120+ C# Interview Questions and Answers to Crack Your Next Interview

Coding interviews often hinge on specific C# skills: explain generics, optimize a LINQ query, or reason about async code and garbage collection. Have you ever frozen on a delegate question or spent too long debugging a live coding task? This article gathers 120+ C# Interview Questions with clear answers, sample code and practical tips across .NET Core, object oriented design, async await, delegates, events, memory management and debugging so you can feel fully prepared and confident by mastering 120+ C# interview questions and answers and successfully land your desired developer job.

To help with that, Interview Coder's AI Interview Assistant gives focused practice, instant feedback and simple explanations tailored to common C# topics so you can target weak spots, sharpen problem solving and track your progress.

40 C# Interview Questions and Answers for Beginners

1. What Is C#? Modern Object-Oriented Language from Microsoft

C# is a modern, object oriented language created by Microsoft for the .NET platform. It targets web, desktop, mobile, and cloud apps and uses static typing so the compiler catches many errors early.

It includes strong type checking, garbage collection, and inheritance, and supports procedural, functional, and event driven styles. Use C# in technical interviews to show familiarity with a common enterprise language and the .NET class libraries.

2. Explain the Basic Syntax of C#: Code Structure and Entry Point

C# follows a C based syntax with namespaces and classes. A program groups code in a namespace, defines classes with the class keyword, and starts execution at static void Main(string[] args). Statements end with semicolons and comments use // or /* */.

Example:

namespace MyApp { class Program { static void Main(string[] args) { Console.WriteLine("Hi"); } } }.

3. What Are Data Types in C#? Value Types and Reference Types

C# has value types that contain data directly such as int, long, short, byte, float, double, decimal, char, and bool. Reference types hold references to data and include string, arrays, classes, interfaces, and delegates. You can also use nullable forms like int? to allow null for value types.

4. Difference Between int and Int32: Keyword Alias vs. Framework Type

int is the C# keyword alias for System.Int32. They are identical at runtime; choose int for idiomatic C# and use Int32 when you want to emphasize the CLR type in API code. Both represent a 32 bit signed integer.

5. How Do You Declare a Variable in C#? Type, Name, and Optional Initializer

• Declare variables by specifying the type and name: int age
• Initialize inline: int age = 25
• You can declare several of the same type on one line: int x = 10, y = 20

Variable names must start with a letter or underscore, contain letters digits or underscores, and are case sensitive.

6. What Are Operators in C#? Basic Arithmetic, Comparison, and Logic

Operators perform actions on values. Common groups are:

• Arithmetic (+ - * / %)
• Relational (== != > < >= <=)
• Logical (&& || !)
• Bitwise (& | ^ ~)
• Assignment (= += -= *=)

Compound assignment combines operation and assign, for example x += 5.

7. Explain the Difference Between == and === in C#: Equality Checks and Reference Equality

C# uses == for equality. There is no === operator in C# like in some other languages. For value types == compares values. For reference types == usually compares references unless the type overrides operator== (for example string compares contents). To check if two references point to the same object use Object.ReferenceEquals(obj1, obj2).

8. What Is a Namespace in C#? Organize Code and Avoid Name Clashes

A namespace groups related types to prevent naming conflicts. Declare with namespace MyApp { ... } and access types by using directives like using MyApp; or by fully qualifying names like MyApp.MyClass. Namespaces can be nested for clearer organization in larger projects.

9. How Do You Handle Exceptions in C#? try, catch, finally, and throw

Wrap risky code in try, catch specific exceptions to handle them, use finally for cleanup, and throw to raise errors.

Example: try { var r = 10 / x; } catch (DivideByZeroException ex) { Console.WriteLine(ex.Message); } finally { /* cleanup */ }.

10. What Is a Constructor? Initialize Instances When Created

A constructor is a special method named after the class that runs when you create an instance. It has no return type.

Example of default and parameterized constructors:

public class MyClass { public MyClass() { } public MyClass(int v) { Value = v; } }

You can chain constructors using this(parameters).

11. What Is an Array in C#? Fixed-Size Sequence of Same-Type Elements

An array holds a fixed number of elements of one type and is zero indexed.

Example:

• Declare and initialize: int[] numbers = {1, 2, 3}
• Allocate: int[] more = new int[5]
• Access elements with brackets like numbers[0]

For dynamic sizing use collections such as List<T>.

12. How Do You Create a List in C#? Dynamic Array Using Generics

Use List<T> from System.Collections.Generic.

Example:

var numbers = new List<int>(); numbers.Add(1); numbers.Add(2); Or initialize with values: var fruits = new List<string> { "Apple", "Banana" };

Lists offer Add, Remove, Insert, Sort, and other helpers useful in coding interviews.

13. Difference Between List<T> and an Array: Dynamic Sizing and Helper Methods

Arrays have fixed size and can be slightly faster for raw access. List<T> resizes automatically and provides many methods like Add, Remove, Contains, and Sort. Use arrays for fixed length data and List<T> when you need flexible collections during algorithm implementation.

14. What Are Properties in C#? Encapsulate Fields with get and set

Properties expose data while allowing control logic. A full property uses a private field with get and set accessors.

Example: private string name; public string Name { get { return name; } set { name = value; } }

For simple cases use auto properties: public string Name { get; set; }.

15. Explain Encapsulation: Bundle Data and Methods and Control Access

Encapsulation groups data and methods inside a class and restricts direct access using access modifiers. Keep fields private and expose behavior via public methods or properties.

Example:

private double balance; public void Deposit(double amount) { if (amount > 0) balance += amount; }

This prevents invalid state changes.

16. What Is an Interface in C#? A Contract for Types to Implement

An interface declares method and property signatures without implementations. A class or struct that implements the interface must define those members.

Example:

public interface IAnimal { void Speak(); } public class Dog : IAnimal { public void Speak() { Console.WriteLine("Woof"); } }

Interfaces help in design for testable and flexible code.

17. Purpose of the using Statement: Two Roles: Namespaces and Resource Cleanup

using brings a namespace into scope so you can use its types without full names. using also manages IDisposable resources so they get disposed automatically.

Example for cleanup:

using (var reader = new StreamReader("file.txt")) { var text = reader.ReadToEnd(); }.

18. How Do You Read from and Write to a File in C#? StreamReader and StreamWriter

Use System.IO classes.

• Write: using (var writer = new StreamWriter("out.txt")) { writer.WriteLine("Hello"); }
• Read: using (var reader = new StreamReader("out.txt")) { string line; while ((line = reader.ReadLine()) != null) Console.WriteLine(line); }

These patterns appear in real world I O code.

19. What Is the Main Method? Program Entry Point for Console Apps

Main is where execution starts in a console application. Typical signatures:

• static void Main(string[] args)
• static int Main(string[] args)

args provides command line parameters.

Example usage:

static void Main(string[] args) { Console.WriteLine("Hello"); }.

20. How Do You Create a Simple Console Application? Use Visual Studio or dotnet CLI

With Visual Studio choose Console App then run. With the dotnet CLI run:

dotnet new console -n MyApp, cd MyApp, then dotnet run.

Edit Program.cs to implement Main and print or read values for quick tests in interviews.

21. What Are Access Modifiers in C#? Control Visibility of Types and Members

Access modifiers determine where members are visible:

• public allows any code to access
• private restricts to the containing class
• protected allows derived classes to access
• internal restricts access to the same assembly
• protected internal combines protected and internal rules

22. Difference Between public, private, protected, and internal: Who Can Access Which Members

• public is visible everywhere
• private is visible only inside the declaring class
• protected is visible inside the class and its subclasses
• internal is visible only inside the same assembly

Use these to hide implementation and expose a clean API for tests or other components.

23. Explain Inheritance: Reuse and Extend Behavior from a Base Class

Inheritance lets a derived class use members of a base class and add or override behavior.

Syntax: public class Dog : Animal { public void Bark() {} }

You can use virtual in the base and override in the derived class to change behavior at runtime.

24. What Is Polymorphism? One Interface, Many Implementations

Polymorphism allows treating different types through a common base type or interface. Compile time polymorphism uses method overloading. Runtime polymorphism uses virtual methods and overrides so the actual method chosen depends on the object type at runtime.

Example:

Animal a = new Dog(); a.Speak(); invokes Dog.Speak.

25. How Do You Implement Method Overloading? Multiple Methods with Same Name but Different Signatures

Define several methods with the same name but different parameter lists.

Example:

public int Add(int a, int b) { } public double Add(double a, double b) { } public int Add(int a, int b, int c) { }

The compiler resolves the correct overload based on arguments.

26. What Is a Static Class? Group Utilities Without Instances

A static class cannot be instantiated and holds only static members. Use it for utility functions.

Example:

public static class MathUtilities { public static int Add(int a, int b) => a + b; }

Call as MathUtilities.Add(3, 4).

27. How Do You Convert a String to an Integer? Parse, TryParse, and Convert

Use int.Parse when the string is valid and you want exceptions on failure. Use int.TryParse to safely attempt conversion and get a bool result: if (int.TryParse(s, out int n)) { /* success */ }.Use Convert.ToInt32 which handles null by returning zero but can throw on invalid formats.

28. What Are Collections in C#? Classes for Grouping Objects

Collections provide data structures like List<T>, Dictionary<TKey, TValue>, HashSet<T>, and older non generic types like ArrayList and Hashtable. Generic collections are type safe and preferred. Collections appear often in interview questions that test algorithms and data structure choices.

29. What Is a Delegate? Type-Safe Reference to a Method

A delegate defines a method signature and can hold references to methods that match it. Declaration: public delegate void Notify(string message); Usage: Notify n = ShowMessage; n("Hi"); Delegates power events and callbacks in frameworks and coding exercises.

30. What Is an Event? Publisher-Subscriber Pattern Using Delegates

An event is a wrapper around a delegate that exposes subscription and unsubscription semantics. Define with event EventHandler MyEvent; A publisher raises the event and subscribers add handlers with +=. Events decouple components, useful in UI and async code.

31. How Do You Create a switch Statement? Choose Code Path Based on a Value

Use switch(expression) with case labels and break to prevent fall through.

Example:

switch (day) { case 1: Console.WriteLine("Mon"); break; case 2: Console.WriteLine("Tue"); break; default: Console.WriteLine("Other"); break; } Recent C# versions support pattern matching in switch for richer scenarios.

32. What Are Tuples in C#? Group Multiple Values into One Object

Tuples hold multiple values without creating a custom type. Use value tuples: var t = (Id: 1, Name: "One"); Access with t.Id and t.Name. You can also use Tuple<T1,T2> but value tuples are lighter and allow element names. They are handy for returning two values from a method.

33. Difference Between a Value Type and a Reference Type: Memory and Copy Behavior

Value types store the data directly and copy on assignment. Examples are int and struct. Reference types store a reference to data on the heap and assignments share that reference.

Example:

int x = 5; int y = x; // copy. Person p1 = new Person(); Person p2 = p1; // same object reference.

34. Explain a Nullable Type: Allow a Value Type to Represent Null

Make a value type nullable with a question mark, for example int? n = null. Check presence with HasValue or compare to null. Use the null coalescing operator to supply a default: int v = n ?? 0. Nullable types are common when mapping database fields or optional inputs in interviews.

35. What Is Garbage Collection? Automatic Memory Cleanup by the Runtime

The .NET garbage collector reclaims memory for objects that are no longer reachable. It uses generations 0 1 and 2 to optimize collection frequency and cost. You can force a collection with GC.

Collect but that is rarely needed and can hurt performance.

36. How Do You Use a foreach Loop? Simple Iteration over Collections

foreach iterates an IEnumerable without managing an index.

Example:

foreach (var fruit in fruits) { Console.WriteLine(fruit); } Use foreach to reduce off by one errors when traversing arrays and lists in coding tasks.

37. What Are Lambda Expressions? Concise Anonymous Functions for Delegates and LINQ

A lambda is written as (parameters) => expression or { statements }.

Example:

numbers.Where(n => n % 2 == 0).ToList(); Lambdas make LINQ queries and inline callbacks concise in algorithm and interview scenarios.

38. How Do You Handle Multiple Exceptions? Specific catch Blocks Then a General One

Place several catch blocks after the same try block to handle specific exception types first: try { } catch (DivideByZeroException ex) { } catch (FormatException ex) { } catch (Exception ex) { } Order matters because the first matching catch executes.

39. What Is LINQ? Language-Integrated Query for Collections and Data

LINQ lets you write expressive queries over objects arrays XML or databases using a uniform syntax and lambda expressions.

Example:

var evens = numbers.Where(n => n % 2 == 0).ToList(); LINQ appears frequently in C# interview questions about filtering mapping and grouping data.

40. Difference Between string and StringBuilder: Immutable vs. Mutable Text Handling

string is immutable so concatenation creates new instances: str += "x" makes a new string. StringBuilder lets you modify text in place and is more efficient for many concatenations: var sb = new StringBuilder("Hi"); sb.Append(" there"); string result = sb.ToString(); Use StringBuilder in loops or heavy text processing to reduce allocations.

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25 C# Interview Questions and Answers for Intermediates

1. Generics: Build Reusable, Type-Safe Containers

Generics let you write classes, interfaces, and methods with a placeholder type so the same code works for many data types while keeping compile-time type checks. That reduces runtime casts and avoids boxing for value types. Use cases include collections, repositories, and helper utilities.

Example generic class:

public class GenericList<T>

{

private T[] items = new T[10];

private int count = 0;

public void Add(T item) { items[count++] = item; }

public T Get(int index) { return items[index]; }

}

Applied Notes

Use generic constraints to require interfaces or base types, for example, where T : IDisposable or where T : class. In repositories, add where T : class, IEntity to ensure entities have identity. Understand covariance and contravariance for delegates and interfaces (IEnumerable<out T> is covariant). Interviewers expect mention of performance benefits and examples like generic caching layers or generic validators.

2. Async and await: Write Code That Stays Responsive

The async keyword marks methods that contain awaits. await pauses the async method until the awaited Task completes, freeing the calling thread to do other work. Async and await simplify asynchronous I/O and concurrency compared with raw threads.

Example:

public async Task<string> DownloadDataAsync(string url)

{

using var client = new HttpClient();

var response = await client.GetStringAsync(url);

return response;

}

Applied Notes

Prefer Task and Task<T> return types over void except for event handlers. Avoid blocking with .Result or .Wait on tasks to prevent deadlocks in UI or ASP.NET contexts. Show understanding of synchronization contexts, ConfigureAwait(false) for library code, and proper exception handling with try catch around await.

3. Dependency injection: Invert Control for Testable Code

Dependency injection supplies a class with its dependencies instead of letting the class construct them. Use constructor injection for required dependencies and factory or property injection for optional dependencies.

Example:

public class Notification

{

private readonly IMessageService _messageService;

public Notification(IMessageService messageService) { _messageService = messageService; }

public void Notify(string message) { _messageService.SendMessage(message); }

}

Applied Notes

Discuss IoC containers like Microsoft DI, Autofac, or Simple Injector and lifetime scopes: transient, scoped, and singleton. In interviews, explain how DI improves unit testing by allowing mocks, and how to avoid service locator anti-patterns.

4. Implementing interfaces: Contracts That Enable Polymorphism

An interface defines a set of members without implementation. A class implements an interface and provides concrete behavior. Interfaces enable loose coupling and multiple inheritance of behavior.

Example:

public interface IAnimal { void Speak(); }

public class Dog : IAnimal { public void Speak() { Console.WriteLine("Woof!"); } }

Applied Notes

Mention explicit interface implementation to resolve name collisions and default interface methods introduced in recent C# versions. Explain when to prefer an abstract base class over an interface, for example, when sharing protected helper logic or backing fields.

5. Reflection: Inspect and Use Types at Runtime

Reflection lets code examine assemblies, types, methods, properties, attributes, and create instances dynamically. It powers plugin systems, serialization customization, and runtime wiring.

Example:

Type type = typeof(Dog);

foreach (var method in type.GetMethods()) { Console.WriteLine(method.Name); }

Applied Notes

Call out performance cost and code complexity. Use reflection for tooling and dynamic factories but prefer compile time approaches like generics or source generators for hot paths. In interviews show knowledge of Activator.CreateInstance, MethodInfo.Invoke, and reading custom attributes.

6. Attributes: Attach Metadata to Code Elements

Attributes are classes derived from System.Attribute that adds metadata to classes, methods, properties, or assemblies. You read them with reflection and use them for validation rules, routing, serialization, and test frameworks.

Example:

[AttributeUsage(AttributeTargets.Class)]

public class DeveloperAttribute : Attribute

{

public string Name { get; }

public DeveloperAttribute(string name) { Name = name; }

}

[Developer("Alice")]

public class SampleClass { }

Applied Notes

Explain how attributes drive frameworks like ASP.NET routing or Entity Framework mapping. Discuss attribute targets, AllowMultiple, and retrieval via GetCustomAttributes. Suggest using attributes sparingly to avoid coupling logic with metadata.

7. IDisposable: Free Resources Deterministically

IDisposable exposes Dispose to release unmanaged or expensive managed resources. Implement the dispose pattern with a virtual Dispose(bool) to handle both explicit disposal and finalizer invocation.

Example:

public class ResourceHolder : IDisposable

{

private bool disposed = false;

public void Dispose()

{

Dispose(true);

GC.SuppressFinalize(this);

}

protected virtual void Dispose(bool disposing)

{

if (disposed) return;

if (disposing) { /* free managed resources */ }

/* free unmanaged resources */

disposed = true;

}

~ResourceHolder() { Dispose(false); }

}

Applied Notes

Use declarations or using statements to ensure Dispose runs. For async disposal implementation, implement IAsyncDisposable and DisposeAsync. In interviews, explain when a finalizer is needed and why GC.SuppressFinalize reduces overhead.

8. Extension Methods: Add Helpers Without Modifying Types

Extension methods are static methods in static classes whose first parameter uses this to extend an existing type. They let you add fluent APIs or utility helpers to framework types.

Example:

public static class StringExtensions

{

public static bool IsNullOrEmpty(this string str) { return string.IsNullOrEmpty(str); }

}

string name = null;

bool isEmpty = name.IsNullOrEmpty();

Applied Notes

Avoid extending types with ambiguous method names. Use extension methods to add LINQ like helpers or conversions. Interviewers may ask about resolution rules and how extension methods interact with instance methods.

9. Threading: Manage Concurrent Work Safely

Use System.Threading for low-level threads and System.Threading.Tasks for higher-level concurrency. Tasks run on the thread pool by default and integrate with async await. Use synchronization primitives to protect shared state.

Example:

Thread start

var thread = new Thread(new ThreadStart(MyMethod));

thread.Start();

Task start

Task.Run(() => MyMethod());

Applied Notes

Talk about ThreadPool, TaskSchedulers, cancellation with CancellationToken, and when to use tasks instead of raw threads. Discuss thread safety strategies: immutability, locks, concurrent collections, and data parallelism via Parallel.ForEach and PLINQ.

10. Task Versus Thread: Choose the Right Abstraction

Thread represents an OS level thread and gives fine-grained control but higher overhead. Task represents an asynchronous operation that uses thread pool threads and higher-level features like continuations and composition.

Example:

Thread t = new Thread(() => { /* work */ }); t.Start();

Task task = Task.Run(() => { /* work */ });

Applied Notes

Prefer Task for I/O-bound work and typical parallelism. Use a dedicated Thread for long-running operations that should not occupy thread pool threads. Mention TaskCompletionSource for custom task control and how Tasks integrate with async await and exception propagation.

11. Lock Statement: Protect Critical Sections

lock ensures only one thread executes a block at a time by acquiring a mutual exclusion lock on an object. Use a private readonly object for locking to avoid external interference.

Example:

private static readonly object _lock = new object();

private static int _counter = 0;

public void IncrementCounter()

{

lock (_lock) { _counter++; }

}

Applied Notes

Avoid locking on this or string instances. Keep lock scopes narrow to reduce contention. For high performance, consider ReaderWriterLockSlim or concurrent collections like ConcurrentDictionary. In interviews, describe deadlocks and how to avoid them by consistent lock ordering.

12. Volatile Keyword: Prevent Stale Cached Reads

volatile tells the runtime and compiler to always read and write a field directly to memory, preventing certain instruction reordering and cached reads. It helps in simple synchronization scenarios but does not replace full locking.

Example:

private volatile bool _isRunning;

public void Stop() { _isRunning = false; }

public void Start()

{

while (_isRunning) { /* work */ }

}

Applied Notes

Use volatile for simple flags. For compound operations like read, modify, write, use locks or Interlocked methods. Explain memory model basics: volatile avoids some reordering but is not a panacea for complex thread safety.

13. JSON Handling: Serialize and Deserialize Reliably

Use System.Text.Json for modern .NET projects or Newtonsoft.Json when you need advanced features. Configure converters for custom types and handle casing, nulls, and reference loops.

Example System.Text.Json:

using System.Text.Json;

string json = JsonSerializer.Serialize(person);

var personObj = JsonSerializer.Deserialize<Person>(json);

Applied Notes

Discuss performance differences, polymorphic serialization limitations, and how to handle circular references with ReferenceHandler.Preserve. Mention DTOs, mapping with AutoMapper, and defensive parsing to avoid injection issues.

14. Lambda Expressions: Concise Anonymous Functions

Lambda expressions define inline functions for delegates and expression trees. They power LINQ queries and event handlers.

Example:

List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

var even = numbers.Where(n => n % 2 == 0).ToList();

Applied Notes

Know the difference between expression lambdas used by IQueryable to build SQL and delegate lambdas used in LINQ to Objects. Show examples using closure variables and explain potential pitfalls like capturing loop variables.

15. Anonymous Types: Quick-Shaped Data for Local Use

Anonymous types let you create unnamed objects with read-only properties for local projections, especially in LINQ queries.

Example:

var person = new { Name = "Alice", Age = 30 };

Console.WriteLine($"Name: {person.Name}, Age: {person.Age}");

Applied Notes

Explain scope limits: you cannot return anonymous types from a public API without using dynamic or projection into known DTOs. Use them for view models inside methods or for test assertions.

16. Asynchronous Programming Implementation: Patterns and Pitfalls

Implement async with Tasks, CancellationToken, and proper exception handling. Use ConfigureAwait(false) in library code to avoid capturing synchronization contexts. For streaming data use IAsyncEnumerable<T>.

Example:

public async Task<string> FetchDataAsync(string url)

{

using var client = new HttpClient();

return await client.GetStringAsync(url);

}

Applied Notes

Cover cancellation support, backpressure for streaming, and combining multiple tasks with Task.WhenAll and Task.WhenAny. Discuss testing async code with async unit tests and mocking asynchronous dependencies.

17. Purpose of Async: Make Asynchronous Code Readable

async enables await inside a method, so you can write asynchronous code that resembles synchronous code. The compiler rewrites async methods into state machines that return Task or Task<T>.

Example:

public async Task<int> CalculateAsync()

{

await Task.Delay(1000);

return 42;

}

Applied Notes

Mention that async methods may run synchronously until they hit the first await that yields. Interviewers may probe for how exceptions flow through async methods and how to propagate cancellation.

18. Prevent Instantiation: Control Object Creation

Prevent instantiation by making constructors private. Use static classes for stateless utilities and singletons when one instance must be shared.

Static utility:

public static class Utility { public static void DoSomething() { } }

Singleton:

public class Singleton

{

private static Singleton _instance;

private Singleton() { }

public static Singleton Instance

{

get

{

if (_instance == null) { _instance = new Singleton(); }

return _instance;

}

}

}

Applied Notes

Discuss thread-safe singleton implementations using Lazy<T> or double check locking. Explain when to prefer dependency injection with a single registration over global singletons to improve testability.

19. Singleton Pattern: Single Instance With Controlled Access

Singleton restricts a class to one instance and provides a global access point. It is helpful for shared resources such as loggers or configuration.

Example:

public class Logger

{

private static Logger _instance;

private Logger() { }

public static Logger Instance

{

get

{

if (_instance == null) { _instance = new Logger(); }

return _instance;

}

}

public void Log(string message) { Console.WriteLine(message); }

}

Applied Notes

Discuss thread safety and initialization overhead. Consider registering a singleton service in an IoC container as an alternative to using a global singleton. Interviewers expect trade-offs between global state and testability.

20. Abstract Classes Versus Interfaces: Choose the Right Contract

Abstract classes can include implemented members, fields, and constructors. Interfaces define a contract that a class must implement and allow multiple inheritance of behavior.

Example abstract class:

public abstract class Animal

{

public abstract void Speak();

public void Sleep() { /* shared behavior */ }

}

Example interface:public interface IAnimal { void Speak(); }

Applied Notes

Choose an abstract class when you want shared state or protected helpers. Choose interfaces for composability and when multiple unrelated classes must implement the same behavior. Mention default interface methods as a limited escape hatch and note versioning risks.

21. Observer Pattern: Notify Many Subscribers of State Changes

Observer implements a one-to-many subscription model. The subject maintains a list of observers and notifies them of state changes. It fits event systems, UI updates, and messaging.

Example:

public interface ISubject { void Attach(IObserver observer); void Detach(IObserver observer); void Notify(); }

public interface IObserver { void Update(string message); }

public class NewsPublisher : ISubject

{

private List<IObserver> _observers = new List<IObserver>();

private string _news;

public void Attach(IObserver observer) => _observers.Add(observer);

public void Detach(IObserver observer) => _observers.Remove(observer);

public void Notify()

{

foreach (var o in _observers) o.Update(_news);

}

public void PublishNews(string news) { _news = news; Notify(); }

}

public class NewsSubscriber : IObserver { public void Update(string message) => Console.WriteLine($"Received news: {message}"); }

Applied Notes

Point out .NET events and IObservable<T> as built-in ways to implement observer behavior. Discuss memory management issues where subscribers forget to unsubscribe, and how weak references can help.

22. IEnumerable Versus IQueryable: Where Evaluation Happens

IEnumerable is best for in-memory enumeration and executes LINQ predicates locally. IQueryable builds expression trees so queries can be translated by providers like Entity Framework into SQL and executed at the data source.

Example IEnumerable:

IEnumerable<int> numbers = new List<int> { 1, 2, 3 }.Where(n => n > 1);

Example IQueryable:

IQueryable<int> queryableNumbers = dbContext.Numbers.Where(n => n > 1);

Applied Notes

Explain deferred execution for both, but stress that IQueryable moves filtering to the database, which reduces memory and network usage. Caution about methods that force evaluation, like ToList or Count, and about writing expressions that can be translated to SQL.

23. Handling Large Data Sets: Minimize Memory and I/O

When dealing with large data sets, use paging, streaming, batching, and asynchronous I/O to reduce memory footprint. Push filters to the database and use projections to retrieve only needed fields.

Techniques:

• Paging with Skip and Take
• Streaming with IAsyncEnumerable<T> or data readers
• Batch processing and bulk inserts

Applied Notes

Mention using IQueryable to let EF Core generate SQL for pagination, using SqlDataReader for streaming rows, and using chunked processing to avoid OOM. For high throughput processes consider message queues and backpressure controls.

24. Circular Reference: Cycles in Object Graphs

A circular reference occurs when objects reference each other, forming a cycle. That can complicate serialization and increase memory retention if not handled.

Example:

public class A { public B BReference { get; set; } }

public class B { public A AReference { get; set; } }

var a = new A(); var b = new B(); a.BReference = b; b.AReference = a;

Applied Notes

Handle circular references in serializers by using reference handling options or DTOs that break cycles. For memory management, consider using WeakReference for caches or event handlers and ensure proper unsubscription to avoid leaks.

25. Yield: Produce Sequences One Item at a Time

yield return enables iterator methods that yield values lazily. The compiler generates a state machine so callers can iterate without allocating the entire collection.

Example:

public IEnumerable<int> GetNumbers(int count)

{

for (int i = 0; i < count; i++) yield return i;

}

foreach (var number in GetNumbers(10)) Console.WriteLine(number);

Applied Notes

Use yield to stream large sequences, to implement custom LINQ operators, or to build generators. Be mindful that exceptions thrown in the iterator body surface during enumeration, not at iterator creation, and that deferred execution interacts with captured variables.

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35 Advanced C# Interview Questions and Answers

1. What Is C# History?

C# started at Microsoft in the late 1990s as a language to combine type-safety and developer productivity with performance close to C++. Anders Hejlsberg led the design; C# 1.0 shipped with .NET in 2002. Since then, the language has evolved through major milestones:

• Generics (C# 2.0)
• LINQ and anonymous types (3.0)
• Async/await (5.0)
• Pattern matching and tuples (7.x)
• Records and top-level statements (9.0)
• Many performance and nullable-reference improvements

The language matured alongside .NET: from the original .NET Framework to .NET Core and the unified, cross-platform .NET 5/6/7.

2. What Are C# Main Features?

C# is object-oriented with strong static typing, garbage collection, and a rich standard library. Key features:

• Classes/structs
• Interfaces
• Generics
• Delegates and events
• LINQ, async/await and the Task-based async model
• Reflection
• Attributes
• Pattern matching
• Tuples
• Robust type system including nullable reference types

It runs on the Common Language Runtime (CLR), which provides memory management and interoperability. For advanced interviews, you should also be ready to discuss performance features like Span<T>, ref returns, and ValueTask.

3. What Is .NET Framework?

The .NET Framework is Microsoft’s original implementation:

• A managed execution environment (CLR)
• Base Class Library (BCL)

It provides cross-language support, JIT compilation, assembly loading, reflection, and a large set of APIs for I/O, networking, UI, threading, and security. Later incarnations split platform concerns:

• .NET Core introduced cross-platform support and performance improvement
• Current unified .NET combines modern runtime improvements and native interop.

Interviewers will probe differences:

• Windows-only APIs
• Deployment models
• How assemblies are resolved at runtime

4. What Is Common Language Runtime (CLR)?

The CLR is the runtime that loads IL, JIT compiles to native code, enforces type safety, runs garbage collection, and provides runtime services such as exception handling, security, and interop with native code. It hosts the thread pool, synchronization primitives, and the GC generations and modes.

For performance questions, discuss the JIT vs ReadyToRun/AOT, how the JIT inlines and optimizes, and how the CLR exposes diagnostic hooks (EventPipe, ETW) for profiling.

5. What Is Base Class Library (BCL)?

The BCL is the set of core assemblies and types you use constantly:

• System.Collections
• System.IO
• System.Threading
• System.Net
• System.Reflection and many others

It defines common patterns (IDisposable, IEnumerable, Task) and provides optimized implementations such as Array, List<T>, ConcurrentDictionary<TKey,TValue>, and Span<T>. In interviews, be prepared to explain trade-offs between using concurrent collections and lock-based code, and why certain APIs exist for high-performance scenarios.

6. What Are keywords in C#?

Keywords are reserved words that have special meaning to the compiler and cannot be used as identifiers unless escaped (for example, @class). Examples:

• class
• struct
• interface
• async
• await
• ref
• out
• in
• const
• readonly
• volatile
• virtual
• override
• sealed
• static

Advanced interview questions may test how keywords like ref, in, and ref readonly affect calling conventions, or how async/await is implemented by the compiler using generated state machines.

7. What Are operators in C#?

Operators in C# include arithmetic (+, -, *, /, %), comparison (==, !=, <, >), logical (&&, ||, !), bitwise (&, |, ^, <<, >>), null-coalescing (??), null-conditional (?.), and pattern-related operators. You can overload many operators for user-defined types.

For the interview depth, explain operator precedence, short-circuiting in boolean operators, and how the null-coalescing assignment (??=) simplifies common patterns. Also show awareness of overflow behavior and the checked/unchecked contexts used to detect arithmetic overflow.

8. What Are Data Types in C#?

C# provides primitive value types (int, long, float, double, bool, char), structs (user-defined value types), and reference types (class, string, array, delegate). There are nullable value types (int?) and advanced types like Span<T> and ReadOnlySpan<T> that avoid allocations.

For interviews, explain differences in memory layout, boxing/unboxing, and how value-type parameter passing differs from reference types when dealing with performance-critical code.

9. How Many Types of Data Types in C#?

C# categorizes types primarily as value types and reference types. Value types store data directly, are usually allocated on the stack (or inline within objects), and copying creates independent values. Reference types store references to heap objects, and copying duplicates the reference. Discuss the practical impacts:

• Default values
• Lifetimes under GC
• Boxing costs
• When to choose struct vs class
• How stack allocation via ref structs can reduce GC pressure

10. What Are Variables in C#?

Variables store names, locations, and bind types to values. Local variables, fields, constants (const), and readonly fields each have lifetimes and semantics. C# allows type inference with var, tuple deconstruction, and ref locals.

Advanced topics include span-based locals (ref struct) and stackalloc for temporary memory. In interviews, you may be asked about capture semantics for variables in closures and how captured variables affect object lifetimes and GC roots.

11. What Is Break Statement in C#?

The break statement exits the nearest loop or switch. Use it to stop iteration when a condition is met. In performance-sensitive loops, consider structured alternatives to avoid complex control flow. When combined with try/finally, remember that finally blocks still execute when break exits a loop, which affects resource cleanup and thread-safety considerations.

12. What Is Continue Statement in C#?

Continue skips the remainder of the current loop body and proceeds to the next iteration. It’s useful for early filtering in loops. In concurrent code, be mindful of using continue in critical sections; always ensure locks are correctly released before continuing, or prefer patterns that reduce lock scope to avoid subtle deadlocks.

13. What Is Goto Statement in C#?

Goto transfers control to a labeled statement within the same method. Modern code rarely needs goto; its use can obscure flow and make reasoning harder. Interviewers might ask when goto is justified, such as in low-level finite state machine implementations where performance and clarity trade off. Avoid goto for normal control flow; prefer well-structured loops, methods, or pattern-based switches.

14. What Is Function in C#?

In C#, functions are methods declared in types. They may be instance or static, have parameters (including ref/out/in/params), generic type parameters, and return values. Advanced topics:

• Expression-bodied members
• Local functions (nested functions with captured state)
• Extension methods
• Async methods that return Task or ValueTask

Be prepared to explain how the compiler transforms async methods into state machines and how that impacts exceptions and stack traces.

15. What Is Ref Parameter in C#?

The ref modifier passes a variable by reference, so the callee can read and write the caller’s storage. Use ref to avoid copies of large structs or to return values via ref returns. Combine with readonly ref to allow efficient read-only access. Interviewers often ask about ref safety, ref locals, and how ref affects the IL and calling conventions.

16. What Is Out Parameter in C#?

The out keyword marks parameters as outputs that the method must assign before returning. It’s commonly used for Try-patterns, e.g., bool TryParse(string s, out int value). Modern C# interviewers also accept alternatives like returning tuples, ValueTuple, or custom result types (Result<T, E>) to express intent more clearly while avoiding mutable out state.

17. What Is In Parameter in C#?

The in modifier (C# 7.2+) passes arguments by reference for read-only access. It avoids copying large structs while guaranteeing immutability inside the callee. Explain differences from ref readonly and when to use in for performance critical APIs. Also note that in parameters can affect calling conventions and JIT optimizations.

18. What Is Array in C#?

Arrays store fixed-size sequences of elements of the same type (T[]). They provide O(1) index access and are allocated on the heap (for reference types or arrays of value types). Discuss multidimensional arrays vs jagged arrays, affinity for contiguous memory, and how arrays interact with GC pinning when used with native interop or unsafe code.

19. Can We Pass Array to Function in C#?

Yes. Passing an array passes a reference to the array object; the callee can modify elements and those changes are visible to the caller. For thread-safe contexts, consider defensive copies or read-only wrappers (ReadOnlyMemory<T> or ImmutableArray<T>). In interviews you may be asked about parameter modifiers and how passing by ref changes semantics for the array reference itself.

20. What Is Multidimensional Array in C#?

A multidimensional array, declared like int[,] or int[,,], represents a rectangular block of memory with consistent row lengths. They offer nicer indexing syntax but sometimes slower access compared to jagged arrays due to CLR representation. For performance-sensitive numeric code prefer Span<T> over multidimensional arrays when possible.

21. What Is Jagged Array in C#?

A jagged array (T[][]) is an array whose elements are arrays, so each row can have different lengths. It maps well to irregular data and can offer better performance for row-major access patterns when rows are allocated independently. Use jagged arrays when you need nonuniform row lengths or when you want to allocate rows lazily.

22. What Are Params in C#?

The params keyword lets a method accept a variable number of arguments of the same type, received as an array. Useful for building flexible APIs (e.g., params object[] args). Be mindful that params creates an array allocation; heavy call sites can avoid allocation by providing an array argument or by offering overloads that accept ReadOnlySpan<T> in performance-sensitive code.

23. What Are the Types of Classes in C#?

C# supports abstract classes, partial classes, sealed classes, and static classes. Abstract classes define reusable behavior and abstract members; sealed classes prevent inheritance; static classes group utility functions and cannot be instantiated; partial classes split implementation across files which is useful for generator-based code. Advanced interviews might also probe inheritance hierarchies, virtual dispatch costs, and how sealed can enable runtime optimizations.

24. What Is Class in C#?

A class groups state (fields, properties) and behavior (methods, events) and is a reference type allocated on the heap. Classes support inheritance, polymorphism, encapsulation, and interfaces. For interview depth, explain how object layout and virtual method tables (vtable) work, how interfaces are dispatched, and how object lifetimes interact with GC roots and finalizers.

25. What Is Constructor in C#?

Constructors initialize new instances. C# supports parameterless, parameterized, and static constructors. Discuss constructor chaining (this(...) and base(...)) and when to prefer factory methods instead of public constructors to control creation and enforce invariants. When talking about dependency injection in interviews, explain how constructors enable required dependencies and how IServiceProvider resolves them.

26. What Is Destructor in C#?

The destructor syntax (~ClassName) defines a finalizer that the GC calls before reclaiming an object, but finalizers run on the finalizer thread and add nondeterministic overhead. Prefer IDisposable and deterministic disposal (using statement or try/finally) for unmanaged resources; implement a finalizer only as a safety net and suppress it (GC.SuppressFinalize) after disposing. Interviews will often probe IDisposable vs finalizer trade-offs and safe handle wrappers.

27. What Is Static Class in C#?

A static class contains only static members and cannot be instantiated or inherited (it’s implicitly sealed). Use static classes for stateless utilities or factories. Be cautious with static mutable state:

• It introduces a global state
• Complicates testing and thread-safety
• Can lead to memory retention via GC roots

28. What Is Static Method in C#?

Static methods operate at the type level and cannot access instance members. Use them for utilities and pure functions. Discuss testability and dependency injection: static methods are harder to mock; prefer instance methods on injected services when behavior needs to be replaceable. Also explain how static methods influence JIT inlining and code layout.

29. What Is Static Constructor in C#?

A static constructor runs once per type before any static members are accessed or before the first instance is created. It’s ideal for complex static initialization that cannot be expressed with simple assignments. Be aware static constructors can throw, which type-initialization exceptions propagate, and that they execute on a thread that triggers initialization, which can cause type initializer deadlocks if not designed carefully.

30. What Is Struct in C#?

Structs are value types allocated inline and copied by value. Use them for small, immutable data where avoiding allocations matters. Avoid large mutable structs because copies become expensive and can cause subtle bugs when methods mutate a copy. Advanced topics: readonly struct, ref struct, and how Span<T> uses ref struct to guarantee stack-only lifetimes.

31. What Is Enum in C#?

An enum is a value type that maps names to underlying integral values. Enums improve readability and can be flagged with [Flags] to combine values bitwise. Interviewers may ask about conversions, parsing enums from strings, and safely versioning enums across APIs.

32. What Is Property in C#?

Properties expose getter and setter logic while hiding backing storage. Auto-properties reduce boilerplate; expression-bodied accessors simplify single-line logic. Advanced use:

• Property patterns
• Init-only setters (init) for immutable initialization
• Thread-safe lazy initialization using Lazy<T>
• Double-check locking with volatile

Consider how property access affects object invariants and concurrency.

33. What Is Inheritance in C#?

Inheritance lets a derived class reuse and extend a base class’s members. Prefer composition over inheritance for many designs to reduce coupling. Interviewers will test virtual methods, abstract classes, the Liskov substitution principle, and how inheritance impacts polymorphic dispatch and runtime performance.

34. How Many Types of Inheritance in C#?

C# supports single inheritance for classes but allows multiple inheritance via interfaces. Common forms discussed in interviews:

• Single
• Multilevel
• Hierarchical
• Hybrid relationships implemented with interfaces

Be prepared to explain interface-based design, explicit interface implementation, and how default interface methods (introduced in recent C# versions) alter implementation patterns.

35. What Is Method Overloading and Method Overriding in C#?

Method overloading defines multiple methods with the same name but different parameter signatures in the same scope; the compiler resolves which one to call at compile time. Method overriding uses virtual in the base class and override in derived classes to replace behavior at runtime via polymorphism.

Interviewers often pair these questions with discussions on overload resolution rules, covariance and contravariance for delegates and generics, and how virtual dispatch affects inlining and performance.

Advanced C# Topics

Additional advanced topics woven into these answers, you should be ready to discuss in C# interviews:

• Threading and synchronization (Thread, ThreadPool, Task, CancellationToken, locks, ReaderWriterLockSlim, concurrent collections)
• Asynchronous programming patterns (Task vs ValueTask, ConfigureAwait, IAsyncDisposable)
• Memory management (GC generations, pinning, weak references, large object heap, GC.Collect trade-offs)
• Delegates and events (event patterns, multicast delegates, preventing memory leaks with weak event patterns)
• Reflection and diagnostics (System.Reflection, Expression trees, dynamic code, profiling APIs).

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30 More Advanced C# Interview Questions and Answers

36. Base Keyword in C#: Reach Into the Parent Class

Use base inside a derived class to access base class constructors, fields, properties, and virtual/overridden methods. Calling base.SomeMethod() invokes the base implementation; using base(...) in a constructor calls a specific base constructor. At IL level, a base method call often compiles to a call (or callvirt for virtuals) to the parent method reference, so understanding base helps when reasoning about vtable dispatch and JIT behavior.

Example:

class Apple : Fruit { public Apple(): base() {} public override void GetFruit(string n) { base.GetFruit(n); } }.

37. Polymorphism in C#: One API, Many Behaviors

Polymorphism lets a single name represent different implementations. Compile-time polymorphism appears as method overloading and operator overloading; the compiler resolves which overload to emit into IL.

Run-time polymorphism uses virtual methods, interface dispatch, and overriding; the CLR dispatches calls using the vtable or interface maps, and the JIT can inline virtual calls only when it can devirtualize. Use polymorphism for flexible APIs and design patterns, but measure performance when you rely heavily on virtual dispatch.

38. Sealed Class in C#: Stop Further Inheritance

Declare class C as sealed to prevent any class from deriving from it. You can also seal an override method to avoid further overriding:

• public sealed override void M()

Sealing can improve performance because the JIT can devirtualize sealed types and inline calls, reducing indirect calls and branch mispredictions. Use sealed to protect invariants or to communicate that a type is complete.

39. Interface in C#: Contracts That Multiple Types Implement

An interface declares method, property, event, and indexer signatures that implementing types must provide. Since C# 8, interfaces can include default implementations, static members, and private helpers, which blurs the line with abstract classes but keeps the contract-first intent.

Interfaces enable multiple inheritance of behavior, support DI patterns, and allow loose coupling for testing and mocking. At runtime, interface calls use interface maps; JIT optimizations for interfaces differ from class virtual calls.

40. Namespace in C#: Logical Grouping of Types

Namespaces group related types to avoid naming collisions and to create a discoverable API surface. Use nested namespaces and clear naming for libraries and assemblies.

The compiler emits fully qualified type names into metadata, which tools and the CLR use for reflection, loading, and versioning. Keep namespaces aligned with assembly boundaries and package names for predictable dependency resolution.

41. Access Modifiers in C#: Define Visibility and Boundaries

Access modifiers control where types and members can be used:

• Public
• Private
• Protected
• Internal
• Protected internal
• Private protected

Choose the narrowest practical scope to enforce encapsulation and to support safer refactoring. At the IL and CLR levels, visibility flags affect metadata and runtime access checks, which also influence how serializers, reflection, and proxy generators interact with your types.

42. Encapsulation in C#: Hide State, Expose Behavior

Encapsulation groups data and operations inside types and hides internal representation using access modifiers and properties. Prefer properties with private setters or full property implementations to control validation and invariants.

Encapsulation reduces coupling, simplifies unit testing, and allows you to change internal structures without breaking callers. Use immutable types when a safe state is critical and performance matters.

43. Abstract Class in C#: Base With Shared Behavior and Deferred Pieces

Abstract classes can contain both abstract members (no implementation) and concrete members. They cannot be instantiated directly and force derived classes to implement abstract members.

Use abstract classes when you want to share implementation and state while requiring derived types to fill specific details. Abstract members compile into metadata that the CLR enforces at type load time.

44. Abstract Class vs Interface: Choose Based on Reuse and Contract Needs

Abstract classes let you provide shared fields, constructors, and default method implementations; classes can inherit only one abstract class. Interfaces express pure contracts and allow multiple implementation inheritance; since C# 8, interfaces can provide default methods, narrowing the distinction. Use an abstract class when you need shared state or base constructors; use interfaces for loose coupling, DI, and version-tolerant contracts.

45. Partial Class in C#: Split a Type Across Files for Clarity

partial allows a single class, struct, or interface to be declared in multiple files. The compiler merges parts into one type. Partial types are helpful for generated code plus handwritten extensions, large feature sets, and separating concerns in the same logical type. Partial methods let a declaration be implemented optionally in another part; if unimplemented, they are removed at compile time.

46. string.Compare vs string.CompareTo: Static Versus Instance Comparisons

string.Compare(strA, strB) is a static API with overloads that accept CultureInfo and StringComparison options and safe null handling. stringInstance.CompareTo(other) is an instance method that uses the instance’s default comparison rules and throws if the instance is null. Prefer string.Compare when you need culture- or case-aware behavior; use CompareTo for quick ordinal comparisons when you know neither side is null.

47. Try Catch Finally in C#: Structured Exception Handling and Cleanup

try holds code that can throw; catch blocks handle specific exception types; finally runs regardless for cleanup. For deterministic disposal, use using or try/finally to call Dispose; catching System.Exception broadly can hide bugs, so catch specific exceptions and rethrow or wrap when appropriate. At runtime, exception handling affects stack unwinding and can influence JIT code generation and performance, so avoid using exceptions for normal control flow.

48. SortedList vs SortedDictionary: Array-Backed Versus Tree-Backed Maps

SortedList stores keys and values in sorted arrays; it provides index-based access, but insertion and removal can be O(n) due to shifting. SortedDictionary uses a balanced tree (red-black) with O(log n) insertions, deletions, and lookups, and no index-based access. Choose SortedList for small, mostly static datasets where memory overhead matters; choose SortedDictionary when you need frequent mutations and predictable insertion cost.

49. Array vs ArrayList: Fixed Typed Array vs Non-Generic Dynamic Collection

Array is a fixed-length, type-safe structure with good performance for primitive operations. ArrayList (legacy) stores objects and requires boxing/unboxing for value types and lacks compile-time type safety. Use List<T> or arrays instead of ArrayList in modern C# for generics, performance, and clarity.

50. Hashtable Class in C#: Key-Value Store Using Hashing

Hashtable (System.Collections) stores object keys and values and uses GetHashCode plus Equals for bucket placement and collision resolution. It predates generics; prefer Dictionary<TKey,TValue> for type safety and performance because it avoids boxing and provides stronger compile-time checks. When using hashtable semantics, ensure stable hash codes and correct equality implementations.

51. Dispose() vs Finalize(): Deterministic vs Fallback Cleanup

Dispose implements IDisposable for deterministic release of unmanaged resources and should be called by clients or via using. Finalize (destructor syntax ~Class()) runs during GC as a nondeterministic fallback and adds to GC overhead because objects with finalizers survive an extra collection. Implement the recommended dispose pattern: provide Dispose(bool) and suppress finalization with GC.SuppressFinalize(this) when cleanup is done.

52. Generics in C#: Type-Safe Reusable Code

Generics let you write collections, algorithms, and delegates that work across types without boxing or repeated code. They generate IL with type parameters preserved for reference types and use runtime generic instantiation for value types, minimizing overhead. Use generic constraints (where T : struct, class, new(), or interfaces) to express capabilities and to enable safer APIs. Generics improve performance, maintainability, and testability.

53. Events in C#: Publish-Subscribe Built Into the Language

Events are a language-level wrapper around delegate fields that enforce add/remove semantics. They implement the observer pattern and support multicast handlers. When designing events, expose EventHandler<TEventArgs> or strongly typed delegates, and keep events nonblocking; consider synchronization and weak references for long-lived publishers to avoid memory leaks.

54. Delegate in C#: Type-Safe Function References

Delegates reference methods with a specific signature and can be combined for multicast invocation. They are the foundation of events, LINQ expressions, and callback patterns. Delegates can be converted from lambdas and method groups and are interoperable with expression trees for building dynamic code or for frameworks that inspect code structure.

55. Boxing and Unboxing: Converting Between Value and Reference Representations

Boxing wraps a value type inside an object on the heap; unboxing extracts the value with a type cast. Boxing allocates and copies memory, so repeated boxing causes GC pressure and slows hot paths. Use generics and Span<T> to avoid boxing. When profiling, watch for boxing hotspots in collections, logging, or reflection.

56. Write vs WriteLine: Control Console Output Flow

Console.Write writes text without a newline; Console.WriteLine appends the current newline. Use Write for inline output composition and WriteLine to finish a line and flush line-oriented buffers. On some runtimes, WriteLine may perform a flush or platform-specific behavior, which can influence I/O performance in tight loops.

57. string vs StringBuilder: Immutable String vs Mutable Buffer

string is immutable; each change allocates a new object. StringBuilder mutates an internal buffer and reduces allocations for many concatenations.

For small, fixed numbers of concatenations, prefer string; for loops or heavy text assembly, use StringBuilder. Also consider interpolated strings and the compiler optimizations that sometimes reduce allocations for simple cases.

58. var vs dynamic: Compile-Time Inference vs Runtime Typing

var instructs the compiler to infer a static type at compile time and preserves type safety and IntelliSense. dynamic defers binding to runtime, skipping compile-time checks and enabling late binding with overhead and potential runtime exceptions.

Use var for cleaner code when the type is obvious; use dynamic only when you must interact with dynamic languages, COM, or runtime-shaped data, and you accept the runtime cost.

59. Nullable Types in C#: Value Types That Can Be Null

Nullable<T> or T? allows value types to represent null. Use HasValue and Value or the null-coalescing operators like x ?? default. Nullable reference types, introduced later, add static analysis to help avoid null-related bugs by marking reference types as nullable or non-nullable. Handle nullability deliberately in APIs and enforce contracts with attributes and static analysis.

60. Iterators in C#: Yield a Simple Custom Enumerator

Iterators using yield return and yield break let you implement IEnumerable<T> without manual state machines. The compiler generates a state machine type, which can be efficient and readable, but be aware of allocation and deferred execution semantics. Each enumeration creates a new enumerator object, and the body executes lazily. Use iterators for streaming sequences and to compose pipelines with LINQ.

61. Reflection in C#: Inspect and Manipulate Metadata at Runtime

Reflection exposes types, members, attributes, and IL metadata via System.Reflection. Use reflection for DI containers, serializers, and tooling, but expect overhead and loss of compile-time safety. For performance-critical scenarios, prefer expressions, compiled delegates, or source generators; reflection works well for startup-level wiring or tooling tasks.

62. JIT Compiler: Convert IL to Native Machine Code at Runtime

The JIT translates MSIL to native code on first use of a method, enabling machine-specific optimizations. RyuJIT supports tiered compilation and profile-guided optimizations; precompilation via ReadyToRun or crossgen can reduce startup by trading portability and peak performance. Understand JIT behavior when you optimize hot paths and consider inlining, devirtualization, and CPU-specific instruction sets.

63. Tuple in C#: Lightweight Multiple-Value Return

ValueTuple syntax (e.g., (string Name, int Age) person = ("John", 30)) provides a simple, allocation-free way to return multiple values from a method and to name components for clarity. Tuples are convenient for quick returns, but for public APIs, prefer small DTOs or records to convey intent and to evolve without breaking callers.

64. is vs as Operators: Safe Type Checks and Casts

is checks whether an object is compatible with a type and yields a boolean; use pattern matching to both test and cast in one step: if (obj is string s) { ... }.

as attempts a safe cast and returns null on failure instead of throwing, so use it when null is an acceptable failure mode. Both operators affect expression trees, nullable handling, and runtime branching; prefer pattern matching for clearer intent and fewer redundancies.

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