C# Coding Interview Checklist

Published 2026-10-16 · Luna Interview

C# can make interview solutions clear and fast—or bury you in LINQ chains and subtle collection costs that eat the clock. If you're using C# in a coding round, the goal is code that's correct, efficient, and easy to explain. This C# coding interview checklist covers collections, idioms, and traps worth reviewing before the day.

First, confirm with your recruiter that C# is allowed, which runtime/.NET version, and whether you can compile and run code.

1. Choose the right collection on purpose

Need	Prefer	Interview note
Dynamic array	`List<T>`	Default workhorse
Hash map	`Dictionary<TKey, TValue>`	`TryGetValue` over indexer for missing keys
Set	`HashSet<T>`	Dedup and membership
Queue (BFS)	`Queue<T>`	FIFO
Stack	`Stack<T>`	LIFO / DFS iterative
Sorted map	`SortedDictionary` / `SortedList`	Rare; know the cost
Priority queue	`PriorityQueue<TElement, TPriority>` (.NET 6+)	Confirm version availability
Linked list	`LinkedList<T>`	Seldom needed; prefer arrays/`List`

Say your reasoning aloud: "I'll use a Dictionary for counts and a Queue for BFS so the algorithm stays obvious."

2. Dictionary gotchas

// Risky if key might be missing
counts[key]++;

// Safer
if (!counts.TryGetValue(key, out var c)) c = 0;
counts[key] = c + 1;

// Or
counts[key] = counts.GetValueOrDefault(key) + 1;

Mention it: "I'm using TryGetValue so missing keys don't throw."

Also watch:

Reference types as keys need correct Equals/GetHashCode if custom
Mutating a key after insert breaks the dictionary (rare in interviews, but know it)
Iterating and modifying the same dictionary throws—collect keys first if needed

3. LINQ: clarity vs cost

LINQ reads well but can hide O(n) passes and allocations.

Pattern	Prefer in interviews	Why
Filter + transform once	One `foreach` or a short LINQ chain	Easier to narrate complexity
Nested `.Where` / `.Select`	Merge into one pass when hot	Avoid multiple enumerations
`.ToList()` mid-chain	Only when you need materialization	Accidental double work
`.OrderBy` then take k	Heap for large n, small k	Sorting whole array is overkill

A good line:

"I'll use a simple loop here so the O(n) pass is obvious; I can switch to LINQ for the final return if you prefer."

4. Priority queues and ordering

If PriorityQueue<TElement, TPriority> is available, practice min-heap usage for Dijkstra and top-k. If not, say so and implement a binary heap on a List<(T, int)> or use SortedSet carefully (unique keys only unless you add a tie-breaker).

Always narrate: "Lower priority value means higher urgency in this min-heap."

5. Strings and spans (keep it practical)

Prefer StringBuilder for repeated concatenation in a loop
string is immutable—slicing creates new strings; that's usually fine at interview scale
For character counts, int[26] or Dictionary<char, int> depending on alphabet

Don't digress into Span<T> unless you're fluent and the interviewer invites low-level talk.

6. Nullability and edge cases

With nullable reference types enabled, be deliberate:

Check null inputs if the problem allows them
Empty arrays and empty strings
Integer overflow if sums can grow (use long when unsure)

Dry-run empty and single-element cases before you claim done.

7. Narration checklist while coding

Restate input/output types
Name the primary collection and why
State brute force complexity, then the plan
Mention any LINQ vs loop choice
Dry-run one path aloud
Close with time and space complexity

Example closing:

"Time is O(n log k) because each of n items is pushed/popped from a size-k heap. Extra space is O(k) for the heap plus O(n) if I build a result list."

8. Quick snippet habits

Prefer foreach when you don't need an index; for when you do
Use var when the type is obvious from the right-hand side
Extract a tiny helper for repeated logic instead of copying loops
Name intermediate collections (var freq = ...) so your narration has nouns

Interviewers follow named steps more easily than a single dense expression.

Soft practice note

Language fluency shows up under time pressure. Timed, out-loud rehearsal in C# beats silent reading. If you want a structured way to practice that loop and stay composed under pressure, Luna Interview is built for rehearsal and prep—before the interview, not as a way around the rules during one.

Day-before checklist

[ ] C# allowed and version confirmed
[ ] Dictionary, HashSet, Queue, Stack, List muscle memory
[ ] TryGetValue habit locked in
[ ] One heap problem solved in C# this week
[ ] One LINQ-heavy rewrite converted to a clear loop
[ ] Complexity sentence ready
[ ] Empty/null edge cases on your mental list

Idiomatic C# in an interview is not maximum cleverness. It's readable collections, honest complexity, and code you can defend while thinking out loud.

Rehearse with an AI interview copilot, then browse commonly reported interview questions.