easyvalue vs reference types
A method declares its parameter as `in int x`. What does that mean for the caller?
void Print(in int x) => Console.WriteLine(x);C# · PRACTICE
C# interview questions.
C# is the .NET language for services, desktop apps and games. Its interviews go deep on value versus reference semantics.
580questions
76easy
258medium
246hard
Structs versus classes, boxing, async and await state machines, LINQ deferred execution, IDisposable, and nullable reference types.
Where it shows up: Enterprise back ends on .NET, Unity game development, and Windows tooling.
Sample C# questions
Twenty real C# questions from the library, code snippet included. Nothing here is paraphrased for search engines. It is the same text a signed-in user sees.
easyexceptions try catch
Inside a catch block, what is the difference between 'throw;' and 'throw ex;'?
try { DoWork(); }
catch (Exception ex)
{
Log(ex);
throw; // vs. throw ex;
}easy
What is the value of result?
var nums = new[] { 1, 2, 3, 4 };
var result = nums.Where(n => n % 2 == 0).Select(n => n * 10);
Console.WriteLine(string.Join(",", result));easyvalue vs reference types
After this runs, what is the value of arr[0]?
int[] arr = { 1, 2, 3 };
int[] copy = arr;
copy[0] = 42;
Console.WriteLine(arr[0]);easynullable types
For `int? x = 5;`, what does `x.GetType()` return?
int? x = 5;
Console.WriteLine(x.GetType());easyLINQ Where and Select
A LINQ query built with Where and Select is stored in a variable but never enumerated (no foreach, ToList, etc.). What happens?
var query = source.Where(x => x > 0).Select(x => Transform(x));
// query is never iteratedeasyvar and type inference
What is the inferred type of `n` here?
var n = 3.0;easygenerics basics
What does this generic method return for the given call?
T First<T>(List<T> items) => items[0];
var nums = new List<int> { 10, 20, 30 };
var result = First(nums);easyusing statement and disposal
In which order do these run when the using block exits normally?
class Res : IDisposable {
public void Dispose() => Console.Write("D");
}
using (var r = new Res()) {
Console.Write("B");
}
Console.Write("A");mediumIEnumerable vs IQueryable
Given this code that builds on top of Entity Framework, what determines where the filtering runs (in the database vs in memory)?
IQueryable<Order> q = db.Orders;
var result = q.Where(o => o.Total > 100)
.AsEnumerable()
.Where(o => o.Notes.Contains("urgent"))
.ToList();easyforeach and iterators
What does the C# compiler require of any object used directly in a foreach loop (the thing after 'in')?
foreach (var item in collection) { /* ... */ }easyasync await basics
What does the await keyword do to the rest of the method after it?
async Task<int> LoadAsync()
{
var data = await FetchAsync();
return data.Length;
}easy
Why does this line fail to compile?
var value = null;easyasync await basics
An async method has signature 'async void Fire()' and throws an exception inside it. What is the practical problem?
async void Fire()
{
await Task.Delay(10);
throw new InvalidOperationException();
}easyforeach and iterators
Inside a foreach over a List<string>, can you reassign the loop variable to change the underlying list element?
var items = new List<string> { "a", "b" };
foreach (var item in items)
{
item = item.ToUpper();
}easygenerics basics
A method takes IEnumerable<object>. You try to pass a List<string>. What happens?
static void Print(IEnumerable<object> items) { }
List<string> names = new() { "a", "b" };
Print(names);easystring interpolation
What does this print, given the formatting in the interpolation?
double rate = 0.5;
Console.WriteLine($"{rate:P0}");easyusing statement and disposal
Two using statements are nested. In what order are the objects disposed?
using (var a = new Res("A"))
using (var b = new Res("B"))
{
// work
}easyList and Dictionary
You need the value for a key only if it exists, without throwing. Which approach is correct and safe?
var scores = new Dictionary<string, int>();
// ...easyexceptions try catch
What happens when an exception is thrown inside a try block that has a matching catch, and the catch block itself runs to completion normally?
int Run()
{
try
{
throw new InvalidOperationException();
}
catch (InvalidOperationException)
{
return 1;
}
finally
{
Console.WriteLine("cleanup");
}
}Sign in to answer
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C# topics covered
49 distinct C# topics are tagged across the library. These are the real topic labels stored on the questions, not a hand-written list.
- events and observer pattern (32)
- IEnumerable vs IQueryable (26)
- expression trees (22)
- Span and Memory (22)
- GC generations and LOH (22)
- volatile and memory barriers (20)
- the CLR and managed execution (19)
- covariance and contravariance (18)
- boxing and unboxing cost (18)
- stackalloc and the stack (16)
- extension methods (16)
- finalization queue and GC.Collect (16)
- custom equality IComparer (15)
- deferred vs immediate LINQ (15)
- reflection and attributes (15)
- ThreadPool and work stealing (14)
- yield return generators (14)
- IDisposable and finalizers (14)
- ref struct and ref returns (14)
- ValueTask vs Task (13)
- ConfigureAwait false (13)
- IL and the JIT compiler (13)
- delegates Func Action (12)
- nullable reference types (12)
- .NET memory model (12)
- string interning and pooling (12)
- ahead-of-time AOT compilation (12)
- struct memory layout (11)
- LINQ Where and Select (10)
- dynamic and ExpandoObject (10)
- source generators (10)
- async/await state machine (9)
- var and type inference (8)
- the async/await state machine (8)
- The .NET memory model (8)
- value vs reference types (7)
- async await basics (7)
- exceptions try catch (6)
- List and Dictionary (5)
- foreach and iterators (4)
- using statement and disposal (4)
- properties and auto-properties (4)
- string interpolation (3)
- generics basics (3)
- interfaces vs abstract classes (3)
- pattern matching switch (3)
- nullable types (2)
- enum basics (2)
- records vs classes (2)
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