type (Object), and unboxing converts a reference type back to a value type. This allows value types to be stored in collections designed for reference types.

27. What is the difference between `==` and `Equals()` in C#?

    • Answer: `==` compares references for reference types and values for value types. `Equals()` is a method that can be overridden to provide custom equality comparisons for objects.

28. What is the `GetHashCode()` method in C#?

    • Answer: `GetHashCode()` returns a hash code for an object, useful for hashing and storing objects in hash tables. It should return the same value for equal objects.

29. What is serialization in C#?

    • Answer: Serialization converts an object's state into a format (like XML or binary) that can be stored or transmitted. Deserialization reverses this process, reconstructing the object from the stored format.

30. What are different ways to serialize objects in C#?

    • Answer: Common ways include using `BinaryFormatter`, `XmlSerializer`, `DataContractSerializer`, and `Json.NET` (a third-party library).

31. What is reflection in C#?

    • Answer: Reflection allows inspecting and manipulating types, methods, and properties at runtime. It's used in dynamic code generation and other advanced scenarios.

32. What are attributes in C#?

    • Answer: Attributes provide metadata about code elements (classes, methods, properties). They are used for various purposes, including code documentation, serialization, and runtime information.

33. Explain the concept of delegates and events in the context of event handling.

    • Answer: Delegates define the signature of event handlers. Events use delegates to manage subscriptions to event handlers, enabling a class to notify other classes about occurrences.

34. How to handle multiple exceptions in a single `try-catch` block?

    • Answer: Use multiple `catch` blocks, each handling a specific exception type. Order the `catch` blocks from most specific to most general.

35. What is the difference between a `foreach` loop and a `for` loop in C#?

    • Answer: `foreach` iterates through elements of a collection, simplifying iteration. `for` provides more control over loop iterations, including starting point, condition, and increment.

36. Explain the difference between value types and reference types in C#.

    • Answer: Value types store data directly in variables. Reference types store references to objects; the objects themselves reside on the heap.

37. What is garbage collection in C#?

    • Answer: Garbage collection automatically reclaims memory occupied by objects that are no longer in use, preventing memory leaks.

38. Explain the concept of IDisposable interface.

    • Answer: `IDisposable` interface defines the `Dispose()` method, used to release unmanaged resources (like file handles or network connections) explicitly. It's crucial for preventing resource leaks.

39. What is the difference between `const` and `readonly` in C#?

    • Answer: `const` variables are compile-time constants; their values must be known at compile time. `readonly` variables are initialized either at declaration or in the constructor; they can't be changed after that.

40. What is the difference between virtual, abstract, and override methods in C#?

    • Answer: `virtual` methods can be overridden by derived classes. `abstract` methods must be overridden; they have no implementation in the base class. `override` methods provide an implementation in a derived class for a virtual or abstract method in the base class.

41. What is the role of the `sealed` keyword in C#?

    • Answer: `sealed` prevents a class from being inherited or a method from being overridden. It improves performance and ensures the class's behavior remains unchanged.

42. Explain the concept of partial classes in C#.

    • Answer: Partial classes split a class definition across multiple files. This is useful for automatically generated code, keeping the generated code separate from manually written code.

43. What is the purpose of the `params` keyword in C#?

    • Answer: `params` allows a method to accept a variable number of arguments of a specific type. The arguments are passed as an array.

44. What is the difference between static and instance methods in C#?

    • Answer: Static methods belong to the class itself, not to specific instances. Instance methods operate on specific instances of the class.

45. Explain the concept of extension methods in C#.

    • Answer: Extension methods add new methods to existing types without modifying the original type. They are defined as static methods in a static class.

46. What is the `null` conditional operator (`?.`) in C#?

    • Answer: The null-conditional operator safely accesses members of an object only if the object is not null, preventing `NullReferenceException`.

47. What is the null-coalescing operator (`??`) in C#?

    • Answer: The null-coalescing operator returns the left-hand operand if it's not null; otherwise, it returns the right-hand operand. It's useful for providing default values.

48. What is the null-coalescing assignment operator (`??=`) in C#?

    • Answer: The null-coalescing assignment operator assigns the right-hand operand to the left-hand operand only if the left-hand operand is null.

49. Explain the difference between `IEnumerable` and `IQueryable` in C#.

    • Answer: `IEnumerable` represents an in-memory collection. `IQueryable` represents a query that can be executed against a remote data source (like a database). `IQueryable` provides deferred execution.

50. What is deferred execution in LINQ?

    • Answer: Deferred execution means that a LINQ query is not executed until its results are actually needed. This improves performance by avoiding unnecessary processing.

51. What is the purpose of the `nameof` operator in C#?

    • Answer: The `nameof` operator returns the name of a variable, property, type, or method as a string. It's useful for debugging and reflection.

52. What is pattern matching in C#?

    • Answer: Pattern matching allows checking the type and structure of an object concisely, improving code readability and maintainability. It includes `is` expressions and switch expressions.

53. What are tuples in C#?

    • Answer: Tuples are lightweight data structures that group multiple values together, each with a name and type. They are useful for returning multiple values from a method.

54. Explain the concept of disposal pattern.

    • Answer: The disposal pattern involves implementing the `IDisposable` interface and using the `using` statement to ensure proper release of unmanaged resources. This is critical for preventing resource leaks.

55. What is the difference between a list and an array in C#?

    • Answer: Arrays are fixed-size collections. Lists are dynamically sized collections, which are more flexible.

56. What is a stack and a queue in C#?

    • Answer: A stack follows the Last-In, First-Out (LIFO) principle. A queue follows the First-In, First-Out (FIFO) principle.

57. How do you create a thread-safe collection in C#?

    • Answer: Use classes like `ConcurrentBag`, `ConcurrentDictionary`, `ConcurrentQueue`, etc. from the `System.Collections.Concurrent` namespace.

58. What is a lock statement in C#?

    • Answer: The `lock` statement provides mutual exclusion, ensuring that only one thread can access a critical section of code at a time, preventing race conditions.

59. What are the different access modifiers available in C#?

    • Answer: `public`, `private`, `protected`, `internal`, and `protected internal` control the accessibility of class members.

60. What is dependency injection in C#?

    • Answer: Dependency injection is a design pattern that promotes loose coupling by providing dependencies to a class from outside, rather than having the class create them internally.

61. What is the purpose of the `System.Diagnostics` namespace in C#?

    • Answer: The `System.Diagnostics` namespace provides classes for performance monitoring, debugging, and process management.

62. How can you measure the execution time of a piece of code in C#?

    • Answer: Use the `Stopwatch` class in the `System.Diagnostics` namespace to measure elapsed time.

63. What is the difference between synchronous and asynchronous programming?

    • Answer: Synchronous programming executes code sequentially; the next instruction doesn't start until the previous one is finished. Asynchronous programming allows multiple operations to proceed concurrently, improving responsiveness.

64. What is the role of the `async` and `await` keywords in C#?

    • Answer: `async` marks a method as asynchronous, allowing it to use `await`. `await` suspends execution of an asynchronous method until an awaited task completes.

65. What is a lambda expression in C#?

    • Answer: A lambda expression is an anonymous function that can be used to create delegates or expression tree types.

66. What is an expression tree in C#?

    • Answer: An expression tree represents code as a tree-like data structure. It's used by LINQ to translate queries into executable code.

67. What is the difference between a regular expression and a wildcard?

    • Answer: Regular expressions provide a more powerful and flexible way to match patterns in strings than wildcards. Wildcards use simple character matching, whereas regular expressions support more complex patterns and operations.

68. How do you handle null values in C#?

    • Answer: Use null-conditional operators (`?.`), null-coalescing operators (`??`), and null checks to handle null values safely and prevent `NullReferenceException`.

69. What is the purpose of the `System.IO` namespace in C#?

    • Answer: The `System.IO` namespace provides classes for working with files and streams.

70. How do you read a file in C#?

    • Answer: Use `StreamReader` or `File.ReadAllText` to read the contents of a file.

71. How do you write to a file in C#?

    • Answer: Use `StreamWriter` or `File.WriteAllText` to write data to a file.

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