Multiple choice practice

C# MCQ questions

Take a short practice test with instant feedback, then revise every question with its answer and the reason behind it.

Questions: 84
From lessons: 64
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Practice test

C# practice testPredict the output Question 1 of 10

What is printed?

var s = new HashSet<int> { 1, 2, 2, 3, 3, 3 };
Console.WriteLine(s.Count);

Score so far: 0/0

All questions with answers

Which keyword declares a constant known at compile time?
- Areadonly
- Bconst
- Cstatic
- Dfinal
Show answer
Answer: B. const
const is compile-time; readonly can be set at runtime in the constructor.
What is the default value of an int field?
- Anull
- B0
- C-1
- Dundefined
Show answer
Answer: B. 0
Value-type fields default to zero values.
Which type is a value type?
- Astring
- Bint[]
- CDateTime
- DList<int>
Show answer
Answer: C. DateTime
DateTime is a struct. Strings and arrays are reference types.
What does int? mean?
- AAn optional parameter
- BA nullable int
- CA pointer to int
- DA dynamic int
Show answer
Answer: B. A nullable int
Nullable<int> can hold a number or null.
What is printed?
C# Illustrative
```
1var name = "Asha";
2Console.WriteLine(
quot;Hello, {name}");
```
- AHello, Asha
- BHello, {name}
- CHello, name
- DA compile error
Show answer
Answer: A. Hello, Asha
The $ prefix makes an interpolated string.
Which operator returns the right side when the left is null?
- A?.
- B??
- C?:
- D!!
Show answer
Answer: B. ??
a ?? b gives b when a is null; ??= assigns only when null.
Which collection prevents duplicates?
- AList<T>
- BHashSet<T>
- CQueue<T>
- DStack<T>
Show answer
Answer: B. HashSet<T>
HashSet<T>.Add returns false for an existing value.
Which keyword lets a method be overridden?
- Aoverride
- Bvirtual
- Csealed
- Dstatic
Show answer
Answer: B. virtual
virtual allows overriding; override replaces it in a subclass.
Which block always runs after try/catch?
- Afinally
- Bcatch
- Celse
- Dend
Show answer
Answer: A. finally
finally runs whether or not an exception was thrown.
What does this LINQ query return?
C# Illustrative
```
1var even = new[] { 1, 2, 3, 4 }.Where(n => n % 2 == 0);
```
- A[1, 2, 3, 4]
- B[2, 4]
- C[1, 3]
- D6
Show answer
Answer: B. [2, 4]
Where keeps even numbers.
Which LINQ method executes the query immediately?
- AWhere
- BSelect
- CToList
- DOrderBy
Show answer
Answer: C. ToList
ToList, Count, First and similar methods enumerate the query.
Which delegate type returns a value?
- AAction<T>
- BFunc<T, TResult>
- CEventHandler
- DAction<T1, T2>
Show answer
Answer: B. Func<T, TResult>
Func returns TResult; Action and EventHandler return void.
What does await do?
- ABlocks the thread until the task finishes
- BAsynchronously waits, freeing the thread
- CStarts a new thread
- DCancels the task
Show answer
Answer: B. Asynchronously waits, freeing the thread
The method is suspended and the thread can do other work.
Which return type should an async method with no result usually have?
- Avoid
- BTask
- Cobject
- DThread
Show answer
Answer: B. Task
async void is only for event handlers because callers cannot await it or catch its exceptions.
What does a record provide automatically?
- ADatabase mapping
- BValue-based equality and ToString
- CThread safety
- DSerialization to XML
Show answer
Answer: B. Value-based equality and ToString
Records also support with-expressions for non-destructive copies.
What does the switch expression return here?
C# Illustrative
```
1int temp = 18;
2var feel = temp switch { < 10 => "cold", < 25 => "mild", _ => "hot" };
```
- A"cold"
- B"mild"
- C"hot"
- DIt does not compile
Show answer
Answer: B. "mild"
Relational patterns are checked in order; 18 is not < 10 but is < 25.
Which interface lets an object be used in a using statement?
- AIEnumerable
- BIDisposable
- CIComparable
- DICloneable
Show answer
Answer: B. IDisposable
using calls Dispose at the end of the scope.
Which attribute marks an xUnit test with inline data?
- A[Fact]
- B[Theory]
- C[Test]
- D[TestMethod]
Show answer
Answer: B. [Theory]
[Theory] with [InlineData(…)] runs once per data row.
What is string immutability?
- AStrings cannot be null
- BOperations return new strings instead of changing the original
- CStrings are stored on the stack
- DStrings are compile-time constants
Show answer
Answer: B. Operations return new strings instead of changing the original
That is why StringBuilder is better for many concatenations.
Which keyword prevents a class from being inherited?
- Astatic
- Bsealed
- Cabstract
- Dprivate
Show answer
Answer: B. sealed
sealed classes cannot be base classes.
When does a LINQ Where(...) query actually run?
- AWhen Where is called
- BWhen the results are enumerated
- CAt compile time
- DNever
Show answer
Answer: B. When the results are enumerated
LINQ uses deferred execution; the query runs when enumerated.
From the lesson: LINQ
What is printed?
C# Illustrative
```
1int[] n = [5, 3, 8, 1];
2Console.WriteLine(string.Join(",", n.Where(x => x > 2).OrderBy(x => x)));
```
- A5,3,8
- B3,5,8
- C8,5,3
- D1,3,5,8
Show answer
Answer: B. 3,5,8
Where keeps 5, 3 and 8; OrderBy sorts them ascending.
From the lesson: LINQ
Which is the best way to check whether any product costs more than 1000?
- Aproducts.Count(p => p.Price > 1000) > 0
- Bproducts.Any(p => p.Price > 1000)
- Cproducts.Where(p => p.Price > 1000).ToList().Count != 0
- Dproducts.First(p => p.Price > 1000) != null
Show answer
Answer: B. products.Any(p => p.Price > 1000)
Any stops at the first match; the others do more work or throw.
From the lesson: LINQ
What does C# compile into before the runtime turns it into machine code?
- AJavaScript
- BIL (Intermediate Language)
- CPython bytecode
- DHTML
Show answer
Answer: B. IL (Intermediate Language)
The C# compiler produces IL; the .NET runtime JIT-compiles IL to machine code.
From the lesson: Introduction to C#
Modern .NET programs can run on Linux and macOS.
- ATrue
- BFalse
Show answer
Answer: A. True
Since .NET Core, .NET is cross-platform.
From the lesson: Introduction to C#