C# | Interview Questions and Answers

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1. What is C#?

C# (pronounced C-sharp) is a modern, object-oriented programming language developed by Microsoft as part of the .NET platform.

2. Differentiate between String and StringBuilder in C#.

• String is immutable, meaning its value cannot be changed after it's created. StringBuilder is mutable and allows for efficient manipulation of strings.

3. Explain the difference between const and readonly in C#.

• const is compile-time constant, and its value must be assigned at the time of declaration. readonly allows the value to be assigned either at the time of declaration or within a constructor.

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

• The using statement is used to include a namespace in the program. It also helps in resource management by automatically disposing of objects when they are no longer needed.

5. Describe 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 converting a reference type back to a value type.

6. What is the significance of the var keyword in C#?

• The var keyword is used for implicitly declaring variables, letting the compiler infer the type based on the assigned value.

7. Explain the purpose of the async and await keywords.

• async is used to declare an asynchronous method, and await is used to asynchronously wait for a task to complete, allowing other tasks to run in the meantime.

8. Differentiate between == and Equals method in C#.

• == is used for comparing the values of two variables, while the Equals method is used for comparing the contents of objects.

9. What is a delegate in C#?

• A delegate is a type that represents references to methods with a specific signature. It is similar to a function pointer in C or C++.

10. Explain the use of LINQ in C#.

• LINQ (Language Integrated Query) is a set of features that extends C# by the addition of query expressions, allowing the manipulation of data from different sources.

11. How does garbage collection work in C#?

• Garbage collection is an automatic memory management process in C# that identifies and collects objects that are no longer needed, freeing up memory.

12. Describe the Singleton design pattern.

• The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance. It involves a private constructor and a static method to retrieve the instance.

13. What is the difference between throw and throw ex in C#?

• throw is used to rethrow the current exception, preserving the original stack trace, while throw ex discards the original stack trace and starts a new one.

14. How does the try, catch, and finally blocks work in C#?

• try is used to enclose the code that might throw an exception. catch is used to handle the exception, and finally is used to specify a block of code that will be executed, regardless of whether an exception is thrown or not.

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

• The sealed keyword is used to prevent a class from being inherited. It ensures that the class cannot be a base class for other classes.

16. Explain the concept of an interface in C#.

• An interface is a contract that defines a set of methods without providing the implementation. Classes that implement an interface must provide concrete implementations for all its methods.

17. How does method overloading work in C#?

• Method overloading allows a class to have multiple methods with the same name but different parameters, enabling the flexibility of using different argument lists.

18. Describe the use of the out keyword in C#.

• The out keyword is used to pass a parameter by reference, allowing the called method to modify the value of the parameter.

19. What is a property in C#?

• A property is a member that provides a flexible mechanism to read, write, or compute the value of a private field.

20. How does the IEnumerable interface work in C#?

• IEnumerable is an interface that provides an enumerator, allowing a collection to be iterated using a foreach loop.

21. Explain the concept of indexers in C#.

• Indexers allow objects to be indexed like arrays, providing a convenient way to access elements within a class.

22. What is the purpose of the using directive in C#?

• The using directive is used to include namespaces in a program, allowing the use of types defined in those namespaces without fully qualifying the type names.

23. Describe the concept of events in C#.

• Events are a mechanism for communication between objects. They allow a class to notify other classes or objects when an action or state change occurs.

24. How does the lock statement work in C#?

• The lock statement is used to synchronize access to a shared resource by acquiring a mutual exclusion lock.

25. What is the role of the params keyword in C#?

• The params keyword allows a method to accept a variable number of parameters, making it more flexible.

26. Explain the concept of nullable types in C#.

• Nullable types allow variables to be assigned a value or null. They are useful when dealing with databases or other situations where values may be missing.

27. What is the purpose of the ref keyword in C#?

• The ref keyword is used to pass a parameter by reference, allowing the called method to modify the value of the parameter.

28. Differentiate between interface and abstract class in C#.

• An interface cannot contain any implementation, while an abstract class can have both abstract and concrete methods. A class can implement multiple interfaces but can inherit from only one abstract class.

29. How does the yield keyword work in C#?

• The yield keyword is used in iterator methods to simplify the implementation of custom iterators.

30. 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 converting a reference type back to a value type.

31. What are the different access modifiers in C#?

• The main access modifiers in C# are public, private, protected, internal, protected internal, and private protected.

32. Describe the use of the base keyword in C#.

• The base keyword is used to access members of the base class from within a derived class.

33. What is the purpose of the this keyword in C#?

• The this keyword is used to refer to the

current instance of the class. It is often used to differentiate between instance variables and parameters with the same name.

34. 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 overloading and method overriding.

35. How does the String.Split method work in C#?

• The String.Split method is used to split a string into an array of substrings based on a specified delimiter.

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

• The Dispose method is used to release unmanaged resources held by an object. It is often implemented by classes that use resources such as files or network connections.

37. Describe the concept of generics in C#.

• Generics allow the creation of classes, interfaces, and methods with placeholder types, providing type safety and code reusability.

38. How does the Nullable<T> struct work in C#?

• The Nullable<T> struct allows value types to have a null value, providing a way to represent the absence of a value.

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

• The volatile keyword is used to indicate that a field might be modified by multiple threads, preventing compiler optimizations that could cause unexpected behavior in a multi-threaded environment.

40. How does the as keyword work in C#?

• The as keyword is used for safe type casting. It attempts to cast an object to a specified type and returns null if the cast fails instead of throwing an exception.

41. Explain the role of the static keyword in C#.

• The static keyword is used to declare a member that belongs to the type itself rather than to a specific instance of the type.

42. What is the difference between a value type and a reference type in C#?

• Value types store the actual data, while reference types store a reference to the memory location where the data is stored.

43. How does the using statement work in C# for implementing IDisposable?

• The using statement ensures that the Dispose method of an object that implements IDisposable is called, providing a convenient way to manage resources.

44. Describe the concept of inversion of control (IoC) in C#.

• Inversion of Control is a design principle where the control of the flow of a program is inverted, typically achieved through dependency injection.

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

• Extension methods allow adding new methods to existing types without modifying them, enhancing the functionality of types from external libraries.

46. How does the try pattern work in C# 7.0 and later?

• The try pattern in C# 7.0 and later allows for pattern matching within the catch block, enabling more expressive and concise exception handling.

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

• The nameof operator is used to obtain the simple (unqualified) string name of a variable, type, or member.

48. How does the Expression class work in C#?

• The Expression class in C# allows representing code as data, enabling operations like compiling and manipulating expressions at runtime.

49. Explain the concept of the async/await pattern in C#.

• The async/await pattern is used for asynchronous programming in C#, making it easier to write asynchronous code without using callbacks or blocking threads.

50. Describe the role of the Thread class in C#.

• The Thread class in C# is used to create and control threads, providing methods for starting, pausing, and terminating threads.

What Next?

These are just a few examples of C# interview questions. Depending on the level of expertise required, interview questions may vary from basic to advanced topics. Good luck with your C# interviews!