The IT world in 2025 is still looking for good C# Developers. So, if you are a fresher and preparing for your first interview. Or, you are a skilled developer seeking a senior-level role then you need to be prepared for C# interview questions.
Basic C# Interview Questions (For Freshers)
Q1. What is C#?
Answer: C#, a contemporary language with safety of types and based on objects, was designed by Microsoft to compile under the .NET environment.
Q2. What are the main features of C#?
Answer: Based on Objects Garbage Collector Implements Type Safety Rich Class Library Provides Cross-Platform .NET Core
Q3. Difference between Value Types and Reference Types?
Answer:
Value Types: They store data (e.g., int, float, struct). Reference Types: References are stored (e.g., class, string, object).
Q4. Explain the Equals() method and == in C# programming?
Answer: Operator == compares values for value types, references for reference types but Equals() compares object contents.
Q5. What is the definition of Constructor in C#?
Answer: A constructor is a unique type of method which appears to be a method that is called automatically when a class object is created. It is primarily used to create the object, which involves setting the object’s fields, properties, and any other logic which requires execution at the time of creation. The name of a constructor must be the same as the name of the class.
Q6. What is the difference between static and instance members in C# programming?
Answer:
static: is associated with the class and is common to all objects. instance: is associated with a particular object.
Q7. What is the difference between const and readonly?
Answer:
const: is a constant which is determined at the time of compilation.
readonly: Assigned once at runtime.
Q8. What is a Destructor in C#?
Answer: It releases resources for an object when it is destroyed. Syntax: ~ClassName() { }
Q9. What is the difference between is and as operators in C# code?
Answer:
is operator Checks object type and confirm that it is suitable for a specific data type.
It returns true if compatible otherwise not.
as operator Converts between reference types or Nullable with types type safely. It returns null if not possible.
Q10. What is an Object in C#?
Answer: To access the properties and method defined in a class. Generally created by new keyword.
Intermediate C# Interview Questions
Q11. What is Inheritance in C#?
Answer: In C# programming, inheritance is a fundamental principle of Object-Oriented Programming (OOP).
It enables you to define a new class (child/derived class) that reuses, extends, or modifies the functionality of another class (parent/base class).
This makes your code more organized, reusable, and maintainable because common functionality can be located in the base class and used by several derived classes.
Why Use Inheritance feature?
- Code Reusability: Write code a single time and reuse multiple times in multiple locations.
- Extensibility: It refers to adding extra features without modifying existing code.
- Maintainability: It means solving issues involving the base class that affect all derived classes immediately.
- Polymorphism: You can use method overriding and overloading polymorphism.
Syntax of Inheritance in C#
class BaseClass
{
// Properties, methods, fields
}
class DerivedClass : BaseClass
{
// Additional properties or methods
}
The colon (
:) symbol is used to define inheritance in C# language.
Q12. What is Polymorphism?
Answer: Polymorphism is a fundamental notion of Object-Oriented Programming (OOP) in C#.
The term polymorphism derives from two Greek words:
Poly means many.
Morph → Forms
So polymorphism means “various forms.” In other words, it enables a single action or method to behave differently depending on the object that invokes it.
Why is Polymorphism useful?
- increases coding flexibility.
- Makes the system flexible (new behavior may be added without affecting existing code).
- Enhances readability and minimizes code duplication.
Polymorphism Type in C#
- Compile-time (method overloading).
- Runtime (method overriding).
Q13. Distinguish between Abstract Class and Interface.
Answer:
Abstract class: Can have implementation.
Interface: Only method signatures (until C# 8 default methods).
Q14. Define Method Overloading and Overriding.
Answer: Key Differences Between Overloading and Overriding
| Feature | Overloading (Compile-time) | Overriding (Runtime) |
|---|---|---|
| Binding Time | Compile time | Runtime |
| Parameters | Must be different | Must be same |
| Keywords Used | None | virtual & override |
| Flexibility | Less flexible | More flexible |
Overloading: Use overloading when methods do similar tasks but need different input parameters.
Overriding: Use overriding when derived classes need to provide their own specific implementation of a method.
Q15. What Encapsulation means in C#?
Answer: Confining and controlling the fields and the accessed methods to the class boundaries using access modifiers.
Q16. What is the difference between Struct and Class in C# language?
Answer:
Struct: Value type, lightweight.
Class: Reference type, supports inheritance.
Q17. What is Boxing and Unboxing?
Answer:
Boxing: Treating a value type as an object.
Unboxing: Retrieving a value type that is stored in an object.
Q18. What Are Properties in C#?
Answer: Properties enable class fields to be accessed in a controlled manner in class with the use of get and set methods.
Q19. What a Namespace is in C#?
Answer: Classes and a Namespace in C# serves as an organizational structure for a set of closely related classes, interfaces, structs, enums, and delegates with respect to a common identifier.
Q20. What is the difference between throw and throw ex in Exception Handling?
Answer:
throw: Preserves stack trace.
throw ex: Resets stack trace.
Q21. What is a Delegate in C#?
Answer: Like a pointer in C++, a Delegate is a type restricted pointer in C# as it allows a dynamic method call and referencing. The use of events, callbacks, and flexible method invocation depends primarily on delegated functions.
Q22. What is an Event in C#?
Answer: An event is a way for a class to announce to other classes that something has occurred.
Q23. What are Generics in C#?
Answer: Generics eliminates the need for boxing/unboxing by enabling classes, methods, and collections to work with any data type.
Q24. Draw the main distinctions between Array and ArrayList in C#.
Answer:
- Fixed vs Flexible Size
Array: If you use an array, the size of the array is fixed. If you declare an array with five elements, you cannot add a sixth one.
ArrayList: The ArrayList may increase or contract as you add or delete objects.
Example:
int[] numbers = new int[3]; // fixed size of 3
// numbers[3] = 50; // Error: index out of range
ArrayList list = new ArrayList();
list.Add(10);
list.Add(20);
list.Add(30);
list.Add(40); // Works fine, no size issue
2. Type Safety
Array: Arrays are strongly typed. It uses the same type of data. If it’s an int[], it can only store integer-type values; other values can’t be held. The compiler enforces it.
ArrayList: Stores elements as objects. This allows you to combine several data types (int, string, bool, and so on), but it weakens type safety and may result in runtime issues.
Example:
int[] marks = new int[2] { 50, 60 }; // only integers allowed
// marks[1] = "Hello"; // Not allowed
ArrayList items = new ArrayList();
items.Add(10);
items.Add("Hello"); // Allowed (but risky)
items.Add(true);
Q25. Explain finalization in contrast with the use of dispose.
Answer:
Finalize(): Called by Garbage Collector (GC). there is no control by coder and it runs automatically.
Dispose(): BY this method, programmer need to manage and takes resources and manually allocates them to the objects.
Q26. Explain in detail the meaning of the terms “early and late binding.”
Answer: Early binding:
Early binding means that the compiler is aware of the referred object, function, or property throughout the creation process at compile time before starting the application. So, everything is decided at compile time, and it’s very fast compared to Late Binding.
Because the compiler is aware of the method in advance, it can ensure that it exists, that the parameters are correct, and that the return type is valid.
Advantages include faster execution, fewer runtime errors, and enhanced support from tools such as IntelliSense in Visual Studio.
Example:
string msgStr = "Hello Rohit";
int length = msgStr.Length; // Compiler knows Length is a property of stringHere, the compiler recognizes that the message is a string and that the length is valid.
Late binding:
Late binding occurs when a decision regarding which object, method, or property to utilize is made only during execution. The compiler does not verify anything in advance, but if you make any mistakes, then it will throw an error at runtime (while running the program).
Example using dynamic:
dynamic objStr = "Hello Rohit";
int lengthObj = objStr.Length; // Compiler won’t check here, error comes only if member not found at runtime
Here, the compiler doesn’t confirm if
objStrhas aLengthproperty. It will only resolve it when the program runs.
Difference – Early Vs Late Binding
Early binding is decided related on function, variable, and class at compile time, making it safer and faster.
Late Binding: Decided at runtime related to a function, variable, or class, which is more flexible but riskier.
Q27. What is the main difference between the terms dynamic, var and object in C# language?
Answer:
var: the type is inferred by compiler at compile time depending on the value given.
dynamic: the type is inferred at run time and you don’t need a specific type for it.
object: This is the base type from which all types are derived.
Q28. What is the difference between out and ref keywords?
Answer:
In the C# language, the ref and out keywords allow us to represent inputs as references rather than values. It suggests that the method can directly change the value of the variable outside of its scope.
1. Initialization Requirements
ref: The variable must be initialized before it is passed to the method.
Reason: Because ref expects an existing value, the method can read and change it.
out: The variable does not have to be initialized before passing.
Reason: The method using out must assign a value before exiting.
2. Data Flow:
ref: Data can run either into and out of the operation.
out: Data can only flow out; the method determines the value; no input value is evaluated.
3. Compiler Enforcement
ref: The compiler ensures that the variable has been assigned before passing it.
out: Compiler forces the method to assign a value before returning.
Example
using System;
class Program
{
static void Main()
{
int a = 10; // Initialized
int b; // Not initialized
// Using ref
UpdateWithRef(ref a);
Console.WriteLine("Value after ref: " + a);
// Using out
UpdateWithOut(out b);
Console.WriteLine("Value after out: " + b);
}
static void UpdateWithRef(ref int number)
{
number += 5; // Can use existing value
}
static void UpdateWithOut(out int number)
{
number = 20; // Must assign before method ends
}
}
Output
Value after ref: 15
Value after out: 20Q29. What is the difference between the two terms IEnumerable and IQueryable?
Answer:
IEnumerable: Iterates in-memory collections like LIST or an Array. It gets all data from dataBase to clients’s memory object and then apply filter.
It is useful for small type of dataset values which already present inside In-memory object.
IQueryable: Supports LINQ-to-SQL, queries executed in database.
It filters data on dabase level and only fetched requested data.
It mostly uses for large datasets stored on remote databases
Q30. In C# programming, state the differences between String and String Builder.
Answer:
String: This is Immutable. It means – once a string object is created then its value we cannot be changed. When we modified string by perform any operation like concatenation and Replace then actually, It creates a new string object in memory with the modified value and previous string object will be remain in memory.
String Builder: This objects are mutable which means we can modify their object value directly without creating new objects in memory and it Designed for effectiveness continue changes.
