C# Interview Questions

In the ever-evolving landscape of software development, mastering a versatile and powerful language like C# can open doors to a myriad of possibilities. Whether you are a seasoned developer looking to enhance your skills or a newcomer navigating the vast world of programming, a solid understanding of C# is essential. In this medium blog post, we embark on a journey through key C# concepts and features, accompanied by illustrative code examples. From asynchronous programming and exception handling to design patterns and LINQ queries, we'll delve into 103 insightful questions to broaden your knowledge and empower you to write robust and efficient C# code. Let's unravel the intricacies of C# together, equipping you with the expertise needed to excel in your programming endeavors.

1. What is C#?

   • C# (pronounced C sharp) is a modern, object-oriented programming language developed by Microsoft.

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

   • Value types store the actual data, while reference types store a reference to the data. Value types are stored in the stack, and reference types are stored in the heap.

3. What is the purpose of the 'using' statement in C#?

   • The 'using' statement is used for automatic resource management, particularly with objects like file streams or database connections. It ensures proper cleanup and releases resources when they are no longer needed.

4. What is the difference between 'const' and 'readonly' in C#?

   • 'const' is used for compile-time constants, and 'readonly' is used for runtime constants. 'const' values are determined at compile-time, while 'readonly' values can be set at runtime but not changed thereafter.

5. Explain the concept of boxing and unboxing in C#.

   • Boxing is the process of converting a value type to a reference type, and unboxing is the reverse process. It involves wrapping a value type in an object (boxing) and extracting the value from the object (unboxing).

6. What is the purpose of the 'virtual' keyword in C#?

   • The 'virtual' keyword is used to indicate that a method or property can be overridden in derived classes. It's a key component of polymorphism in object-oriented programming.

7. How does exception handling work in C#?

   • C# uses a try-catch block for exception handling. Code within the 'try' block is monitored for exceptions, and if an exception occurs, it's caught and handled in the 'catch' block.

8. What is the 'async' and 'await' keywords used for in C#?

   • 'async' and 'await' are used for asynchronous programming. 'async' denotes an asynchronous method, and 'await' is used to pause execution until the awaited task completes, without blocking the thread.

9. Explain the purpose of the 'interface' keyword in C#.

   • Interfaces define a contract for classes to implement. They consist of method signatures, properties, events, and indexers. A class that implements an interface must provide implementations for all its members.

10. What is the 'LINQ' in C#?

    • LINQ (Language Integrated Query) is a set of language extensions that add query capabilities to C#. It provides a consistent way to query various data sources, such as databases, collections, XML, etc.

11. Differentiate between 'StringBuilder' and 'String' in C#.

    • 'StringBuilder' is mutable, allowing for dynamic modifications, while 'String' is immutable, meaning its value cannot be changed after creation. StringBuilder is more efficient for concatenating or modifying strings in a loop.

12. What are delegates in C#?

    • Delegates are reference types that hold references to methods. They are used to create type-safe function pointers and to implement events and callbacks.

13. Explain the concept of polymorphism in C#.

    • Polymorphism allows objects of different types to be treated as objects of a common base type. It includes method overriding, interfaces, and operator overloading.

14. How does garbage collection work in C#?

    • Garbage collection is the automatic process of reclaiming memory occupied by objects that are no longer in use. The .NET runtime has a garbage collector that identifies and releases memory from unreferenced objects.

15. What is the purpose of the 'yield' keyword in C#?

    • 'yield' is used in iterator methods to simplify the process of creating iterators. It indicates where a value should be returned in the sequence.

16. Explain the concept of generics in C#.

    • Generics allow you to create classes, interfaces, and methods with placeholders for data types. They provide type safety and enable code reusability without sacrificing performance.

17. What is the difference between 'var' and 'dynamic' in C#?

    • 'var' is implicitly typed at compile time, while 'dynamic' is resolved at runtime. 'var' is used for statically-typed variables, and 'dynamic' is used for late-bound types.

18. How does the 'using' directive differ from the 'using' statement in C#?

    • The 'using' directive is used to include a namespace in the program, while the 'using' statement is used for resource management, particularly with the 'IDisposable' interface.

19. Explain the concept of an abstract class in C#.

    • An abstract class is a class that cannot be instantiated on its own and may contain abstract methods. It serves as a base class for other classes, providing a common interface.

20. What is the purpose of the 'sealed' keyword in C#?

    • The 'sealed' keyword is used to prevent further inheritance of a class. Once a class is marked as 'sealed,' it cannot be extended by other classes.

21. What are lambda expressions in C#?

    • Lambda expressions are concise and anonymous methods that allow you to write inline functions. They are often used in LINQ queries and functional programming constructs.

22. Explain the 'async/await' pattern and its benefits in C#.

    • 'async/await' is used for asynchronous programming, making it easier to write asynchronous code without the complexities of traditional callback-based approaches. It improves code readability and maintainability.

23. What is the purpose of the 'lock' statement in C#?

    • The 'lock' statement is used to synchronize access to a resource by preventing multiple threads from executing a specific code block simultaneously. It helps in avoiding race conditions.

24. How does C# support multiple inheritance?

    • C# supports multiple interface inheritance, allowing a class to implement multiple interfaces. However, it does not support multiple inheritance for classes to avoid the diamond problem.

25. What is the significance of the 'try-finally' block in C# exception handling?

    • The 'try-finally' block ensures that code within the 'finally' block is executed, regardless of whether an exception is thrown or not. It is commonly used for cleanup operations.

26. Explain the difference between 'ArrayList' and 'List' in C#.

    • 'ArrayList' is a non-generic collection, storing objects of any type, while 'List' is a generic collection, providing type safety by allowing storage of a specific type.

27. How does C# support properties, and what is the advantage of using them?

    • Properties allow encapsulation of class fields and provide controlled access to them. They have get and set methods and enable additional logic when getting or setting values.

28. What is the purpose of the 'volatile' keyword in C#?

    • The 'volatile' keyword is used to indicate that a field may be modified by multiple threads, ensuring that the latest value is always read from and written to memory.

29. Explain the concept of events in C#.

    • Events are a mechanism for communication between objects. They enable a class to notify other classes or objects when something of interest occurs, allowing for loosely coupled designs.

30. What is the 'using' statement used for with databases in C#?

    • The 'using' statement is used with database connections to ensure that the connection is properly closed and resources are released, even if an exception occurs during database operations.

31. How does C# support reflection?

    • Reflection allows inspecting and interacting with types, methods, properties, and other members of assemblies at runtime. C# provides the 'System.Reflection' namespace for reflection.

32. Explain the concept of indexers in C#.

    • Indexers allow instances of a class or struct to be indexed like arrays. They provide a way to access elements or data members using square bracket notation.

33. What is the purpose of the 'is' and 'as' operators in C#?

    • The 'is' operator is used for type checking, and 'as' is used for safe type casting. 'is' returns a boolean indicating whether an object is of a specified type, while 'as' performs a safe cast.

34. How does C# support anonymous types?

    • Anonymous types allow you to create objects without explicitly defining a class. They are often used in LINQ queries to return structured data without the need for a predefined class.

35. Explain the 'StringBuilder' capacity and performance considerations.

    • Setting an initial capacity for 'StringBuilder' can improve performance by reducing the number of reallocations. It's especially beneficial when appending large amounts of text.

36. What is the role of the 'Web API' in C#?

    • Web API in C# is a framework for building HTTP services that can be consumed by various clients, such as web browsers and mobile devices. It follows RESTful principles for communication.

37. How does C# support conditional compilation?

    • Conditional compilation allows code to be included or excluded during compilation based on specified conditions. The '#if', '#else', and '#endif' directives are used for conditional compilation in C#.

38. What is the purpose of the 'Dispose' method in C#?

    • The 'Dispose' method is part of the 'IDisposable' interface and is used for releasing unmanaged resources, such as file handles or database connections. It's commonly employed within the 'using' statement.

39. How does C# handle value types and reference types in method parameters?

    • Value types are passed by value, meaning a copy of the actual data is passed, while reference types are passed by reference, allowing modifications to the actual object.

40. Explain the concept of 'extension methods' in C#.

    • Extension methods allow adding new methods to existing types without modifying them. They are static methods defined in a static class and are called as if they were instance methods of the extended type.

41. What is the difference between 'IEnumerable' and 'IEnumerator' in C#?

    • 'IEnumerable' is an interface that represents a collection of objects that can be enumerated, while 'IEnumerator' is an interface used to iterate through the elements of a collection.

42. Explain the concept of dependency injection in C#.

    • Dependency injection is a design pattern in which the dependencies of a class are provided from the outside, typically through constructor injection or property injection. It enhances modularity and testability.

43. How does C# handle exceptions in a multi-threaded environment?

    • In a multi-threaded environment, unhandled exceptions in a thread terminate the thread but not the entire application. Each thread can have its own exception handling mechanism.

44. What is the purpose of the 'var' keyword in C#?

    • The 'var' keyword is used for implicit typing, allowing the compiler to determine the type of a variable based on the assigned value. It is often used in LINQ queries and when the type is evident from the right-hand side of the assignment.

45. Explain the concept of 'serialization' in C#.

    • Serialization is the process of converting an object's state into a format that can be easily stored, transmitted, or reconstructed. C# supports serialization through the 'System.Runtime.Serialization' namespace.

46. How does C# support operator overloading?

    • Operator overloading allows defining custom behaviors for operators. In C#, you can overload operators by providing special methods in your class with names like 'operator+' or 'operator==.'

47. What is the 'yield return' statement used for in C#?

    • The 'yield return' statement is used in iterator methods to return each element of the collection one at a time without blocking the calling thread. It is often used in conjunction with 'foreach' loops.

48. Explain the concept of 'Nullable Types' in C#.

    • 'Nullable Types' allow value types to have a value of 'null' in addition to their normal range of values. They are declared using a question mark syntax, like 'int?' or 'double?'.

49. What is the purpose of the 'dynamic' keyword in C#?

    • The 'dynamic' keyword allows you to work with objects whose type is not known until runtime. It enables late binding and is often used in scenarios where the type is determined dynamically.

50. How does C# support encryption and decryption?

    • C# provides classes in the 'System.Security.Cryptography' namespace for encryption and decryption. Commonly used classes include 'AesManaged' and 'RSA'.

51. Explain the concept of 'asynchronous programming' in C# and its benefits.

    • Asynchronous programming in C# allows non-blocking execution of code, enhancing responsiveness. 'async/await' keywords simplify the development of asynchronous code by providing a more readable and maintainable structure.

52. What is the 'Lazy' class in C# and when would you use it?

    • 'Lazy' is a class in C# that defers the creation of an object until it is actually needed. It is useful when creating the object is resource-intensive, and you want to postpone its creation until it's necessary.

53. How does C# handle method overloading?

    • Method overloading in C# allows defining multiple methods with the same name but different parameter lists. The compiler selects the appropriate method based on the number and types of arguments provided.

54. What is the role of the 'Task' class in C#?

    • The 'Task' class represents an asynchronous operation in C#. It is used in conjunction with the 'async/await' keywords to perform tasks asynchronously without blocking the main thread.

55. Explain the concept of 'shallow copy' and 'deep copy' in C#.

    • A 'shallow copy' duplicates an object and its references but not the objects referenced. A 'deep copy' creates a new object and copies all the objects referenced by the original object recursively.

56. What is the purpose of the 'nameof' operator in C#?

    • The 'nameof' operator returns the name of a variable, type, or member as a string at compile time. It helps avoid hardcoding names, reducing the likelihood of errors during refactorings.

57. Explain the difference between 'Singleton' and 'Static' classes in C#.

    • 'Singleton' is a design pattern that ensures a class has only one instance and provides a global point of access to it. 'Static' classes, on the other hand, cannot be instantiated and are used for grouping related methods or properties.

58. How does C# handle memory management?

    • C# uses automatic memory management through a garbage collector. The garbage collector identifies and releases memory that is no longer in use, helping developers avoid manual memory management issues like memory leaks.

59. What are attributes in C# and how are they used?

    • Attributes provide metadata about the code elements, such as classes, methods, or properties. They are used for adding declarative information, and they are accessed using reflection at runtime.

60. Explain the concept of 'boxing' and 'unboxing' in C#.

    • 'Boxing' is the process of converting a value type to a reference type, and 'unboxing' is the reverse process. It involves wrapping a value type in an object (boxing) and extracting the value from the object (unboxing).

61. What is the role of the 'app.config' or 'web.config' file in a C# application?

    • 'app.config' or 'web.config' files are used to store configuration settings for an application. They allow developers to store key-value pairs, connection strings, and other configuration information in a centralized location.

62. Explain the purpose of the 'finally' block in C# exception handling.

    • The 'finally' block is used to specify code that should be executed regardless of whether an exception is thrown or not. It is often used for cleanup operations, ensuring resources are released.

63. What are anonymous types in C#? Provide an example.

    • Anonymous types allow you to create objects without explicitly defining a class. For example:

    var person = new { Name = "John", Age = 30 };

1. What is the 'Action' delegate in C#?

   • The 'Action' delegate represents a method that takes parameters and returns void. It is a predefined generic delegate and is often used for encapsulating and passing around methods as parameters.

2. Explain the difference between 'Task.Run' and 'Task.Factory.StartNew' in C#.

   • Both methods are used to run a delegate asynchronously. However, 'Task.Run' is generally preferred for its simplicity and better exception handling, while 'Task.Factory.StartNew' provides more control over the task creation options.

3. What is the 'using static' directive in C#?

   • The 'using static' directive allows you to use static members of a class without specifying the class name. It simplifies the code by eliminating the need to prefix every static member with the class name.

4. How does C# handle memory leaks in long-running applications?

   • C# uses a garbage collector to automatically reclaim memory from objects that are no longer in use. Properly disposing of unmanaged resources and using tools like profilers can help identify and address memory leaks.

5. What is the purpose of the 'params' keyword in C#?

   • The 'params' keyword allows a method to accept a variable number of parameters of a specified type. It simplifies calling methods with varying numbers of arguments.

6. Explain the 'DataContract' attribute in C#.

   • The 'DataContract' attribute is used in the context of WCF (Windows Communication Foundation) to indicate that a class or structure is part of a data contract. It helps in defining the structure of data that can be exchanged between services.

7. What is the purpose of the 'ref' and 'out' keywords in C#?

   • Both 'ref' and 'out' keywords are used for passing arguments by reference. 'ref' indicates that the parameter may be modified, while 'out' is used when the parameter is expected to be assigned a value within the method.

8. Explain the 'using' statement with regard to IDisposable objects.

   • The 'using' statement is used to ensure the proper disposal of objects that implement the 'IDisposable' interface. It automatically calls the 'Dispose' method on the object when the block is exited, ensuring resource cleanup.

9. What is the purpose of the 'nameof' operator in C# 6.0 and later versions?

   • The 'nameof' operator returns the name of a variable, type, or member as a string at compile time. It is useful in scenarios where string literals are used, reducing the risk of naming-related errors during code changes.

10. Explain the 'Checked' and 'Unchecked' keywords in C#.

    • The 'Checked' keyword is used to enable overflow checking for integral-type arithmetic operations, while 'Unchecked' is used to disable overflow checking. By default, overflow checking is enabled in C#.

11. What is the 'in' parameter modifier in C# 7.2 and later versions?

    • The 'in' modifier is used to indicate that a parameter is intended to be read-only. It is particularly useful when passing large structures to avoid unnecessary copying.

12. What is the purpose of the 'async void' return type in C#?

    • 'async void' is used for asynchronous methods that don't have a return value. It is commonly used for event handlers where the asynchronous operation doesn't need to report a result.

13. Explain the concept of 'pattern matching' in C#.

    • Pattern matching is a feature that allows developers to test a value against a pattern and conditionally extract information from it. It simplifies code and enhances readability in scenarios like 'switch' statements and 'is' expressions.