C# Quiz

1. What does the C# compiler turn your source code into?

• Native machine code for one processor only
• IL (intermediate language), which the .NET runtime compiles to machine code as it runs
• JavaScript
• Python bytecode

The JIT compiler in the runtime turns IL into native code; Native AOT can also do it ahead of time (lesson 01).

2. How do you run a single file called app.cs with .NET 10?

• csc app.cs
• dotnet build app.cs --exe
• dotnet run app.cs
• You must create a project first

Single-file apps need no project file (lessons 03–04).

3. What does Console.WriteLine(7 / 2); print?

• 3.5
• 3
• 4
• 3.0

Both operands are integers, so the fraction is discarded (lesson 08).

4. Which type should you use for money?

• double
• float
• decimal
• int

decimal stores base-10 values exactly, so 0.1 + 0.2 is exactly 0.3 (lesson 08).

5. What does Math.Round(2.5) return by default?

• 2
• 3
• 2.5
• It throws an exception

The default is banker's rounding: halves round to the nearest even number. Pass MidpointRounding.AwayFromZero for 3 (lesson 08).

6. Why is int.TryParse usually better than int.Parse for user input?

• It's the only one that handles negative numbers
• It returns false for invalid text instead of throwing an exception
• It rounds decimals automatically
• It works without a using directive

(lesson 09)

7. What is the value of name ?? "guest" when name is null?

• null
• An empty string
• "guest"
• It throws a NullReferenceException

?? returns the right-hand side when the left is null (lesson 10).

8. What happens if a case in a C# switch statement has no break before the next case?

• Execution falls through to the next case, like in C
• It's a compile error
• The switch exits silently
• It loops back to the first case

C# forbids implicit fall-through (lesson 13).

9. Which loop always runs its body at least once?

• while
• for
• do … while
• foreach

The condition is checked after each pass (lesson 14).

10. What does name.ToUpper(); on its own line do to name?

• Nothing — strings are immutable and the new string is thrown away
• Changes name to upper case
• Throws an exception
• Changes only the first letter

You need name = name.ToUpper(); (lesson 16).

11. In $"{price,8:F2}", what does 8 mean?

• 8 decimal places
• A minimum width of 8 characters, right-aligned
• Multiply by 8
• Show at most 8 digits

The full form is {value,alignment:format} (lesson 18).

12. What's the difference between ref and out parameters?

• There is none
• out can only be used with numbers
• A ref argument must be initialised by the caller; an out parameter must be assigned by the method
• ref parameters are read-only

(lesson 22)

13. What is values[^1]?

• The first element
• The last element
• An error: indexes can't be negative
• The element before the first

^n counts from the end (lesson 30).

14. Which collection should you choose to look values up by a key quickly?

• List<T>
• Queue<T>
• Dictionary<TKey, TValue>
• Stack<T>

(lesson 28)

15. What happens if you remove items from a List inside a foreach over that same list?

• The removed items are skipped safely
• An InvalidOperationException is thrown
• The loop restarts from the beginning
• It's a compile error

Use RemoveAll, a backwards for loop, or build a new list (lesson 27).

16. After var b = a; where a is an object of a class, what happens when you change b.Name?

• Only b changes
• a.Name changes too, because both variables refer to the same object
• It's a compile error
• a becomes null

Classes are reference types (lessons 07 and 31). A struct would be copied instead (lesson 40).

17. What does { get; private set; } on a property mean?

• The property can't be read
• Anyone can read it, but only code inside the class can change it
• It can only be set in the constructor
• It's the same as a public field

(lesson 33)

18. What does the C# 14 field keyword refer to inside a property accessor?

• A field with the same name as the class
• The compiler-generated backing field of that property
• The first field declared in the class
• The property's default value

It lets you add logic like validation without declaring a separate field (lesson 33).

19. Which statement about static members is true?

• Each object gets its own copy of a static field
• A static method can't use instance fields, because there's no current object
• Static classes can be instantiated with new
• Static members can only be private

(lesson 34)

20. For a method to be overridden in a derived class, the base method must be marked…

• new
• sealed
• virtual (or abstract)
• static

Methods aren't virtual by default in C# (lesson 37).

21. What's a key difference between an abstract class and an interface?

• A class can implement many interfaces but inherit from only one class
• Interfaces can have constructors; abstract classes can't
• Abstract classes can't contain any implemented methods
• Interfaces can only be used with structs

(lessons 38–39)

22. Two records new Point(1, 2) and new Point(1, 2) are compared with ==. The result is…

• true, because records use value equality
• false, because they're different objects
• A compile error
• It depends on the platform

(lesson 40)

23. Why does [Flags] matter on an enum like Permissions.Read | Permissions.Write?

• It's required to use the | operator
• It makes ToString print the combined names, such as "Read, Write"
• It stops invalid values being stored
• It makes the enum a reference type

Each member should also have its own bit value: 1, 2, 4, 8… (lesson 41).

24. With nullable reference types enabled, what does string? mean?

• A string that can't be empty
• A string variable that may be null; the compiler warns if you use it without checking
• A string that's created lazily
• A nullable value type

(lesson 43)

25. Which pattern matches an array whose first element is 1, whatever its length (at least one element)?

• { 1, * }
• [1, ..]
• (1, _)
• [1, _]

.. matches any number of elements; [1, _] requires exactly two (lesson 44).

26. What does where T : IComparable<T> do in a generic method?

• Makes T default to IComparable
• Restricts T to types that implement IComparable<T>, so the method can call CompareTo
• Converts every argument to IComparable
• Has no effect at compile time

A constraint (lesson 45).

27. When does a LINQ query like numbers.Where(n => n > 5) actually run?

• Immediately, when Where is called
• When the result is enumerated, for example by foreach or ToList() — and again each time
• Only once, the first time it's used
• In the background on another thread

Deferred execution (lesson 49).

28. What's the right way to rethrow a caught exception while keeping its original stack trace?

• throw ex;
• throw;
• return ex;
• catch (ex) { }

throw ex; resets the stack trace (lesson 52).

29. What does a using declaration such as using var file = new StreamWriter("a.txt"); guarantee?

• The file is created in a temporary folder
• Dispose is called when the variable goes out of scope, even if an exception is thrown
• The file is read-only
• The object is never garbage collected

(lesson 53)

30. Two independent async downloads each take 1 second. How do you run them so the total is about 1 second?

• Await the first, then start and await the second
• Start both, then await Task.WhenAll(...)
• Call .Result on both
• Wrap them in a lock

Starting a task returns immediately; awaiting them together lets them overlap (lesson 57).