27 Most Important .NET Fundamentals Interview Questions and Answers (2025)

Blog / 27 Most Important .NET Fundamentals Interview Questions and Answers (2025)

.NET fundamentals are the bedrock beneath every C# line you write from the Common Language Runtime’s JIT magic to the type system, garbage collector, and base class libraries that tie it all together.

Grasping how these pieces interact not only sharpens your day-to-day coding instincts; it also gives you the interview edge, letting you explain why your code behaves, scales, and performs the way it does when everyone else is stuck at “it just works.”

Q1.

What is the .NET Framework?

Junior

The .NET Framework is a comprehensive software development platform created by Microsoft.
It provides a runtime environment called the Common Language Runtime (CLR) and a rich set of class libraries, supporting multiple programming languages like C#, VB .NET, and F# for building various applications on Windows and other platforms with .NET Core and later versions.

Q2.

What are the key components of the .NET Framework?

Junior

The two main components are:

Common Language Runtime (CLR), which manages the execution of .NET applications by providing services like memory management and security.
.NET Class Library (BCL), a vast collection of pre-built classes, interfaces, and types offering a wide range of functionalities.

Q3.

Explain the difference between value types and reference types in .NET.

Junior

Value types (e.g., int, bool, struct) store their actual data directly in memory, typically on the stack. Each variable holds its own copy of the data.
Reference types (e.g., class, string, array) store a reference (memory address) to the actual data, which is located on the heap. Multiple variables can point to the same data in memory.

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Q4.

What is managed code in .NET?

Junior

Managed code is code that is executed under the control of the Common Language Runtime (CLR). The CLR provides services such as automatic memory management (garbage collection), type safety, and exception handling.Languages like C# and VB .NET compile to managed code.

Q5.

Explain boxing and unboxing in .NET.

Junior

Boxing is the implicit conversion of a value type to a reference type (specifically, to an object or an interface). The value type is copied onto the heap.
Unboxing is the explicit conversion of a reference type back to a value type. It involves checking the type and copying the value from the heap back to the stack.

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Q6.

What is garbage collection in .NET?

Junior

Garbage collection is an automatic memory management process performed by the CLR.
It periodically identifies and reclaims memory occupied by objects that are no longer referenced by the application, thus preventing memory leaks.

Q7.

What is the difference between String and StringBuilder in C#?

Junior

String objects are immutable, meaning their value cannot be changed after creation. Any operation that seems to modify a string actually creates a new String object.
StringBuilder is mutable, allowing modifications to the string without creating a new object each time, making it more efficient for frequent string manipulations.

Q8.

What are the main principles of Object-Oriented Programming (OOP)?

Junior

The four main principles are:

Encapsulation: Encapsulation is the principle of bundling data (attributes or properties) and the methods that operate on that data into a single unit, or class. It also involves hiding the internal implementation details of an object and controlling access to its data.
Abstraction: Abstraction is the principle of showing only the essential or relevant information to the user while hiding the complex underlying details. It focuses on what an object does rather than how it achieves it, often using abstract classes and interfaces.
Inheritance: Inheritance is a mechanism that allows a new class (derived or child class) to inherit properties and behaviors (methods) from an existing class (base or parent class). This promotes code reusability and helps in establishing hierarchical relationships between classes.C# supports single inheritance for classes but multiple inheritance for interfaces.
Polymorphism: Polymorphism, meaning "many forms," enables objects of different classes to respond to the same method call in their own specific way. This can be achieved through method overriding and method overloading.

Q9.

Explain assemblies in .NET.

Junior

An assembly is a compiled unit of code (.exe for applications, .dll for libraries) that contains Intermediate Language (IL), metadata (manifest), and resources. It's the basic unit of deployment, versioning, and security in .NET.
Assemblies can be private (application-specific) or shared (stored in the Global Assembly Cache - GAC).

Q10.

What are delegates in .NET?

Junior

A delegate is a type-safe function pointer that represents a reference to a method with a specific signature (return type and parameters).
Delegates allow you to treat methods as objects, enabling them to be passed as arguments to other methods.

Q11.

What is the Common Language Runtime (CLR)?

Mid

The CLR is the virtual machine component of the .NET Framework that manages the execution of .NET applications. It provides services like memory management, security, and exception handling.

Q12.

What is Just-In-Time (JIT) compilation?

Mid

JIT compilation is a process where the Common Language Runtime (CLR) converts the Intermediate Language (IL) code of a .NET assembly into native machine code at runtime, just before it's executed.
This allows for platform independence while achieving efficient execution.

Q13.

Explain generics in .NET and their benefits.

Mid

Generics allow you to define type-safe data structures and algorithms without specifying the exact data type at compile time.
Type parameters are used as placeholders, and the actual type is specified when the generic type or method is used.
Benefits include improved type safety, code reusability, and better performance (by avoiding boxing/unboxing for value types).

Q14.

How does exception handling work in .NET?

Mid

Exception handling in .NET uses try blocks to enclose code that might throw an exception. catch blocks are used to handle specific types of exceptions that occur in the try block.
The optional finally block contains code that is guaranteed to execute regardless of whether an exception occurred or was caught, often used for cleanup operations.

Q15.

What is LINQ and how is it used?

Mid

LINQ (Language Integrated Query) provides a unified syntax within C# to query data from various sources like collections, databases, and XML.
It uses a set of extension methods to perform operations like filtering, sorting, and selecting data directly in the code, improving readability and type safety.

Q16.

What is the difference between IEnumerable and IQueryable in LINQ, and when to use each?

Mid

IEnumerable represents a sequence that can be iterated over, typically used for in-memory collections. Operations on IEnumerable are executed in memory.
IQueryable also represents a sequence but is designed for querying data sources outside of memory (like databases). It allows for query translation and execution at the data source, improving performance for large datasets.
Use IQueryable when querying databases (e.g., with Entity Framework) and IEnumerable for in-memory collections.

Q17.

What is dependency injection (DI) and its benefits?

Mid

Dependency Injection is a design pattern where a class receives its dependencies from an external source rather than creating them itself.
This promotes loose coupling, improves testability (by allowing the use of mock objects), increases code reusability, and enhances maintainability.

Q18.

How do you optimize performance in .NET applications?

Mid

Performance optimization involves various techniques such as choosing the right data structures, minimizing boxing and unboxing, using StringBuilder for string manipulation, leveraging asynchronous programming, optimizing database queries (indexing, stored procedures), implementing caching, and profiling the application to identify bottlenecks.

Q19.

Describe common design patterns used in .NET.

Mid

Common design patterns include creational patterns like Singleton and Factory, structural patterns like Adapter and Decorator, and behavioral patterns like Observer and Strategy.
Architectural patterns like MVC are also widely used, especially in ASP .NET Core.

Q20.

How do you handle concurrency and multithreading in .NET?

Mid

.NET provides several ways to handle concurrency, including the Thread class, the Thread Pool, the Task and async/await pattern, the Parallel class for parallel execution, and thread-safe collections in the System.Collections.Concurrent namespace.
Proper synchronization mechanisms like locks are crucial to avoid race conditions and deadlocks.

Q21.

What is reflection in .NET and when would you use it?

Senior

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Q22.

How do you ensure security in .NET applications?

Senior

Q23.

Describe different approaches to caching in .NET applications and when to use each.

Senior

Q24.

What is the difference between await AsyncMethod() and AsyncMethod().Result?

Senior

Q25.

What are code contracts in .NET?

Senior

Q26.

How does the Task Parallel Library (TPL) improve parallel programming?

Senior

Q27.

Explain the concept of marshaling in .NET.

Senior