Top 50 C# Interview Questions & Answers (2021 Edition) – Freshers & Experienced Developers

C# is a versatile, object‑oriented language that runs on the .NET framework. It powers ASP.NET web applications, desktop software, and even games via Unity. If you’re pursuing a career in C#, mastering these interview questions will help you demonstrate both depth and breadth of knowledge to hiring managers.

Below is a curated list of the most frequently asked C# interview questions, ranging from fundamentals for freshers to advanced topics for seasoned developers. Each question is followed by a concise, authoritative answer that reflects industry best practices.

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C# Interview Questions for Freshers & 2/3/5/10 Years Experience

These questions cover basic concepts, syntax, and common pitfalls that interviewers expect candidates to master.

1. What is C#?

C# is a statically typed, type‑safe, object‑oriented language compiled to Microsoft Intermediate Language (MSIL) by the .NET runtime.

2. Explain the types of comments in C# with examples

• Single‑line: // This is a single‑line comment
• Multi‑line: /* This is a multi‑line comment It can span several lines */
• XML documentation: /// <summary>Set error message</summary>

3. Can multiple catch blocks be executed?

No. Once a matching catch block executes, control passes to the finally block (if present) and then to the code following the try‑catch structure.

4. What is the difference between public, static, and void?

public – Accessible from any code that references the type.
static – Belongs to the type itself, not to an instance.
void – Indicates a method does not return a value.

5. What is an object?

An object is an instance of a class, created with the new keyword. It provides access to the class’s members (methods, properties, fields).

6. Define constructors.

A constructor is a special method that shares its name with the class. It runs automatically when an instance is created, initializing fields and performing setup.

7. What are jagged arrays?

Jagged arrays are arrays of arrays. Each element can reference an array of a different length, allowing irregular data structures.

8. What is the difference between ref & out parameters?

ref requires the variable to be initialized before the call. out does not; the callee must assign a value before returning.

9. What is the use of the using statement in C#?

The using block ensures that an IDisposable resource is automatically released when the block exits, even if an exception occurs.

10. What is serialization?

Serialization converts an object into a byte stream for storage or transmission. Deserialization reverses the process, recreating the object. Types must be marked with [Serializable] or implement ISerializable.

11. Can the this keyword be used within a static method?

No. this refers to the current instance, which static methods do not have.

12. What is the difference between constants and read‑only fields?

Constants are compile‑time values that cannot change. Read‑only fields are assigned once at runtime (either inline or in a constructor) and cannot be modified thereafter.

C# Advanced Interview Questions for 3/5/10 Years Experience

13. What is an interface? Provide an example.

An interface declares a contract of methods and properties without implementation. Example:

public interface IGreeter
{
    void SetName(string name);
    string Greet();
}

public class FriendlyGreeter : IGreeter
{
    private string _name;

    public void SetName(string name) => _name = name;

    public string Greet() => $"Hello, {_name}!";
}

14. What are value types and reference types?

Value types (e.g., int, struct) store data directly in the stack. Reference types (e.g., class, string) hold a reference to the object on the heap.

15. What are Custom Controls and User Controls?

Custom controls are compiled into DLLs and can be dropped onto forms. User controls (.ascx) are composite controls built from existing controls, useful for reusable UI components.

16. What are sealed classes in C#?

A sealed class cannot be inherited. The sealed modifier prevents subclassing, ensuring the class’s behavior remains unchanged.

17. What is method overloading?

Method overloading lets multiple methods share a name but differ in parameter lists. The compiler selects the appropriate overload based on arguments.

18. What is the difference between Array and ArrayList?

An Array has a fixed size and homogeneous element types. ArrayList (now largely replaced by List<T>) is dynamic and stores objects.

19. Can a private virtual method be overridden?

No. A private method is not visible to derived classes, so it cannot be overridden.

20. Describe the accessibility modifier protected internal.

Members marked protected internal are accessible within the same assembly or from derived classes in other assemblies.

21. What are the differences between System.String and System.Text.StringBuilder?

String is immutable; each modification creates a new instance. StringBuilder is mutable, designed for efficient string manipulation.

22. Difference between Array.CopyTo() and Array.Clone()?

Clone() creates a shallow copy of the entire array. CopyTo() copies elements into an existing destination array.

23. How can we sort an array in descending order?

Use Array.Sort() followed by Array.Reverse(), or provide a custom comparer.

24. Write C# syntax to catch an exception.

Example:

try
{
    GetAllData();
}
catch (Exception ex)
{
    // Handle error
}

25. Difference between an interface and an abstract class?

Interfaces declare only signatures; all members are implicitly public and abstract. Abstract classes can provide concrete implementations and private members.

26. Difference between Finalize() and Dispose()?

Dispose releases unmanaged resources deterministically. Finalize (destructor) is called by the GC and is non‑deterministic.

27. What are circular references?

Two or more objects reference each other, potentially preventing garbage collection and causing memory leaks.

28. What are generics in C#?

Generics enable type‑safe, reusable code (e.g., List<T>), reducing runtime casting and improving performance.

29. What is an object pool in .NET?

An object pool keeps a set of initialized objects ready for reuse, minimizing costly allocations and garbage collection.

30. List commonly used exception types in .NET.

ArgumentException, ArgumentNullException, ArgumentOutOfRangeException, ArithmeticException, DivideByZeroException, OverflowException, IndexOutOfRangeException, InvalidCastException, InvalidOperationException, IOException, NullReferenceException, OutOfMemoryException, StackOverflowException, etc.

31. What are custom exceptions?

Custom exceptions are user‑defined types derived from Exception, tailored to specific error scenarios.

32. What are delegates?

Delegates are type‑safe function pointers that enable callbacks and event handling.

33. How do you inherit a class in C#?

Use the colon syntax:

public class Derived : Base
{ }

34. What is the base class from which all .NET classes derive?

System.Object.

35. Difference between method overriding and overloading?

Overriding changes a virtual method’s implementation in a derived class. Overloading creates multiple methods with the same name but different signatures.

36. What are the different ways a method can be overloaded?

By varying parameter type, order, or count.

37. Why can’t you specify accessibility for methods inside an interface?

Interface members are implicitly public; specifying accessibility would be redundant.

38. How can you allow a class to be inherited but prevent a method from being overridden?

Declare the method as sealed inside a non‑sealed class.

39. What happens if inherited interfaces have conflicting method names?

Implement the method once in the derived class, ensuring it satisfies all interface contracts.

40. Difference between a struct and a class?

Structs are value types stored on the stack, cannot inherit from other structs, and are immutable by convention. Classes are reference types stored on the heap.

41. How to use nullable types in .NET?

Append ? to a value type:

int? id = null;
if (id.HasValue)
{ /* use id.Value */ }

42. How can you create an array with non‑default values?

Use Enumerable.Repeat or collection initializers.

43. Difference between is and as operators?

is checks type compatibility and returns a boolean. as casts and returns null if the cast fails.

44. What’s a multicast delegate?

A delegate that references multiple methods; invoking it calls all attached methods in order.

45. What are indexers in C#?

Indexers let objects be indexed like arrays:

public int this[int index] { get { … } set { … } }

46. Difference between throw and throw ex?

throw preserves the original stack trace; throw ex resets it, making debugging harder.

47. What are C# attributes and their significance?

Attributes add metadata to types and members; they can be queried via reflection at runtime.

48. How to implement a singleton design pattern?

Example:

public sealed class Singleton
{
    private static readonly Singleton _instance = new Singleton();
    public static Singleton Instance => _instance;
    private Singleton() { }
}

49. Difference between DirectCast and CType?

DirectCast requires the runtime type to match the target type exactly. CType performs a conversion defined by the target type, potentially invoking conversion operators.

50. Is C# managed or unmanaged code?

C# is managed code; the CLR compiles it to MSIL and handles memory management.

51. What is a console application?

A console application runs in a command‑prompt window and is often the first step for .NET beginners.

52. Example of removing an element from a Queue

Use Dequeue() to remove the front element:

Queue queue = new Queue();
queue.Enqueue(1);
queue.Enqueue(2);
queue.Enqueue(3);
int removed = queue.Dequeue(); // removed = 1