Dot Net Developer Interview Preparation Guide

Basics of .NET Framework

Q1: What is the .NET Framework?
The .NET Framework is a software development platform created by Microsoft that helps developers build various types of applications like web apps, desktop programs, and mobile apps. Think of it as a toolbox that gives you everything you need to create software that runs on Windows.

Q2: What’s the difference between .NET Framework and .NET Core?
.NET Framework works only on Windows and is heavier in size, while .NET Core is lightweight, works on Windows, Mac, and Linux, and is more modern. .NET Core is like the updated, flexible version that companies prefer nowadays because it can run anywhere.

Q3: What does CLR stand for and what does it do?
CLR means Common Language Runtime. It’s the engine that actually runs your .NET programs. When you write code, CLR converts it into machine language that computers understand, manages memory, handles errors, and keeps your application secure. Basically, it’s the brain behind your program.

Q4: What is FCL in .NET?
FCL stands for Framework Class Library. It’s a huge collection of pre-written code that Microsoft provides for common tasks like working with files, connecting to databases, creating user interfaces, and much more. Instead of writing everything from scratch, you can use these ready-made tools.

Q5: Why do we use C# for .NET development?
C# is the primary language for .NET because it’s powerful yet easy to learn, object-oriented, type-safe, and specifically designed to work perfectly with the .NET ecosystem. It has clean syntax that makes code readable and maintainable.

Data Types and Variables

Q6: What are the different data types in C#?
C# has value types like int for whole numbers, float and double for decimals, bool for true/false, char for single characters, and reference types like string for text, arrays, and classes. Choosing the right data type helps save memory and makes your code efficient.

Q7: What’s the difference between int and float?
Int stores whole numbers without decimals like 10, 100, -50. Float stores decimal numbers like 10.5, 3.14, -2.75. Use int when you don’t need decimal points, like counting items. Use float when you need precision, like calculating prices.

Q8: What is a nullable type in C#?
Nullable types let you assign null to value types that normally can’t be null. For example, int? allows you to store numbers or null. This is helpful when working with databases where some values might be missing.

Q9: Explain static variables versus instance variables.
Static variables belong to the class itself and are shared across all objects. Instance variables belong to individual objects, so each object has its own copy. Think of static as one shared locker for everyone, and instance as personal lockers for each person.

Q10: What is the var keyword in C#?
The var keyword lets you declare variables without explicitly specifying the type. The compiler figures out the type automatically based on the value you assign. For example, var name = “John” automatically makes name a string. It’s convenient but can reduce code clarity if overused.

Q11: What’s the difference between const and readonly?
Const values must be assigned when you declare them and can never change. Readonly values can be assigned either when declared or in the constructor, and they stay constant after that. Use const for values that truly never change like pi, and readonly when you need flexibility in initialization.

Q12: What are dynamic types in C#?
Dynamic types bypass compile-time type checking. The type is resolved at runtime instead. This is useful when working with external data sources or COM objects, but use it carefully because you lose type safety and can get runtime errors.

Object-Oriented Programming Concepts

Q13: What is a class in C#?
A class is a blueprint for creating objects. It defines what properties and behaviors objects will have. For example, a Car class might have properties like color and model, and methods like Start and Stop. The class is the design, the object is the actual car built from that design.

Q14: What is an object?
An object is an instance of a class. When you create an object from a class, you’re bringing the blueprint to life. For example, if Car is the class, then myCar would be an object with specific values like red color and Honda model.

Q15: Explain constructors and their types.
Constructors are special methods that run automatically when you create an object. They initialize the object’s properties. Default constructors take no parameters. Parameterized constructors accept values to set properties during creation. Static constructors initialize static members and run only once.

Q16: What is a destructor?
A destructor is a method that runs when an object is destroyed or garbage collected. It’s used to clean up resources like closing file connections or database connections. In C#, you rarely need to write destructors because garbage collection handles most cleanup automatically.

Q17: What are the four pillars of OOP?
Encapsulation means bundling data and methods together and hiding internal details. Abstraction means showing only essential features and hiding complex implementation. Inheritance means creating new classes based on existing ones. Polymorphism means one interface with multiple implementations.

Q18: What is encapsulation and why is it important?
Encapsulation means wrapping data and methods into a single unit and controlling access using access modifiers like private, public, and protected. It protects your data from unauthorized access and makes code more maintainable. For example, keeping salary as private and accessing it through methods.

Q19: How do you achieve encapsulation in C#?
Use private fields to store data and public properties or methods to access them. Properties let you add validation logic before setting values. This way, you control how data is accessed and modified, preventing invalid data from entering your system.

Q20: What is abstraction?
Abstraction means hiding complex implementation details and showing only what’s necessary. For example, when you use a phone, you don’t need to know how the circuits work internally. You just use the interface like buttons and touchscreen. Same concept in programming.

Q21: What’s the difference between abstract class and interface?
Abstract classes can have both implemented and abstract methods, can have constructors, fields, and access modifiers. Interfaces can only have method signatures without implementation in earlier C# versions, but now can have default implementations. A class can inherit only one abstract class but implement multiple interfaces.

Q22: When should you use an abstract class versus an interface?
Use abstract classes when you have common functionality to share among related classes. Use interfaces when you want to define a contract that unrelated classes can implement. For example, Vehicle as abstract class, but IFlyable interface can be implemented by Bird, Plane, Drone.

Q23: What is inheritance?
Inheritance lets you create new classes based on existing classes. The new class automatically gets all the properties and methods of the parent class. This promotes code reuse. For example, SavingsAccount and CurrentAccount can inherit from BankAccount class.

Q24: What are the types of inheritance in C#?
C# supports single inheritance where a class inherits from one parent class, multilevel inheritance where class C inherits from B which inherits from A, and interface inheritance where a class can implement multiple interfaces. C# doesn’t support multiple class inheritance to avoid complexity.

Q25: What is polymorphism?
Polymorphism means many forms. It allows methods to behave differently based on the object calling them. Compile-time polymorphism happens through method overloading. Runtime polymorphism happens through method overriding using virtual and override keywords.

Q26: Explain method overloading with examples.
Method overloading means having multiple methods with the same name but different parameters. For example, Add(int a, int b), Add(int a, int b, int c), and Add(double a, double b) are overloaded methods. The compiler decides which method to call based on arguments provided.

Q27: Explain method overriding with examples.
Method overriding happens when a child class provides a different implementation for a method already defined in the parent class. Use virtual keyword in the parent method and override keyword in the child method. For example, Animal has virtual Speak() method, Dog overrides it to return “Bark”.

Q28: What is method hiding in C#?
Method hiding uses the new keyword to hide a parent class method in the child class. Unlike overriding, hiding doesn’t use virtual or override. The method called depends on the reference type, not the object type. It’s less common than overriding.

Q29: What are access modifiers in C#?
Public means accessible from anywhere. Private means accessible only within the same class. Protected means accessible within the class and derived classes. Internal means accessible within the same assembly. Protected internal combines protected and internal access.

Q30: What is the purpose of the sealed keyword?
Sealed prevents inheritance. When you mark a class as sealed, no other class can inherit from it. When you mark a method as sealed, it prevents further overriding in child classes. Use it when you want to stop the inheritance chain for security or design reasons.

Advanced C# Concepts

Q31: What is boxing and unboxing?
Boxing converts a value type to a reference type by wrapping it in an object. Unboxing converts it back. For example, int i = 10; object o = i; is boxing, and int j = (int)o; is unboxing. Boxing has performance overhead, so avoid it in loops or performance-critical code.

Q32: What are delegates in C#?
Delegates are type-safe function pointers that hold references to methods. They let you pass methods as parameters to other methods. Think of delegates as middlemen that can call methods on your behalf. They’re used for event handling and callback mechanisms.

Q33: What are the types of delegates?
Single-cast delegates point to one method. Multicast delegates point to multiple methods and call them in sequence. Func delegates return a value. Action delegates don’t return a value. Predicate delegates return bool. Anonymous delegates let you define inline methods without naming them.

Q34: What is an event in C#?
Events are notifications sent by objects when something happens. They’re built on delegates. For example, a button has a Click event that fires when someone clicks it. You subscribe to events by attaching event handler methods that run when the event triggers.

Q35: What’s the difference between delegates and events?
Delegates are function pointers. Events are wrapper around delegates that provide encapsulation. With events, outside code can only subscribe or unsubscribe, but cannot invoke the event or clear subscribers. Events provide better control and safety.

Q36: What are anonymous methods?
Anonymous methods are inline unnamed methods defined using the delegate keyword. They’re convenient for short, one-time-use functionality. For example, delegate(int x) { return x * 2; }. Lambda expressions are more modern and concise replacements for anonymous methods.

Q37: What are lambda expressions?
Lambda expressions are concise syntax for writing anonymous methods. They use the => operator. For example, x => x * 2 is a lambda that doubles a number. They’re heavily used with LINQ queries and make code cleaner and more readable.

Q38: What is exception handling in C#?
Exception handling is managing runtime errors gracefully using try-catch-finally blocks. Try block contains code that might fail. Catch block handles the error. Finally block runs regardless of whether an error occurred, useful for cleanup like closing connections.

Q39: What’s the difference between throw and throw ex?
Throw preserves the original stack trace, helping you debug where the error actually started. Throw ex resets the stack trace to the current location, losing information about the original error source. Always use throw to maintain debugging information.

Q40: What are custom exceptions?
Custom exceptions are your own exception classes created by inheriting from Exception class. Create them when built-in exceptions don’t accurately describe your error. For example, InsufficientBalanceException for banking applications. They make error handling more meaningful.

Q41: What is the purpose of the finally block?
The finally block always executes whether an exception occurs or not. Use it for cleanup code like closing database connections, file handles, or releasing resources. Even if return statements appear in try or catch, finally still runs.

Q42: What are generics in C#?
Generics let you write classes and methods that work with any data type while maintaining type safety. Instead of writing separate code for int list, string list, you write one generic List<T> that works for all types. They improve code reusability and performance.

Q43: What are the benefits of using generics?
Type safety because compiler checks types at compile time. Performance because no boxing/unboxing needed. Code reusability because one generic class works for multiple types. Better IntelliSense support and fewer runtime errors.

Q44: What are constraints in generics?
Constraints limit which types can be used with generics. For example, where T : class means T must be a reference type. Where T : struct means value type. Where T : new() means T must have parameterless constructor. Constraints ensure type has required features.

Q45: What is the difference between Array and ArrayList?
Array is fixed-size and type-safe, storing only one type of data. ArrayList is dynamic-size but not type-safe, storing any type as objects. ArrayList requires boxing for value types. Arrays are faster. Modern code prefers List<T> over ArrayList.

Q46: What is a collection in C#?
Collections are data structures for storing and managing groups of objects. They include lists, dictionaries, queues, stacks, and sets. Collections handle resizing automatically, provide methods for searching and sorting, and are more flexible than arrays.

Q47: Explain List<T> and its advantages.
List<T> is a dynamic array that automatically grows when needed. It’s generic so it’s type-safe. Provides methods like Add, Remove, Contains, Sort, and Find. Much better than ArrayList because no boxing and compile-time type checking.

Q48: What is Dictionary<TKey, TValue>?
Dictionary stores key-value pairs where each key is unique. It provides fast lookups by key. For example, Dictionary<int, string> employees where employee ID is key and name is value. It’s like a real dictionary where you look up words to find meanings.

Q49: What’s the difference between List and LinkedList?
List uses array internally, fast for accessing elements by index, but inserting in middle is slow. LinkedList uses nodes with pointers, fast for inserting anywhere, but slow for accessing by index. Use List for most cases, LinkedList when frequent insertions/deletions in middle.

Q50: What is a Stack collection?
Stack is Last-In-First-Out (LIFO) structure. Push adds elements, Pop removes and returns the top element, Peek returns top without removing. Like a stack of plates where you take from the top. Useful for undo functionality and expression evaluation.

Multithreading and Asynchronous Programming

Q51: What is multithreading?
Multithreading means running multiple operations simultaneously within a single program. Each thread is an independent execution path. It improves application responsiveness and utilizes multiple CPU cores efficiently. For example, downloading a file while still letting users interact with the UI.

Q52: What’s the difference between process and thread?
Process is a complete program running independently with its own memory space. Thread is a lightweight unit within a process sharing the same memory space. Creating threads is faster than creating processes. Threads share data easily but processes are isolated.

Q53: How do you create a thread in C#?
Create a Thread object passing a ThreadStart delegate that points to the method you want to run. Call Start() to begin execution. For example, Thread t = new Thread(MyMethod); t.Start();. Modern code prefers Task-based approaches over direct threading.

Q54: What is thread synchronization and why is it needed?
Thread synchronization ensures that only one thread accesses shared resources at a time, preventing data corruption. Without it, multiple threads modifying the same data simultaneously cause race conditions. Use lock keyword to create critical sections where only one thread can enter.

Q55: What is the lock keyword?
Lock creates a critical section that only one thread can enter at a time. Other threads wait until the lock is released. For example, lock(obj) { sharedVariable++; } ensures thread-safe increment. Always lock on private objects, never on types or strings.

Q56: What is a deadlock?
Deadlock occurs when two or more threads wait for each other indefinitely, creating a circular dependency. Thread A waits for resource held by Thread B, while Thread B waits for resource held by Thread A. Avoid deadlocks by acquiring locks in consistent order and using timeouts.

Q57: What are thread priorities?
Thread priorities tell the scheduler which threads should get more CPU time. Priorities range from Lowest to Highest. Setting priority to High doesn’t guarantee the thread runs faster, it just makes it more likely to get scheduled. Use sparingly as it can cause unpredictable behavior.

Q58: What is the ThreadPool in C#?
ThreadPool is a collection of reusable threads managed by .NET. Instead of creating new threads, you borrow from the pool, use them, and return them. This reduces overhead. Use ThreadPool.QueueUserWorkItem for short-lived operations. Thread pool threads are background threads.

Q59: What’s the difference between foreground and background threads?
Foreground threads keep the application alive. The app won’t exit until all foreground threads finish. Background threads don’t prevent app shutdown. When all foreground threads complete, background threads are terminated. UI threads are foreground, ThreadPool threads are background.

Q60: What is async and await in C#?
Async marks methods that perform asynchronous operations. Await pauses method execution until the asynchronous operation completes without blocking the calling thread. This keeps UI responsive. For example, await DownloadFileAsync() downloads without freezing the interface.

File Handling and Serialization

Q61: How do you read a text file in C#?
Use File.ReadAllText(path) to read entire file as string, or File.ReadAllLines(path) to get array of lines. For large files, use StreamReader for line-by-line reading to avoid loading everything into memory. Always handle exceptions for file not found or access denied.

Q62: How do you write to a text file?
Use File.WriteAllText(path, content) to write string to file, or File.WriteAllLines(path, lines) for arrays. For appending, use File.AppendAllText. For more control, use StreamWriter. Always close files properly using using statement to prevent resource leaks.

Q63: What is serialization?
Serialization converts objects into a format that can be stored or transmitted, like JSON or XML or binary. Deserialization converts it back to objects. It’s essential for saving application state, caching, sending data over network, or storing configuration.

Q64: What are the types of serialization in .NET?
Binary serialization creates compact binary format but isn’t human-readable and has compatibility issues. XML serialization produces readable XML but is verbose. JSON serialization using libraries like Newtonsoft.Json or System.Text.Json is most popular now, readable and widely supported.

Q65: What is the difference between StreamReader and File.ReadAllText?
File.ReadAllText reads entire file into memory at once, simple but memory-intensive for large files. StreamReader reads line by line, memory-efficient, gives more control, but requires manual closing. Use StreamReader for large files, File methods for small files.