100 Must-Know C# Interview Questions and Answers 2025

C# Fundamentals

1.
What is C# and what are its key features?
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2.
Explain the basic structure of a C# program.
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3.
What are the different types of data types available in C#?
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4.
What is the difference between value types and reference types?
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5.
What are nullable types in C#?
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6.
Can you describe what namespaces are and how they are used in C#?
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7.
Explain the concept of boxing and unboxing in C#.
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8.
What is Type Casting and what are its types in C#?
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9.
What are operators in C# and can you provide examples?
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10.
What is the difference between == operator and .Equals() method?
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11.
What is the purpose of the var keyword in C#?
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12.
What are the differences between const and readonly keywords?
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13.
How does checked and unchecked context affect arithmetic operations?
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14.
What are the different ways to handle errors in C#?
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15.
Explain the role of the garbage collector in .NET.
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Object-Oriented Programming in C#

16.
Define Object-Oriented Programming and its principles.
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17.
What is a class and how is it different from a struct?
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18.
Explain the concept of inheritance and its use in C#.
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19.
What is polymorphism, and can you give a C#_ example?
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20.
What is encapsulation and how is it implemented in C#?
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21.
What are abstract classes and interfaces, and when do you use each?
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22.
Can you explain what a virtual method is in C#?
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23.
What is method overloading and method overriding?
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24.
Can you describe the base keyword?
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25.
What is an access modifier and what are the different types of access modifiers?
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C# Advanced Concepts

26.
What are indexers in C#?
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27.
Explain the concept of delegates in C#.
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28.
What are events and how are they different from delegates?
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29.
What are Lambda expressions and where would you use them?
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30.
Can you explain what extension methods are and how to use them?
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31.
What are generics and how do they provide type safety?
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32.
Define LINQ and mention its advantages.
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33.
What is the difference between IEnumerable and IQueryable?
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34.
What are async and await keywords and how do they work?
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35.
What is the purpose of the using statement?
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C# Collections and Data Structures

36.
What are collections in C#?
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37.
What is the difference between arrays and collections?
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38.
Explain the different types of collections in .NET.
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39.
What is the difference between List and LinkedList?
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40.
Can you discuss the IDictionary interface and its implementation?
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41.
What are HashTable and Dictionary and how do they differ?
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42.
How does a C# HashSet work and what are its benefits?
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43.
What are Enumerable and Queryable collections?
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44.
When would you use a Queue vs a Stack?
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45.
How do you sort elements in a collection?
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C# Exception Handling

46.
What is exception handling and why is it necessary?
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47.
What are the common exception types in C#?
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48.
How do you create custom exceptions in C#?
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49.
What is the use of the finally block?
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50.
Can you explain exception filters introduced in C# 6?
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C# Asynchronous Programming

51.
What is the Task Parallel Library (TPL)?
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52.
Explain the difference between synchronous and asynchronous operations.
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53.
How do you cancel an asynchronous operation?
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54.
What is the difference between Task and Thread?
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55.
Discuss the use of the Parallel class in C#.
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C# File I/O and Serialization

56.
How do you read from and write to a text file in C#?
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57.
What are the file handling classes in C#?
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58.
Explain serialization and deserialization in the context of C#.
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59.
What is the difference between XML Serialization and JSON Serialization?
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60.
How do you use streams in C#?
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C# Attributes and Reflection

61.
What are attributes in C#?
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62.
How do you define a custom attribute?
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63.
What is reflection and why is it useful?
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64.
Explain how to use reflection to inspect an assembly’s metadata.
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65.
How do you use reflection to create an instance of a class at runtime?
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C# Memory Management

66.
Describe the stack and heap in .NET’s memory management.
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67.
What are the finalizers in C#?
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68.
How do you force a garbage collection?
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69.
Explain the IDisposable interface and the Dispose pattern.
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70.
What is a memory leak in .NET and how can it be prevented?
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C# Debugging and Diagnostic

71.
How do you debug a C# application?
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72.
What are breakpoints and how are they used?
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73.
Explain the use of the Debug and Trace classes.
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74.
Discuss the techniques to analyze a memory dump.
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75.
How can you profile a C# application to identify performance bottlenecks?
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C# Concurrency and Parallelism

76.
What is a deadlock and how can it be prevented?
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77.
Discuss the reader-writer lock pattern in C#.
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78.
Explain how the lock keyword ensures thread safety.
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79.
What are Mutexes, Semaphores, and Monitors?
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80.
How do you achieve parallelism using PLINQ?
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C# Unit Testing and Test Driven Development (TDD)

81.
What is unit testing and what frameworks do you use for it in C#?
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82.
Explain the concept of Test-Driven Development (TDD).
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83.
How do you mock objects in C# unit tests?
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84.
What are the common attributes used in a test method?
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85.
How do you test asynchronous code in C#?
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C# Best Practices and Design Patterns

86.
Why are SOLID principles important in C#?
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87.
Can you describe some common design patterns and their applications in C#?
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88.
How do you ensure your C# code is maintainable and readable?
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89.
What strategies do you use for error handling and exception management?
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90.
Discuss the concept of dependency injection and how it’s used in C#.
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91.
What are the new features introduced in the latest version of C#?
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92.
How has pattern matching evolved in recent C# versions?
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93.
Explain how C#8 nullable reference types work.
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94.
What is the switch expression and how does it differ from the switch statement?
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95.
How do you take advantage of tuples in C#?
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C# Interoperability

96.
How can you call unmanaged code using C#?
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97.
What is the role of P/Invoke in C#?
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98.
How do you interface with COM objects in C#?
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99.
Discuss C# and .NET Core inter-platform capabilities.
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C# and .NET Framework/Core Future

100.
How is C#_ evolving with .NET 5 and beyond?
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100 C# interview questions

What is C# and what are its key features?

Explain the basic structure of a C# program.

What are the different types of data types available in C#?

What is the difference between value types and reference types?

What are nullable types in C#?

Can you describe what namespaces are and how they are used in C#?

Explain the concept of boxing and unboxing in C#.

What is Type Casting and what are its types in C#?

What are operators in C# and can you provide examples?

What is the difference between == operator and .Equals() method?

What is the purpose of the var keyword in C#?

What are the differences between const and readonly keywords?

How does checked and unchecked context affect arithmetic operations?

What are the different ways to handle errors in C#?

Explain the role of the garbage collector in .NET.

Define Object-Oriented Programming and its principles.

What is a class and how is it different from a struct?

Explain the concept of inheritance and its use in C#.

What is polymorphism, and can you give a C#_ example?

What is encapsulation and how is it implemented in C#?

What are abstract classes and interfaces, and when do you use each?

Can you explain what a virtual method is in C#?

What is method overloading and method overriding?

Can you describe the base keyword?

What is an access modifier and what are the different types of access modifiers?

What are indexers in C#?

Explain the concept of delegates in C#.

What are events and how are they different from delegates?

What are Lambda expressions and where would you use them?

Can you explain what extension methods are and how to use them?

What are generics and how do they provide type safety?

Define LINQ and mention its advantages.

What is the difference between IEnumerable and IQueryable?

What are async and await keywords and how do they work?

What is the purpose of the using statement?

What are collections in C#?

What is the difference between arrays and collections?

Explain the different types of collections in .NET.

What is the difference between List and LinkedList?

Can you discuss the IDictionary interface and its implementation?

What are HashTable and Dictionary and how do they differ?

How does a C# HashSet work and what are its benefits?

What are Enumerable and Queryable collections?

When would you use a Queue vs a Stack?

How do you sort elements in a collection?

What is exception handling and why is it necessary?

What are the common exception types in C#?

How do you create custom exceptions in C#?

What is the use of the finally block?

Can you explain exception filters introduced in C# 6?

What is the Task Parallel Library (TPL)?

Explain the difference between synchronous and asynchronous operations.

How do you cancel an asynchronous operation?

What is the difference between Task and Thread?

Discuss the use of the Parallel class in C#.

How do you read from and write to a text file in C#?

What are the file handling classes in C#?

Explain serialization and deserialization in the context of C#.

What is the difference between XML Serialization and JSON Serialization?

How do you use streams in C#?

What are attributes in C#?

How do you define a custom attribute?

What is reflection and why is it useful?

Explain how to use reflection to inspect an assembly’s metadata.

How do you use reflection to create an instance of a class at runtime?

Describe the stack and heap in .NET’s memory management.

What are the finalizers in C#?

How do you force a garbage collection?

Explain the IDisposable interface and the Dispose pattern.

What is a memory leak in .NET and how can it be prevented?

How do you debug a C# application?

What are breakpoints and how are they used?

Explain the use of the Debug and Trace classes.

Discuss the techniques to analyze a memory dump.

How can you profile a C# application to identify performance bottlenecks?

What is a deadlock and how can it be prevented?

Discuss the reader-writer lock pattern in C#.

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

What is the purpose of the `var` keyword in C#?

What are the differences between `const` and `readonly` keywords?

How does `checked` and `unchecked` context affect arithmetic operations?

What is the purpose of the `using` statement?

Explain the use of the `Debug` and `Trace` classes.

Explain how the `lock` keyword ensures thread safety.