C# & .NET Scenario-Based MCQ

Scenario-based multiple choice questions covering core C# and .NET topics.


Table of Contents

  1. Fundamentals & Type System
  2. Classes, OOP & Inheritance
  3. Generics & Collections
  4. Delegates, Events & Lambda
  5. LINQ
  6. Async / Await & Multithreading
  7. Exception Handling
  8. Memory Management & GC
  9. Interfaces & Patterns
  10. File I/O & Serialization
  11. ASP.NET Core & Web API
  12. Entity Framework Core
  13. C# Language Features
  14. Records, Structs & Value Types
  15. Dependency Injection & Testing


1. Fundamentals & Type System

Q. A developer writes the following code and is surprised by the output. What does the program print?

int a = 5;
object b = a;
int c = (int)b;
b = 10;
Console.WriteLine(a + " " + c);

Answer: B Boxing copies the value of a into a heap-allocated object. Unboxing copies that value back to c. Assigning b = 10 creates a new box; it does not change a or c. Both a and c remain 5.

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Q. What is the output of the following code?

string s1 = "hello";
string s2 = "hello";
Console.WriteLine(object.ReferenceEquals(s1, s2));

Answer: B The CLR string intern pool ensures that identical string literals share the same object at runtime. object.ReferenceEquals therefore returns true for two variables holding the same literal.

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Q. A team member writes this code expecting it to swap two integers:

void Swap(int x, int y) { int t = x; x = y; y = t; }
int a = 1, b = 2;
Swap(a, b);
Console.WriteLine(a + " " + b); // expected: 2 1

What is actually printed and why?

Answer: B int is a value type. Parameters x and y are copies. Modifying them inside the method has no effect on a and b. To fix, use ref parameters or a tuple swap: (a, b) = (b, a).

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Q. A developer needs a variable that can hold either an int or null. Which declaration is correct in C#?

Answer: C int? is shorthand for Nullable<int>. It allows the value type int to hold null. Assigning null directly to a non-nullable int causes a compile error.

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Q. What does the following expression print?

Console.WriteLine(10 / 3);
Console.WriteLine(10.0 / 3);

Answer: B Integer division truncates toward zero — 10 / 3 is 3. When either operand is double, floating-point division is used — 10.0 / 3 produces the full double-precision result 3.3333333333333335.

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Q. A developer declares the following and attempts to use it. What is the result?

const double Pi = 3.14159;
Pi = 3.14; // line B

Answer: B const values are compile-time constants and are embedded directly into the IL. Any attempt to assign to them causes a compile-time error.

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2. Classes, OOP & Inheritance

Q. A developer writes the following class hierarchy and runs the code. What is printed?

class Animal
{
    public virtual string Speak() => "...";
}
class Dog : Animal
{
    public override string Speak() => "Woof";
}
Animal a = new Dog();
Console.WriteLine(a.Speak());

Answer: B virtual/override enables runtime polymorphism. The CLR dispatches to the most-derived override at runtime. Although a is declared as Animal, it holds a Dog instance, so Dog.Speak() is called.

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Q. A developer uses new instead of override. What does the code print?

class Base   { public virtual  string Name() => "Base"; }
class Derived : Base { public new string Name() => "Derived"; }

Base obj = new Derived();
Console.WriteLine(obj.Name());

Answer: B new hides the base method rather than overriding it. Because the reference is typed as Base, the virtual dispatch table still points to Base.Name(). If the variable were typed as Derived, it would print Derived.

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Q. A class needs to prevent instantiation but still provide shared helper methods to derived classes. Which modifier achieves this?

Answer: C An abstract class cannot be instantiated directly but can be inherited. It may contain a mix of abstract members (no implementation, must be overridden) and concrete members (with implementation).

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Q. What is the output of the following code?

class Counter
{
    public static int Count = 0;
    public Counter() { Count++; }
}
var c1 = new Counter();
var c2 = new Counter();
Console.WriteLine(Counter.Count);

Answer: C Count is a static field shared by all instances. Each constructor call increments it. After two instantiations, Count is 2.

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Q. A developer wants a class that can only be used as a base class inside its own assembly, but can be used and instantiated normally outside. Which combination is correct?

Answer: D A public class with a protected constructor cannot be instantiated directly from outside (no accessible constructor) but derived classes inside or outside the assembly can call it. internal abstract is only visible inside the assembly.

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Q. What is printed by the following code?

class A
{
    public A()        => Console.Write("A ");
    public A(int x)   => Console.Write($"A{x} ");
}
class B : A
{
    public B() : base(1) => Console.Write("B ");
}
new B();

Answer: C Base constructors always run before derived constructors. B() chains to A(int x), printing A1 , then B() body prints B .

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3. Generics & Collections

Q. A developer writes the following generic method and calls it:

T Max<T>(T a, T b) where T : IComparable<T>
    => a.CompareTo(b) >= 0 ? a : b;

Console.WriteLine(Max(3, 7));
Console.WriteLine(Max("apple", "mango"));

What is printed?

Answer: A The where T : IComparable<T> constraint allows calling CompareTo. Both int and string implement IComparable<T>. 7 > 3 and "mango" > "apple" lexicographically, so 7 and mango are returned.

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Q. A developer needs a collection that:

Which .NET collection best fits?

Answer: B In .NET 5+, Dictionary<K,V> preserves insertion order as an implementation detail (though not guaranteed by specification). For a guaranteed ordered dict use SortedList/SortedDictionary (sorted by key, not insertion) or OrderedDictionary (non-generic, legacy). The most idiomatic modern answer is Dictionary<K,V> for key lookup; if strict insertion-order guarantee is needed List<(K,V)> or a purpose-built OrderedDictionary<K,V> (.NET 9) should be used.

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Q. What is the output of the following code?

var list = new List<int> { 1, 2, 3 };
var copy = list;
copy.Add(4);
Console.WriteLine(list.Count);

Answer: B List<T> is a reference type. Assigning copy = list copies the reference, not the data. Both variables point to the same List<int> instance. Adding to copy is visible through list.

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Q. A developer needs fast membership testing for a large set of string tags with no duplicates. Which collection offers O(1) average-case lookup?

Answer: C HashSet<T> uses a hash table internally, giving O(1) average Contains and Add. List is O(n), SortedSet is O(log n), LinkedList is O(n).

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Q. What does this code print?

var stack = new Stack<int>();
stack.Push(1); stack.Push(2); stack.Push(3);
Console.WriteLine(stack.Pop());
Console.WriteLine(stack.Peek());

Answer: B Stack<T> is LIFO. Push(1), Push(2), Push(3) — top is 3. Pop() removes and returns 3. Peek() returns the new top without removing it — 2.

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Q. A developer uses covariance on a generic interface:

IEnumerable<Dog> dogs = new List<Dog>();
IEnumerable<Animal> animals = dogs;

Why does this compile without a cast?

Answer: B IEnumerable<out T> is covariant. The out keyword means T only appears in output (return) position. When Dog derives from Animal, IEnumerable<Dog> is safely assignable to IEnumerable<Animal>.

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4. Delegates, Events & Lambda

Q. What is the output of the following code?

Func<int, int> doubler = x => x * 2;
Func<int, int> addTen  = x => x + 10;
Func<int, int> combined = x => addTen(doubler(x));
Console.WriteLine(combined(5));

Answer: A doubler(5) = 10, then addTen(10) = 20.

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Q. A developer builds a multicast delegate and notices only the last return value is received:

Func<int> d = () => 1;
d += () => 2;
d += () => 3;
Console.WriteLine(d());

What is printed and why?

Answer: C Multicast delegates invoke all targets in order but only propagate the return value of the last invoked delegate. If you need all results, iterate GetInvocationList().

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Q. What is the classic closure variable-capture bug in the following code?

var actions = new List<Action>();
for (int i = 0; i < 3; i++)
    actions.Add(() => Console.Write(i + " "));
actions.ForEach(a => a());

Answer: B All three lambdas close over the same i variable. After the loop finishes, i is 3. Fix: int copy = i; actions.Add(() => Console.Write(copy + " "));

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Q. A junior developer accidentally does this:

public class Publisher
{
    public Action<string>? MessageSent;  // public field delegate
}
var pub = new Publisher();
pub.MessageSent += s => Console.WriteLine("A: " + s);
pub.MessageSent  = s => Console.WriteLine("B: " + s); // line X
pub.MessageSent?.Invoke("hello");

What is a problem with this design, and what does line X demonstrate?

Answer: B A public delegate field exposes the assignment operator to external code. = on line X replaces the entire invocation list. Using event restricts external code to += and -= only, protecting subscribers.

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Q. What is the difference between Action<string> and Func<string, bool>?

Answer: B Action<T> delegates always return void. Func<T, TResult> delegates return the last type parameter (bool here). Choose Action for side-effect operations and Func for transformations/predicates.

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5. LINQ

Q. What does this query return?

int[] numbers = { 1, 2, 3, 4, 5, 6 };
var result = numbers.Where(n => n % 2 == 0).Select(n => n * n);
foreach (var x in result) Console.Write(x + " ");

Answer: B Where filters to even numbers (2, 4, 6). Select squares each: 4, 16, 36.

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Q. A developer writes the following LINQ and is surprised that the database is queried five times. Why?

IQueryable<Product> query = db.Products.Where(p => p.Price > 100);
for (int i = 0; i < 5; i++)
    Console.WriteLine(query.Count());

Answer: B IQueryable<T> uses deferred execution. The query is not executed when defined; it executes each time a terminal operator (Count, ToList, First, etc.) is called. Fix: int count = query.Count(); before the loop.

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Q. What does the following code print?

var words = new[] { "banana", "apple", "cherry", "avocado" };
var result = words
    .Where(w => w.StartsWith('a'))
    .OrderBy(w => w.Length)
    .FirstOrDefault();
Console.WriteLine(result);

Answer: A Where filters to words starting with a: ["apple", "avocado"]. OrderBy(Length) sorts by length: ["apple" (5), "avocado" (7)]. FirstOrDefault returns apple.

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Q. A developer needs to flatten a list of orders where each order contains multiple items, and get a distinct list of all product names. Which LINQ operators should be used?

Answer: B SelectMany flattens a sequence of sequences (each order's items into a single sequence). Distinct then removes duplicate product names.

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Q. What is the difference between First() and FirstOrDefault() in LINQ?

Answer: B First() throws when no element matches or the sequence is empty. FirstOrDefault() returns default(T) in those cases. Prefer FirstOrDefault() when an empty result is a valid scenario.

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Q. What is printed by this GroupBy query?

var data = new[] { "cat", "car", "bat", "bar", "bee" };
var groups = data.GroupBy(w => w[0]);
foreach (var g in groups)
    Console.WriteLine($"{g.Key}:{g.Count()}");

Answer: C Words starting with c: cat, car (count 2). Words starting with b: bat, bar, bee (count 3). Output is c:2 and b:3 on separate lines.

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6. Async / Await & Multithreading

Q. A developer writes this async method. What is wrong with it?

public async Task<string> GetDataAsync()
{
    Thread.Sleep(2000); // simulate work
    return "done";
}

Answer: B Thread.Sleep is a synchronous blocking call. It blocks the thread for 2 seconds even inside an async method, defeating the purpose of async. await Task.Delay(2000) yields the thread to the pool while waiting.

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Q. What is printed by the following code?

async Task<int> ComputeAsync()
{
    await Task.Delay(0);
    return 42;
}
var t = ComputeAsync();
Console.WriteLine(t.IsCompleted ? "Done" : "Running");
await t;
Console.WriteLine("Finished: " + t.Result);

Answer: A await Task.Delay(0) still schedules a continuation, so the task may not be complete by the time the IsCompleted check runs. After await t the task is complete and Result is 42.

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Q. A developer writes a fire-and-forget async method as async void. A tester reports that exceptions from the method are swallowed silently. Why?

Answer: A async void raises exceptions on the SynchronizationContext rather than returning a faultable Task. If there is no handler, the exception can crash the process or be silently lost. Use async Task and always await the result.

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Q. What is the risk of the following code in a high-throughput ASP.NET Core application?

public string GetData() => GetDataAsync().Result;

Answer: B In contexts with a SynchronizationContext (classic ASP.NET, WPF, WinForms), .Result blocks the thread, while the awaited continuation needs that same thread to resume — causing a deadlock. Use await throughout or ConfigureAwait(false).

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Q. Two threads share a counter and increment it 10,000 times each. The final value is often less than 20,000. What is the cause and fix?

int counter = 0;
void Increment() { for (int i = 0; i < 10_000; i++) counter++; }
var t1 = Task.Run(Increment);
var t2 = Task.Run(Increment);
Task.WaitAll(t1, t2);
Console.WriteLine(counter);

Answer: B counter++ is three operations: read, add 1, write back. Two threads can both read the same value and write the same incremented value, losing updates. Interlocked.Increment is CPU-atomic and fixes the race.

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Q. When should ValueTask<T> be preferred over Task<T>?

Answer: B Task<T> always allocates a heap object. ValueTask<T> is a struct that avoids the allocation when the result is already available synchronously. Prefer it in hot-path methods like caching layers. Do not await a ValueTask<T> more than once without converting to Task first.

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7. Exception Handling

Q. What is the difference between throw and throw ex in the following code?

try { DoWork(); }
catch (Exception ex)
{
    // Option A:
    throw;
    // Option B:
    throw ex;
}

Answer: B throw (bare) re-throws the current exception with its original stack trace intact. throw ex creates a new throw point — the stack trace appears to originate from the catch block, hiding the real source of the error.

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Q. Given this code, does the finally block execute?

try
{
    Console.WriteLine("try");
    return;
}
finally
{
    Console.WriteLine("finally");
}

Answer: B finally blocks execute regardless of how the try block exits — normal flow, return, break, continue, or a handled exception. Exceptions: Environment.FailFast() and StackOverflowException can bypass finally.

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Q. A developer defines a custom exception. Which constructor pattern is most important to include?

public class OrderException : Exception
{
    // Which constructors should be included?
}

Answer: C Best practice is to provide the three standard constructors. The (string, Exception) constructor is especially important: it allows wrapping a lower-level exception as an InnerException, preserving the full exception chain for debugging.

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Q. A developer uses an exception filter. What is an advantage of the when clause?

catch (HttpRequestException ex) when (ex.StatusCode == HttpStatusCode.NotFound)
{
    // handle 404 only
}

Answer: B The when predicate is evaluated in the context of the original throw site. If it returns false, the stack is not unwound, allowing debuggers to inspect the original state. It also cleanly routes exception handling without re-throwing.

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Q. What happens when Task.WhenAll is awaited and two of three tasks fail?

Task[] tasks = [
    Task.Run(() => throw new Exception("E1")),
    Task.Run(() => throw new Exception("E2")),
    Task.Run(() => Console.WriteLine("ok")),
];
try { await Task.WhenAll(tasks); }
catch (Exception ex) { Console.WriteLine(ex.Message); }

Answer: B await Task.WhenAll(...) waits for all tasks and then re-throws the first exception from the underlying AggregateException. To see all failures, inspect tasks[i].Exception for each faulted task after the catch.

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8. Memory Management & GC

Q. A developer notices that an object with a finalizer takes two garbage collection cycles to be reclaimed. Why?

Answer: B When an unreachable finalizable object is discovered in cycle 1, the GC queues it for finalization rather than reclaiming it immediately. The finalizer thread runs the ~Destructor, and the next GC cycle reclaims the now-finalized object. This is why GC.SuppressFinalize(this) in Dispose() improves performance.

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Q. Which code pattern correctly implements the Dispose pattern to handle both managed and unmanaged resources?

public class Resource : IDisposable
{
    private bool _disposed;

    // Option A:
    public void Dispose() { CloseHandle(); }

    // Option B:
    protected virtual void Dispose(bool disposing)
    {
        if (_disposed) return;
        if (disposing) { /* free managed */ }
        CloseHandle(); // free unmanaged
        _disposed = true;
    }
    public void Dispose() { Dispose(true); GC.SuppressFinalize(this); }
    ~Resource() { Dispose(false); }
}

Answer: B Option B is the canonical Dispose pattern: Dispose(bool) separates managed from unmanaged cleanup, GC.SuppressFinalize prevents a redundant finalizer call when Dispose() is called explicitly, and the _disposed guard prevents double-dispose.

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Q. A developer writes a cache using strong references and observes memory keeps growing. What alternative avoids holding objects alive unnecessarily?

Answer: B WeakReference<T> allows the GC to collect the referenced object. When TryGetTarget returns false, the cache misses and recomputes. This avoids holding large objects alive longer than necessary under memory pressure.

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Q. A developer finds a memory leak in a long-running service. The most likely root cause from the following list is:

Answer: B Unsubscribed event handlers are one of the most common managed memory leaks. The publisher holds a delegate reference to the subscriber. As long as the publisher lives, the subscriber cannot be collected. Always unsubscribe in Dispose() or use WeakEventManager.

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Q. When is it acceptable to call GC.Collect() explicitly?

Answer: B Forcing a GC disrupts the GC's self-tuning heuristics, promotes objects to higher generations unnecessarily, and introduces latency pauses. It is almost always the wrong choice in production application code.

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9. Interfaces & Patterns

Q. A developer defines two interfaces with the same method name and implements both:

interface IA { string Hello(); }
interface IB { string Hello(); }
class C : IA, IB
{
    public string Hello() => "shared";
    string IB.Hello() => "IB explicit";
}
C c = new C();
Console.WriteLine(c.Hello());
Console.WriteLine(((IB)c).Hello());

What is the output?

Answer: B The public Hello() satisfies IA and is accessible via the class reference. IB.Hello() is an explicit interface implementation, only accessible via an IB reference, and returns IB explicit.

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Q. A developer wants to add a default implementation to an interface method so existing implementors do not need to be updated. Which C# feature supports this?

Answer: B C# 8 introduced default interface method implementations. Existing classes that implement the interface inherit the default without change. Classes can still override the default.

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Q. Which design pattern is implemented in this code?

public abstract class DataProcessor
{
    public void Process()
    {
        ReadData();
        TransformData();
        WriteData();
    }
    protected abstract void ReadData();
    protected abstract void TransformData();
    protected virtual void WriteData() => Console.WriteLine("Writing...");
}

Answer: C The Template Method pattern defines the skeleton of an algorithm in a base class (Process()), deferring specific steps to subclasses via abstract or virtual methods. The overall structure is fixed; the details vary.

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Q. A developer wants to ensure only one instance of a ConfigurationManager class exists throughout the application. Which pattern should be used?

Answer: B The Singleton pattern restricts instantiation to one object. In .NET, the thread-safe lazy singleton is commonly implemented with Lazy<T>: private static readonly Lazy<ConfigurationManager> _instance = new(() => new());

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Q. A developer wants to add logging and timing behaviour to an existing IOrderService without modifying its implementation. Which pattern applies?

Answer: C The Decorator pattern wraps an existing object with a new class implementing the same interface, adding behaviour before/after delegating to the inner implementation — no modification of the original class needed.

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10. File I/O & Serialization

Q. A developer reads a large file line by line using File.ReadAllLines. A colleague suggests replacing it with File.ReadLinesAsync in .NET 9. What is the key difference?

Answer: A File.ReadAllLines allocates an array containing every line. For a 10 GB log file this can exhaust memory. File.ReadLinesAsync (returning IAsyncEnumerable<string>) yields one line at a time, allowing processing with await foreach and minimal memory usage.

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Q. A developer serializes an object to JSON:

var order = new Order { Id = 1, CustomerName = "Alice" };
string json = JsonSerializer.Serialize(order);

The output is {"Id":1,"CustomerName":"Alice"} but the API contract requires {"id":1,"customer_name":"Alice"}. What is the simplest fix?

Answer: B [JsonPropertyName("id")] on a property overrides the serialized name. For a global policy, JsonNamingPolicy.SnakeCaseLower (added in .NET 8) converts all property names to snake_case automatically.

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Q. A developer needs to process a 2 GB JSON file without loading it all into memory. Which .NET API is most appropriate?

Answer: B Utf8JsonReader is a forward-only, low-allocation JSON reader that processes tokens from a ReadOnlySpan<byte> or ReadOnlySequence<byte>. JsonSerializer.DeserializeAsync with a Stream also streams the document. Both avoid loading the entire file into memory.

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Q. A developer is writing a file and wants to ensure the data is flushed to disk even if the application crashes immediately after. Which approach is correct?

Answer: B FileOptions.WriteThrough (or flushToDisk: true) bypasses the OS page cache and writes directly to the storage device. This is important for write-ahead logs and databases where durability is critical. Normal Flush() only moves data from .NET buffers to the OS, not necessarily to disk.

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11. ASP.NET Core & Web API

Q. An ASP.NET Core controller action is registered with [HttpPost] but the client receives 405 Method Not Allowed when sending a POST request. What is the most likely cause?

Answer: B 405 Method Not Allowed means the URL matched a route but the HTTP method was not allowed. The most common causes are: the route matches a different action that only allows GET, or attribute routing has a mismatch between the declared HTTP method and the client's request method.

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Q. A developer registers a service as Singleton but injects it into a Scoped service, which is then injected into a Transient service. What is the problem?

Answer: B A Singleton outlives a Scope. If a singleton captures a Scoped service, that scoped service is never released at the end of the request, causing memory leaks and stale state. ASP.NET Core's DI container throws a InvalidOperationException in Development mode to catch this.

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Q. A minimal API endpoint is defined as:

app.MapGet("/users/{id}", (int id) => Results.Ok($"User {id}"));

A client calls GET /users/abc. What response does the client receive?

Answer: B When a route parameter cannot be converted to the declared parameter type (int), the ASP.NET Core model binding system returns 400 Bad Request automatically. With [ApiController] or minimal APIs, this is handled before the handler runs.

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Q. A developer registers two middleware components in this order:

app.Use(async (ctx, next) => { Console.Write("A1 "); await next(); Console.Write("A2 "); });
app.Use(async (ctx, next) => { Console.Write("B1 "); await next(); Console.Write("B2 "); });
app.Run(async ctx => Console.Write("C "));

What is printed for a single request?

Answer: A The middleware pipeline is a chain of nested calls. A1 executes, calls next → B1 executes, calls next → C runs (terminal). Unwinding: B2 runs, then A2.

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Q. A Web API returns 200 OK for a newly created resource. A reviewer says this violates REST conventions. What should be returned instead?

Answer: B REST convention for a successful resource creation is 201 Created with a Location header containing the URI of the new resource. Use CreatedAtAction or CreatedAtRoute in ASP.NET Core controllers, or Results.Created(uri, value) in minimal APIs.

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Q. A developer forgets to add app.UseAuthentication() but has app.UseAuthorization(). What happens when a protected endpoint is accessed with a valid JWT?

Answer: B UseAuthentication() must come before UseAuthorization(). Authentication middleware reads and validates the JWT, setting HttpContext.User. Without it, User is an unauthenticated principal and every [Authorize] check fails with 401.

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12. Entity Framework Core

Q. A developer writes the following LINQ query against an EF Core DbContext. What is the issue?

var names = db.Products
    .ToList()
    .Where(p => p.Price > 100)
    .Select(p => p.Name)
    .ToList();

Answer: B ToList() materialises the query immediately, pulling all rows into memory. Any LINQ operators applied after ToList() run in-process (LINQ to Objects), not as SQL. Move Where and Select before the final ToList() to generate an efficient SQL query.

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Q. A developer loads a list of orders with their items:

var orders = db.Orders.ToList();
foreach (var order in orders)
    Console.WriteLine(order.Items.Count);

The app is extremely slow when there are 1,000 orders. What is the problem?

Answer: B Without .Include(o => o.Items), EF Core uses lazy loading (if enabled) or throws a NullReferenceException. With lazy loading enabled, each order.Items access fires a new SQL query — 1,000 extra queries. Fix: db.Orders.Include(o => o.Items).ToList().

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Q. A developer calls db.SaveChangesAsync() without calling AsNoTracking(). A colleague adds AsNoTracking() to a read-only query. What is the benefit?

Answer: B EF Core's change tracker stores a snapshot of each loaded entity for change detection. For read-only scenarios (reports, APIs that only return data), this snapshot wastes memory and CPU. AsNoTracking() skips this overhead.

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Q. Two concurrent requests both read an order with Status = "Pending" and try to set it to "Processing". Only one should succeed. Which EF Core mechanism handles this?

Answer: B Optimistic concurrency adds a concurrency token (e.g., [Timestamp] public byte[] RowVersion { get; set; }). EF Core includes the original token in the WHERE clause of UPDATE. If another request has changed the row, the WHERE matches zero rows and EF throws DbUpdateConcurrencyException.

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13. C# Language Features

Q. What does the following pattern matching code print?

object obj = 42;
if (obj is int n and > 10 and < 100)
    Console.WriteLine($"In range: {n}");
else
    Console.WriteLine("Out of range");

Answer: B C# 9 conjunctive patterns use and to combine conditions. obj is int n and > 10 and < 100 checks that obj is an int, assigns it to n, and verifies 10 < n < 100. 42 satisfies all conditions.

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Q. A developer uses a switch expression. What does the following return for shape = new Circle(5)?

double Area(Shape shape) => shape switch
{
    Circle  c => Math.PI * c.Radius * c.Radius,
    Rectangle r => r.Width * r.Height,
    _ => throw new ArgumentOutOfRangeException()
};

Answer: B The switch expression matches Circle with radius 5 and computes π × 5 × 5 ≈ 78.54. Switch expressions in C# 8+ allow concise pattern-based dispatch without break statements.

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Q. What does the null-coalescing assignment operator do in the following code?

List<string>? tags = null;
tags ??= new List<string>();
tags.Add("dotnet");
Console.WriteLine(tags.Count);

Answer: B The ??= operator (C# 8+) assigns the right-hand value to the left-hand variable only when the variable is null. After ??=, tags is a new list. Adding "dotnet" makes Count = 1.

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Q. What is the output of the following using C# string interpolation with format specifiers?

decimal price = 1234.5m;
Console.WriteLine($"Price: {price:C2}");
Console.WriteLine($"Pi: {Math.PI:F4}");

Answer: B {value:C2} formats as currency with 2 decimal places (locale-dependent — $1,234.50 in en-US). {value:F4} formats as fixed-point with 4 decimal places — 3.1416 (rounded).

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Q. A developer uses Span<T> instead of array slicing. What is the advantage?

int[] data = { 1, 2, 3, 4, 5 };
Span<int> slice = data.AsSpan(1, 3); // { 2, 3, 4 }

Answer: B Span<T> is a stack-allocated struct that represents a contiguous region of memory. .AsSpan(1, 3) creates a zero-copy window into data. Mutations via slice are reflected in the original array. It is allocation-free, unlike creating a new int[] slice.

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Q. A developer uses required on a property:

public class Config
{
    public required string ConnectionString { get; init; }
}
var cfg = new Config(); // line A

What happens at line A in C# 11+?

Answer: B The required modifier (C# 11) enforces that the property is set in an object initializer. Omitting it is a compile-time error: CS9035: Required member 'Config.ConnectionString' must be set in the object initializer or attribute constructor.

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14. Records, Structs & Value Types

Q. What does the following code print?

record Point(int X, int Y);
var p1 = new Point(1, 2);
var p2 = new Point(1, 2);
Console.WriteLine(p1 == p2);
Console.WriteLine(ReferenceEquals(p1, p2));

Answer: C Records use value-based equality: p1 == p2 compares properties and returns true. But records are still reference types (unless record struct), so ReferenceEquals returns false — they are distinct objects on the heap.

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Q. A developer uses a record with the with expression. What does this print?

record Person(string Name, int Age);
var alice = new Person("Alice", 30);
var bob   = alice with { Name = "Bob" };
Console.WriteLine(alice.Name + " " + bob.Name);

Answer: C The with expression creates a new record copying all properties from the original and overriding specified ones. alice is unchanged (Alice, 30); bob is a new record (Bob, 30). Records are immutable — with produces a copy, not a mutation.

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Q. What is the key difference between a class and a struct in C#?

Answer: B The fundamental distinction: class → reference type → assignment copies reference → heap allocation. struct → value type → assignment copies the entire value → typically stack/inline. Structs cannot inherit from other structs/classes (except interfaces) and are sealed implicitly.

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Q. A developer defines a large mutable struct and passes it by value repeatedly. A performance profiler shows excessive copying. What is the most appropriate fix?

Answer: B Passing a large struct by value copies all its bytes. ref parameters pass a reference to the original, avoiding copying. in passes a read-only reference (no mutation, no copy). readonly struct also allows the JIT to pass by reference automatically in some cases.

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15. Dependency Injection & Testing

Q. A developer wants to write a unit test for OrderService which calls IEmailSender. The test must verify that SendAsync was called once without actually sending email. Which technique achieves this?

Answer: B Mocking frameworks create test doubles that implement the interface. After execution, Verify asserts that specific methods were called with specific arguments. This is the standard approach to testing method interactions without side effects.

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Q. A developer registers a service in Program.cs and needs to resolve it in a test without starting a full web host. Which approach is correct?

Answer: B WebApplicationFactory<T> spins up the application in memory using the same Program.cs configuration. Tests can make HTTP requests via CreateClient() or resolve services from the container, enabling integration tests without a real network connection.

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Q. A developer writes this test and it always passes even when the code is broken. What is wrong?

[Fact]
public async Task GetOrder_ReturnsOrder()
{
    var service = new OrderService();
    var order = service.GetOrderAsync(1); // missing await
    Assert.NotNull(order);
}

Answer: B Without await, order is a non-null Task<Order?> — not the actual order value. The assertion checks the task object, not the result. Fix: var order = await service.GetOrderAsync(1);

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Q. A developer uses the Arrange-Act-Assert pattern in a test. A reviewer suggests extracting repeated Arrange steps. Which xUnit attribute supports shared setup that runs once before all tests in a class?

Answer: B xUnit uses IClassFixture<T> for class-scoped setup (created once, shared by all tests in the class). [SetUp] and [TestInitialize] are NUnit/MSTest attributes. xUnit's per-test setup is done in the test class constructor.

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Q. A developer wants to run the same test with multiple input values:

[Theory]
[InlineData(2, 4)]
[InlineData(3, 9)]
[InlineData(-1, 1)]
public void Square_ReturnsCorrectValue(int input, int expected)
{
    Assert.Equal(expected, input * input);
}

What xUnit feature is being used and how many test cases will be run?

Answer: B [Theory] marks a data-driven test. Each [InlineData(...)] attribute provides one set of arguments, resulting in 3 individual test cases. This is xUnit's equivalent of parameterised tests in NUnit ([TestCase]) and MSTest ([DataRow]).

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