Top 65 Common C# Interview Questions with Answers for All Levels

You’re halfway through a coding interviews and suddenly they ask something like, “Can you explain how async/await actually works under the hood?” Your brain freezes. You knew this stuff last night, but now it’s gone. I’ve been there.

Back when I was prepping for my Amazon interview, I bombed questions about delegates and threading so severely I thought I’d never get another callback. That pain pushed me to figure out how to study smarter, not longer.

This guide breaks down the C# questions that keep showing up, from OOP to LINQ to design patterns—that separate “yeah, I can code” from “hire this person now.” I’ll also show you how I used Interview Coder, the AI Interview Assistant I built, to practice these questions until I could explain them in my sleep.

If you’ve ever blanked out mid-question or felt like interview prep was just random chaos, stick around. This will help you fix that.

When I prepped for my first C# interview, memorizing syntax and slapping some code together would be enough. It wasn’t. I got hit with questions I didn’t even know how to reason through about memory models, object lifetimes, and even random .NET trivia. Embarrassing.

Fast forward a few years: I’ve landed internships at Amazon, Meta, and TikTok. Not because I magically got smarter, but because I got systematic. InterviewCoder helped me stop guessing and start practicing the exact questions that show up under pressure, with structure, and with feedback that doesn't sugarcoat.

This post? It’s a list of real C# questions that freshers like you actually get asked, along with how I think about them now, not how a textbook would. If you’re tired of spinning your wheels with random prep, start here.

Interviewers don’t care if you can rattle off definitions. They want to see if you actually understand how C# behaves under the hood, including memory management, object lifetimes, type safety, and platform choices. This guide breaks down 21 questions that come up again and again, along with how I’d answer them if I were sitting in your chair.

C# is like C’s clean-cut cousin who got into .NET and learned about object-oriented programming. C is procedural and close to the metal. C# is a higher-level, garbage-collected language that requires the CLR to run.

They want to know if you understand memory, ownership, and runtime environments. Not just what public and static mean.

Yes to inheritance. No to multiple inheritance. C# lets a class inherit from one base class but enables you to slap on as many interfaces as you want.

If you're in an interview and you say, "C# supports multiple inheritance," you're cooked. Say "No, but we can compose behavior with interfaces" and then back it up with a short code example.

Fixed size, type-safe, fast.

Has a flexible size, is slow, and accepts any object.

If you say you’d use ArrayList in modern C# code, expect a raised eyebrow. Prefer List<T> — it’s generic, type-safe, and does what you want 99% of the time.

Generics let you write type-safe code without repeating yourself. They're the reason List<int> doesn’t throw boxing overhead, while ArrayList does.

You might get hit with questions on where T: class, or why covariance matters in interfaces like IEnumerable<out T>. Don’t just memorize syntax; understand why generics exist.

Extension methods let you add methods to existing types, like sticking a Post-It on someone else’s notebook and pretending it was always there.

You want to add utility methods to a type you didn’t write (think: string.ToTitleCase()), but you don’t want to mess with inheritance.

The caller must initialize; the method can read/write

The caller doesn’t need to initialize; the technique must write

They both pass by reference, but with different rules.

In interviews, be ready to show TryParse() with out, or an algorithm that needs to return multiple values cleanly.

This one comes up a lot. Don’t regurgitate definitions.

Back it up with: “In real life, I’d use IComparable as an interface, but Shape as an abstract class because all shapes need base behavior like Draw().”

CLR handles memory.

You handle memory (and shoot yourself in the foot if you’re not careful)

Interviewers care if you know when to drop down to unmanaged (interop, performance, legacy reasons).

You’ve got:

utility methods can’t be instantiated

spread class across files (sound in codegen)

base logic + contracts

can’t be inherited (used for perf, predictability)

Most people forget why sealed exists. Mention inlining and you’ll win points.

Think of the GC like a janitor. When objects are no longer referenced, they are swept from memory. C# has three generations (Gen 0, 1, 2) for this.

Don’t call GC.Collect() unless you know what you’re doing.

The thing that makes C# run. Handles memory, type safety, JIT compilation, and security.

.NET is the playground. C# is the language. CLR is the referee.

That’s the pipeline.

You don’t have to know how to write a JIT compiler, but you do need to understand how your C# code becomes instructions that the Central processing unit (CPU) can run, mainly for debugging performance problems.

Quick rundown:

Old, Windows-only, enterprise graveyard

Cross-platform, modular

The modern, unified future

Don’t build new stuff on .NET Framework unless your manager tells you to (and even then… push back).

Common Intermediate Language. The bytecode your C# gets compiled to before it hits the CLR.

If you’re using reflection or dynamic features, you’re basically controlling CIL behavior at runtime. Nerdy, but valuable to mention.

Yeah, I’m repeating this on purpose. Managed code is your default. But know when you’re calling out to unmanaged code (e.g., P/Invoke, C++ DLLs).

Interviewers love asking this to see if you understand what happens outside your happy C# bubble.

3 stages:

Mark

Sweep

Compact

Avoid significant object heap churn. Avoid allocating stuff in tight loops. Avoid pretending GC doesn’t exist.

If you mention Span<T> or pooling, you're in advanced territory. Good.

If you don’t know this, you’ll mess up when writing high-performance code.

Yeah, again. Because they’ll ask twice, this is how they gauge your architectural instincts.

“I’d use an interface if I need multiple types to agree on a method contract. I’d use an abstract class when shared implementation matters more than flexibility.”

Think of a type-safe function pointer.

Delegates let you assign a method to a variable and call it later, like events, callbacks, and commands.

Mention multicast delegates, Action, Func, and Predicate for bonus points.

Everyone thinks they know async. Few do.

That’s it. But what matters is how this avoids blocking the main thread and how it interacts with ConfigureAwait(false) and Task.WhenAll, and CancellationToken.

Prepare for follow-ups on deadlocks, exception handling, and performance in async-heavy code.

I built InterviewCoder because I got tired of seeing brilliant engineers fail interviews they could’ve passed just because they didn’t know what to expect or how to practice under pressure.

You don’t need another PDF. You need reps. Try Interview Coder today, it’s free to get started.

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Have you ever gotten hit with a C# question and suddenly forget everything except public static void Main? Been there. Mid-level interviews don’t care if you skimmed a cheat sheet; they want to know if you’ve actually solved problems with C# under constraints. This list is built for that level.

These aren’t trivia flashcards. These are the follow-up traps, real-world angles, and interview “gotchas” that catch devs who only skim the docs. I’ve missed a few of these myself. Then I built InterviewCoder to make sure I didn’t miss them again. Now you won’t either.

If you’re prepping C# interviews at the intermediate level, stop just memorizing what StringBuilder does. These 26 questions are reused across tech screens. But more importantly, I break down how interviewers push beyond surface-level answers and what to say when they do. Practice smarter. No fluff.

Think of indexers as “syntactic sugar” that makes your object act like an array without looking like one. Clean APIs, no leaking internals.

“Give me an example where this helped encapsulate a data structure.” Then they’ll push on performance. Can the indexer hide a cache? Are you boxing too much? Are you allocating for every access?

You write C#. The compiler spits out IL. The JIT (Just-In-Time compiler) turns that into machine code the first time it runs. No magic, just steps.

They’ll ask about startup time, tiered compilation, and why pre-JIT (like NGEN or ReadyToRun) sometimes beats it in microservices. Be ready to discuss cold-start penalties.

Strings in C# are frozen. Change one, and you’re just making a new object. StringBuilder is your way out of unnecessary heap allocations when you’re doing tons of edits.

If you’re in a loop modifying strings hundreds of times, using + is a rookie move. Interviewers want you to explain the heap churn and how StringBuilder avoids it.

.NET cleans up after you with GC. It’s like a janitor that walks the heap, figures out what’s dead, and frees it.

Gen 0 vs Gen 2 promotions, when GC.Collect() is a red flag, and how the large object heap behaves is a bonus point for telling a story about reducing memory pressure.

Base with partial implementations

Split code across files

It can’t be inherited

Can’t be instantiated.

Explain when each is useful, sealed to enforce no inheritance, and possibly nudge JIT. Partial for codegen separation. Static for utility classes. Don’t over-explain, just show you’ve used them.

Don’t memorize the table. Just know:

“What if both define the same method name?” Talk about conflicts, explicit implementations, and C# default interface methods.

Let you bolt a new method onto a type you don’t own. Looks like it’s part of the type that saves you from wrapper hell.

They're not magic. Don’t abuse them to patch poor design. In interviews, explain how you use them to clean up utility code without making everything static.

Used primarily with code generation. Let's auto-generated files and your logic live side-by-side without merge hell.

Mention how you keep partials cohesive and how they’re helpful with tools like Entity Framework or WPF.

Name one case where late binding is worth it (e.g., plugin loading). Then explain why you'd avoid it in performance-critical code.

Same method name, different signatures. But you can’t overload on just the return type. C# needs unique parameter patterns.

Describe how you refactor ambiguous overloads to make client code less brittle.

Peek at types and methods at runtime. Useful for plugins, not for hot paths.

Reflection is slow. Cache it if you have to use it. Otherwise, prefer source gen or compile-time bindings.

Mention P/Invoke overhead, and when crossing the managed/native line, it becomes a problem, especially in tight loops.

Multiple threads are running in the same app. Welcome to chaos.

Describe a real race condition you fixed. What sync primitive did you use (lock, Monitor, SemaphoreSlim)? Why?

Query data in C# like you're writing structured query language (SQL), but it’s just method chains under the hood.

Deferred vs immediate execution. Show how you prevent double enumeration or unnecessary allocations.

Pointers to methods. Let you swap logic at runtime. Used in events, callbacks, and plugin systems.

Thread safety when invoking. Handle nulls. Avoid exceptions blowing up the call chain.

It can’t be inherited. Helps prevent misuse. Sometimes helps JIT inline better.

“Would you seal a class by default?” No. Only seal when you're sure it's done and shouldn’t be extended.

Calling one constructor from another in the same class using: this().

Centralizes init logic and reduces duplication. But handle exceptions early if chaining depends on inputs.

When do you use internal over public? Talk about hiding internals from consumers and about safely versioning.

Base class method you want to let derived classes override.

Mention the performance hit of virtual dispatch. Talk about when you’d prefer interface default methods or sealed overrides.

Streams, readers, writers. Most devs screw up file locks or async reads.

Describe how you test file I/O under flaky conditions, such as disk full, permission errors, and retry logic.

Tell a story where you replaced exception-based control flow with safer checks.

One instance, globally accessible. Use cases: logging, config, etc.

Talk about how it hurts testability and introduces hidden state. Then suggest alternatives like DI containers.

Use Lazy<T> in .NET—it’s safer than writing your own double-check lock logic.

Be prepared to sketch it. Then explain why Lazy<T> avoids locking overhead and is race-safe.

You raise, they listen.

Bring up weak references or explain how you unsubscribe in Dispose() to avoid lingering subscriptions.

Explain how you implement the full IDisposable pattern. Avoid resurrecting objects.

A delegate pointing to multiple methods. When invoked, all subscribers run.

“What happens when one fails?” Talk about exception handling, ordering, and short-circuiting.

Look, most engineers skim lists like this, nod, and move on. That’s not how you get hired. Pull 5 of these today, write real answers in your own words, and run them in a mock interview. That’s what I did. That’s why Interview Coder exists.

Want an easier way to get unstuck and track your weak spots? Download Interview Coder for free and start practicing like it’s game day.

If you're prepping for a senior C# role and think you can coast on years of experience, good luck. I’ve seen too many brilliant engineers walk into technical interviews with “senior” titles and still get wrecked by basic follow-ups.

I’m Roy. I built InterviewCoder after bombing more interviews than I’d like to admit, and eventually landed internships at Amazon, Meta, and TikTok. What changed? I stopped winging it and started treating prep like actual reps.

This list isn’t just trivia. These are the kinds of questions that come up when interviewers want to see if you’ve shipped real code, not just solved LeetCode puzzles. They’re the traps that separate “wrote it once in a tutorial” from “debugged it at 2 AM in prod.”

So if you’re tired of memorizing syntax and still getting stumped when it matters, keep reading.

Sometimes one file just isn’t enough. Maybe multiple devs are working on the same class, or maybe auto-generated code is cluttering up your logic. Partial classes let you split a class across multiple files. It’s still one class at compile time, just spread out.

public partial class MyClass

{

// part of the class

}

Super valuable for WinForms or when you're messing with code generators.

Strings in C# are like clingy exes; they never let go. Every time you change one, a new object is created. If you’re doing that in a loop, you’re burning memory for no reason.

StringBuilder skips all that drama and just mutates the same instance.

Bad way:

string s = "";

for (int i = 0; i < 1000; i++) {

s += i + " ";

}

Better:

var sb = new StringBuilder();

for (int i = 0; i < 1000; i++) {

sb.Append(i).Append(" ");

}

Less garbage collection. More performance. Use it when strings change a lot.

const = locked at compile time. readonly = can change in the constructor.

public const int MaxUsers = 100;

public readonly string ConfigPath;

public MyClass(string config) {

ConfigPath = config; // valid for readonly, not const

}

Oh, and const is implicitly static. readonly isn’t. Know the difference; interviewers love to throw this one in just to see you sweat.

Reflection is how C# lets you inspect types and members at runtime. Think, "What's inside this object even though I don’t know its type right now?"

Type type = typeof(string);

Console.WriteLine(type.FullName); // System.String

You can even invoke methods or grab private values. It’s useful for plugins, serializers, or frameworks, but it’s also slow. Don’t overuse it unless you need to.

Same method name, different parameter signatures.

public void Send(string message) { }

public void Send(string message, bool urgent) { }

Change the number, order, or types of parameters. That’s it. Just don’t overload Main(); the runtime only calls one.

Both compare values… until they don’t.

For value types like int, they behave the same:

int x = 5;

int y = 5;

Console.WriteLine(x == y); // True

Console.WriteLine(x.Equals(y)); // True

For reference types like objects or strings? == can check reference, while Equals() checks value (unless overridden).

Know which one you’re using. Interviewers love this trap.

Indexers let you treat an object like an array.

public string this[int i] {

get { return internalList[i]; }

set { internalList[i] = value; }

}

You can now do myObject[5] like it’s an array, just syntactic sugar, but cool when building collections.

Arrays are just fixed-length containers of the same type.

string[] drinks = { "coffee", "tea", "water" };

They’re reference types, zero-indexed, and live on the heap. If you need dynamic resizing, switch to List<T>.

Compile-time = early binding (faster, safer) Runtime = late binding (more flexible, slower)

// Early binding

var calc = new Calculator();

calc.Add(2, 3);

// Late binding via reflection

Type t = typeof(Calculator);

object obj = Activator.CreateInstance(t);

MethodInfo mi = t.GetMethod("Add");

mi.Invoke(obj, new object[] { 2, 3 });

If you use reflection a lot, just know it comes at a performance cost.

They're like fancy public fields with access control.

public int Age { get; set; }

Behind the scenes, it's a private variable with get/set methods. You can add logic to getters/setters too.

public int Age {

get => age;

set {

if (value < 0) throw new Exception("Nice try");

age = value;

}

}

Boxing = value → objectUnboxing = object → value

int num = 42;

object obj = num; // boxing

int unboxed = (int)obj; // unboxing

Avoid this in tight loops. It’s slow and adds GC pressure.

Quick and dirty:

bool IsPrime(int n) {

if (n < 2) return false;

for (int i = 2; i <= Math.Sqrt(n); i++)

if (n % i == 0) return false;

return true;

}

That Math.Sqrt(n) trick saves you unnecessary checks.

Generate all substrings of a string.

void PrintSubstrings(string str) {

for (int i = 0; i < str.Length; i++)

for (int j = i + 1; j <= str.Length; j++)

Console.WriteLine(str.Substring(i, j - i));

}

Basic version:

bool IsPalindrome(string s) {

for (int i = 0, j = s.Length - 1; i < j; i++, j--)

if (s[i] != s[j]) return false;

return true;

}

Case-sensitive. Add.ToLower() or regex cleanup if needed.

string ReverseWords(string sentence) {

var words = sentence.Split(' ');

Array.Reverse(words);

return string.Join(" ", words);

}

string Reverse(string s) {

return new string(s.Reverse().ToArray());

}

Or do it manually if the interviewer wants pointer-level stuff.

If your class creates its own dependencies, it’s glued together. That’s a mess to test or scale.

DI flips it: dependencies get passed in.

public class OrderService {

private readonly IPaymentService _paymentService;

public OrderService(IPaymentService paymentService) {

_paymentService = paymentService;

}

}

ASP.NET Core handles all this behind the scenes with:

builder.Services.AddScoped<IPaymentService, PaymentService>();

Interviewers ask this to see if you’ve built maintainable systems, not just toy apps.

Instead of copy-pasting logic for int, string, and float, use generics:

public void Print<T>(T value) {

Console.WriteLine(value);

}

Safer, reusable, and cleaner. You’ll see them in List<T>, Dictionary<K,V>, etc.

Language-Integrated Query (LINQ) lets you write queries on collections without a bunch of loops.

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

You can use it on:

Loops through stuff in memory

Builds DB queries, executes in DB

A real collection with all items in memory

For EF or remote queries. Use List<T> when you already have the data.

Add methods to existing types without editing their source:

public static class StringExtensions {

public static string ToTitle(this string s) =>

CultureInfo.CurrentCulture.TextInfo.ToTitleCase(s);

}

Now you can do "hello world".ToTitle(). This is how LINQ methods like.Where() and .Select() work under the hood.

Want to get info about a type or call a method dynamically?

Type t = typeof(Console);

var method = t.GetMethod("WriteLine", new[] { typeof(string) });

method.Invoke(null, new object[] { "Yo" });

Useful in tooling, frameworks, or anything that needs meta-programming. Slow, so don’t abuse it.

I’ve been that guy sweating through LeetCode with four tabs open and Slack pinging in the background. Praying I don't accidentally alt-tab during a screen share and look like I'm cheating. It messes with your head if you’ve felt that stress, yeah, same.

So I built something I wish I had back then: Interview Coder. Start with the free plan. Zero dollars. Get used to how it works. If it clicks, grab the Pro version for $25/month (annual). It’s like having a calm brain next to you when your real one’s freaking out.