C# Interview Questions and Answers for freshers

1. What is C#?

   • Answer: C# (C Sharp) is a modern, object-oriented programming language developed by Microsoft. It's part of the .NET framework and is used to build a wide variety of applications, from desktop software to web applications and games.

2. What is the .NET framework?

   • Answer: The .NET framework is a software framework developed by Microsoft. It provides a runtime environment (CLR - Common Language Runtime) and a large class library (FCL - Framework Class Library) that simplifies application development in various languages, including C#.

3. Explain the difference between `System.Int32` and `int` in C#.

   • Answer: `System.Int32` and `int` are aliases for the same data type; they both represent a 32-bit signed integer. `System.Int32` is the full type name, while `int` is a more concise keyword alias provided for convenience.

4. What are value types and reference types in C#? Give examples.

   • Answer: Value types (e.g., `int`, `float`, `bool`, `struct`) store their data directly on the stack. Reference types (e.g., `class`, `string`, `array`) store a reference to the data on the heap. When you assign a value type, a copy is made. When you assign a reference type, only the reference is copied.

5. What is a `struct` in C#? When would you use it?

   • Answer: A `struct` is a value type that is used to group related data together. Use `struct` for small, lightweight data structures that don't require inheritance or a complex lifecycle. They're more efficient than classes for small data items because they're stored on the stack.

6. What is a class in C#?

   • Answer: A class is a blueprint for creating objects. It defines the data (fields/properties) and behavior (methods) of objects. Classes are reference types.

7. Explain the concept of object-oriented programming (OOP).

   • Answer: OOP is a programming paradigm based on the concept of "objects," which contain data (attributes) and code (methods) that operate on that data. Key principles include encapsulation, inheritance, and polymorphism.

8. What is encapsulation?

   • Answer: Encapsulation is the bundling of data and methods that operate on that data within a class, protecting the data from outside access and misuse. This is typically achieved through access modifiers like `public`, `private`, `protected`, and `internal`.

9. What is inheritance?

   • Answer: Inheritance is a mechanism where a class (derived class or subclass) inherits properties and methods from another class (base class or superclass). It promotes code reusability and establishes an "is-a" relationship.

10. What is polymorphism?

    • Answer: Polymorphism allows objects of different classes to be treated as objects of a common type. This enables flexibility and extensibility in code. It's often implemented through method overriding and interfaces.

11. What is an interface in C#?

    • Answer: An interface defines a contract that classes can implement. It specifies a set of methods, properties, and events that the implementing classes must provide. Interfaces promote loose coupling and polymorphism.

12. What is an abstract class in C#?

    • Answer: An abstract class cannot be instantiated directly. It serves as a base class for other classes, providing a common blueprint and potentially containing abstract methods (methods without implementation) that derived classes must implement.

13. Explain the difference between `abstract` and `virtual` methods.

    • Answer: `abstract` methods have no implementation in the base class and *must* be implemented by derived classes. `virtual` methods have an implementation in the base class but can be overridden by derived classes.

14. What are access modifiers in C#?

    • Answer: Access modifiers (`public`, `private`, `protected`, `internal`, `protected internal`) control the accessibility of members (fields, methods, properties) within a class or struct. They determine which parts of the code can access those members.

15. What is a constructor in C#?

    • Answer: A constructor is a special method in a class that is automatically called when an object of that class is created. It's used to initialize the object's fields.

16. What is a destructor in C#?

    • Answer: A destructor (finalizer) is a special method in a class that is automatically called when an object is garbage collected. It's used to release unmanaged resources (like file handles or network connections).

17. What is garbage collection in C#?

    • Answer: Garbage collection is an automatic memory management feature in C#. It reclaims memory occupied by objects that are no longer being used, preventing memory leaks.

18. What is a `static` member?

    • Answer: A `static` member (field, method, property) belongs to the class itself, not to any specific instance of the class. There's only one copy of a static member shared by all instances.

19. What is the difference between `==` and `Equals()`?

    • Answer: `==` compares references for reference types and values for value types. `Equals()` is a method that can be overridden to provide custom comparison logic, often comparing the content of objects rather than just references.

20. What is boxing and unboxing?

    • Answer: Boxing is converting a value type to a reference type (e.g., converting an `int` to an `object`). Unboxing is the reverse process.

21. What is a delegate in C#?

    • Answer: A delegate is a type that represents a reference to a method. Delegates enable callbacks and event handling.

22. What is an event in C#?

    • Answer: An event is a notification mechanism. It allows objects to communicate with each other when something significant happens (e.g., a button click).

23. Explain exception handling in C#.

    • Answer: Exception handling uses `try`, `catch`, and `finally` blocks to handle runtime errors. The `try` block contains code that might throw an exception. `catch` blocks handle specific exceptions. `finally` blocks contain cleanup code that always executes.

24. What is a `try-catch-finally` block?

    • Answer: A structured way to handle exceptions. `try` encloses code that might throw an exception, `catch` handles specific exception types, and `finally` executes regardless of whether an exception occurred.

25. What are the different types of exceptions in C#?

    • Answer: Many! Common ones include `System.Exception`, `System.ArgumentException`, `System.NullReferenceException`, `System.IO.FileNotFoundException`, `System.OutOfMemoryException`, and many more, categorized in a hierarchy.

26. What is LINQ?

    • Answer: LINQ (Language Integrated Query) is a powerful querying technology that allows you to query data from various sources (databases, XML, collections) using a consistent syntax.

27. What are lambda expressions?

    • Answer: Lambda expressions are anonymous functions that can be used to create delegates or expression tree types. They provide a concise way to write short methods.

28. What is a generic type?

    • Answer: A generic type is a type that can work with different data types without being explicitly written for each one. This increases code reusability and type safety.

29. What is the difference between `List` and `Array`?

    • Answer: `List` is a dynamic array that can resize itself as needed. `Array` has a fixed size determined at creation.

30. What is a property in C#?

    • Answer: A property provides a controlled way to access and modify the fields of a class. It can include getter and setter methods to perform validation or other operations.

31. What is an indexer in C#?

    • Answer: An indexer allows a class to be accessed like an array, using index notation (e.g., `myObject[0]`).

32. What is operator overloading?

    • Answer: Operator overloading allows you to redefine the behavior of operators (like +, -, *, /) for custom types.

33. What is asynchronous programming in C#?

    • Answer: Asynchronous programming allows you to perform long-running operations without blocking the main thread, improving responsiveness. Keywords like `async` and `await` are used.

34. What is the `async` and `await` keywords?

    • Answer: `async` marks a method as asynchronous. `await` pauses execution of the method until an asynchronous operation completes.

35. Explain the concept of IDisposable interface.

    • Answer: `IDisposable` is an interface that defines a method `Dispose()`. Classes implementing it should release unmanaged resources in the `Dispose()` method, usually using a `using` statement.

36. What is a using statement?

    • Answer: The `using` statement ensures that IDisposable objects are properly disposed of, even if exceptions occur.

37. What are namespaces in C#?

    • Answer: Namespaces are used to organize code and prevent naming conflicts. They provide a hierarchical structure for classes and other types.

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

    • Answer: Use null checks (e.g., `if (myObject != null)`), the null-conditional operator (`?.`), the null-coalescing operator (`??`), and the null-coalescing assignment operator (`??=`).

39. What is the difference between `string` and `StringBuilder`?

    • Answer: `string` is immutable (cannot be changed after creation). `StringBuilder` is mutable (can be modified efficiently), making it better for string manipulation involving many changes.

40. What is reflection in C#?

    • Answer: Reflection allows you to inspect and manipulate types and their members at runtime.

41. What is serialization?

    • Answer: Serialization is the process of converting an object into a stream of bytes that can be stored or transmitted. Deserialization is the reverse process.

42. What is deserialization?

    • Answer: Deserialization is the process of reconstructing an object from a stream of bytes created by serialization.

43. What is multithreading in C#?

    • Answer: Multithreading allows you to execute multiple parts of a program concurrently, improving performance, especially on multi-core processors.

44. How do you create a thread in C#?

    • Answer: Using the `Thread` class and its constructor, or using Task Parallel Library (TPL).

45. What is the Task Parallel Library (TPL)?

    • Answer: TPL simplifies parallel programming in C#, providing higher-level abstractions for managing threads and tasks.

46. What is deadlock?

    • Answer: A deadlock occurs when two or more threads are blocked indefinitely, waiting for each other to release resources.

47. How do you prevent deadlocks?

    • Answer: Careful resource ordering, avoiding circular dependencies, using timeouts, and employing proper synchronization mechanisms.

48. What is a race condition?

    • Answer: A race condition occurs when the outcome of an operation depends on the unpredictable order in which multiple threads execute.

49. How do you handle race conditions?

    • Answer: Using locks (mutexes), semaphores, or other synchronization primitives to control access to shared resources.

50. What is a mutex?

    • Answer: A mutex (mutual exclusion) is a synchronization primitive that allows only one thread to access a shared resource at a time.

51. What is a semaphore?

    • Answer: A semaphore is a synchronization primitive that controls access to a shared resource by a limited number of threads.

52. What is the difference between a mutex and a semaphore?

    • Answer: A mutex allows only one thread access; a semaphore allows a limited number of threads (specified by its count).

53. What are delegates and events used for?

    • Answer: Delegates allow methods to be passed as arguments; events use delegates to provide a notification mechanism (publish-subscribe).

54. What is the difference between a method and a function?

    • Answer: In C#, the terms are often used interchangeably. A method is a function that is a member of a class or struct.

55. What is the purpose of the `params` keyword?

    • Answer: `params` allows a method to accept a variable number of arguments of a specified type.

56. What is the `out` keyword?

    • Answer: `out` parameters don't need to be initialized before being passed to a method; the method is responsible for assigning a value.

57. What is the `ref` keyword?

    • Answer: `ref` parameters must be initialized before being passed to a method; changes made to the parameter in the method affect the original variable.

58. What is the difference between `out` and `ref` keywords?

    • Answer: `out` parameters don't require initialization before the method call, while `ref` parameters do. Both allow the method to modify the original variable.

59. What is the role of the `finally` block in exception handling?

    • Answer: The `finally` block guarantees that code within it will always execute, regardless of whether an exception occurred, allowing for cleanup (e.g., closing files).

60. How do you work with files and directories in C#?

    • Answer: Using classes in the `System.IO` namespace, such as `File`, `Directory`, `StreamReader`, `StreamWriter`.

61. What is the difference between `FileStream` and `StreamReader`?

    • Answer: `FileStream` provides low-level access to files; `StreamReader` provides higher-level access for reading text from files.

62. How do you handle file I/O exceptions?

    • Answer: Using `try-catch` blocks to catch exceptions like `FileNotFoundException`, `IOException`, etc.

63. How can you ensure thread safety in C#?

    • Answer: Using locking mechanisms like `lock` statements, mutexes, semaphores, or other synchronization primitives to protect shared resources.

64. Explain the concept of immutability.

    • Answer: Immutability means that an object's state cannot be modified after it's created. This helps prevent unintended side effects and simplifies concurrency.

65. What are some examples of immutable types in C#?

    • Answer: `string`, value types (int, float, bool, etc.)

66. What is a design pattern?

    • Answer: A reusable solution to a commonly occurring design problem in software development.

67. Can you name a few common design patterns?

    • Answer: Singleton, Factory, Observer, MVC (Model-View-Controller), Strategy, etc.

68. What is the difference between `foreach` and `for` loops?

    • Answer: `foreach` is used to iterate over collections; `for` provides more control over the iteration process (e.g., starting index, ending condition, increment).

69. What is the purpose of the `is` operator?

    • Answer: The `is` operator checks if an object is compatible with a given type.

70. What is the purpose of the `as` operator?

    • Answer: The `as` operator performs a type cast; if the cast fails, it returns `null` instead of throwing an exception.

71. What is the difference between `is` and `as` operators?

    • Answer: `is` checks for type compatibility; `as` performs a type cast safely, returning `null` on failure.

72. Explain the concept of Dependency Injection.

    • Answer: A design principle where dependencies are provided to a class instead of being created within the class, improving testability and modularity.

73. What are some benefits of using Dependency Injection?

    • Answer: Improved testability, loose coupling, better code organization, easier maintenance.

74. What is the role of the `using` directive?

    • Answer: `using` directives add namespaces to the current scope, making it easier to reference types within those namespaces without fully qualifying their names.

75. How do you handle null exceptions?

    • Answer: Use null checks (`if (obj != null)`), the null-conditional operator (`?.`), the null-coalescing operator (`??`), and handle potential exceptions.

76. What are extension methods?

    • Answer: Extension methods allow you to add new methods to existing types without modifying the original type's source code.

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