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C# & .NET (C# 14, .NET 10 LTS) Module 14

LINQ interview questions

Interview questions on LINQ: short warm-ups, then what each harder question tests, the first step of a good answer and its follow-ups.

19 questions: 5 warm-ups and 14 with model answers · Levels: Fresher to senior

Warm-up questions

Short questions with short answers, each answered in full.

  1. Fresher

    What is the difference between First, FirstOrDefault and Single in LINQ?

    Answered in the lesson LINQ: your first queries

  2. Fresher

    What is the difference between Select and SelectMany?

    Answered in the lesson LINQ projection, filtering, sorting and paging

  3. Fresher

    What is the difference between GroupBy and ToLookup?

    Answered in the lesson LINQ grouping and aggregation

  4. Fresher

    What is the difference between Join and GroupJoin?

    Answered in the lesson LINQ joins: Join, GroupJoin, LeftJoin and RightJoin

  5. Fresher

    What is the difference between IEnumerable and IQueryable in C#?

    Answered in the lesson IQueryable and expression trees

Questions with model answers

What each question tests, the first step of a good answer and the follow-ups to expect.

  1. Fresher to mid

    A checkout looks up the customer with customers.Single(c => c.Email == email). Since a bulk import, it throws "Sequence contains more than one matching element" for a few users. What does the error tell you, and what do you change, in the code and in the data?

    What it tests
    What Single promises, reading a LINQ exception as evidence about the data, and fixing the rule that let duplicates in rather than hiding them with First.
    First step
    Read the message as a finding about the data, not a bug in the query: two customers share one email. Before you touch the code, count the emails that now occur more than once, and ask whether an email was meant to be unique.
    Follow-ups

    From the lesson LINQ: your first queries

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    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  2. Mid to senior

    A service builds var overdue = invoices.Where(i => i.Due < today) once at start-up, where today is a local, and reads overdue on every request. New invoices show up in it, yet after midnight the invoices due the day before are still missing. Explain both behaviours and fix the code.

    What it tests
    Deferred execution, what a lambda captures, and deciding what a long-lived service computes per request and what it may keep.
    First step
    Separate the two things the query holds. The source, invoices, is read again on every enumeration, which is why new invoices appear. today is a variable captured when the service started and never assigned again, so the cut-off date is frozen at start-up.
    Follow-ups

    From the lesson LINQ: your first queries

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  3. Fresher to mid

    A sales report sorts orders with .OrderBy(o => o.Region).OrderBy(o => o.Date). Users say the regions are mixed together, although within one date the regions look sorted. Explain exactly what the query does, fix it, and say how a test would have caught it.

    What it tests
    How OrderBy and ThenBy combine, why the result looks almost right, and test data for an order with ties.
    First step
    Read the second OrderBy as a new sort, not as a second key: it sorts every order by date. Then work out what decides the order of two orders that share a date, and compare that with what the users see.
    Follow-ups

    From the lesson LINQ projection, filtering, sorting and paging

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  4. Mid to senior

    An export merges two CSV files into a List<Customer> and calls Distinct() on it, but the duplicate customers stay. Customer is a class. Explain why, and compare three ways to remove the duplicates.

    What it tests
    How Distinct decides that two items are the same, reference against value equality, and choosing between a record, DistinctBy and a comparer.
    First step
    Ask what Distinct compares. It calls Equals and GetHashCode, and for a class that does not override them they compare references: every object built from a CSV row is a different object, even when all its values match another row's.
    Follow-ups

    From the lesson LINQ projection, filtering, sorting and paging

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  5. Mid to senior

    A feed loads 20 posts a page with posts.OrderByDescending(p => p.CreatedAt).Skip(page * 20).Take(20). In busy hours users see the same post twice while scrolling, and now and then a post never appears. Explain both and redesign the paging.

    What it tests
    Offset against keyset paging, what moves an offset page while a user scrolls, and a sort key that is unique and stable.
    First step
    Write down what Skip(page * 20) counts: positions, not posts. Then replay two page requests on paper, once with a post published between them and once with a post deleted, and note which post each second page starts with.
    Follow-ups

    From the lesson LINQ projection, filtering, sorting and paging

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  6. Fresher to mid

    A dashboard shows each store's average sale with stores.Select(s => new { s.Name, Avg = s.Sales.Average(x => x.Amount) }). The morning a new store opens, the whole page fails with InvalidOperationException. Explain the cause, fix it, and say what the page should show for that store.

    What it tests
    What the aggregates do with an empty sequence, the nullable overloads and DefaultIfEmpty, and choosing an honest value to show.
    First step
    Find the store whose Sales is empty. Average has no value to return for it, so it throws, and because the page reads the query as a whole, one empty store breaks every row.
    Follow-ups

    From the lesson LINQ grouping and aggregation

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  7. Mid to senior

    A support report groups the month's tickets with tickets.GroupBy(t => t.Status) and prints a count per status. In quiet months the Escalated row disappears instead of showing 0, and the chart built on it shifts its colours. Explain why and fix it.

    What it tests
    That grouping finds only the keys present in the data, deciding which rows a report must show, and a count of 0 for a key with no items.
    First step
    Notice where the rows come from: GroupBy makes a group only for a status that occurs in the tickets, so a month without escalations has no Escalated group.
    Follow-ups

    From the lesson LINQ grouping and aggregation

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  8. Mid to senior

    A nightly job reads 50 million log lines lazily, groups them with GroupBy(l => l.StatusCode) and counts each group. It fails with OutOfMemoryException. What does GroupBy keep, roughly how much memory is that, and what do you use instead?

    What it tests
    What GroupBy holds in memory, estimating it from measured numbers, and CountBy, AggregateBy or a dictionary as the fix.
    First step
    List what GroupBy must keep: every line, in its group, until the last line has been read, because it hands on no group before then. Counting needs one number per status code, not the lines.
    Follow-ups

    From the lesson LINQ grouping and aggregation

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  9. Fresher to mid

    A revenue report joins customers to orders with Join and counts the customers in the result. The count is 1,200 lower than the CRM's, and finance asks which customers are missing. Explain the gap and write the query that lists them.

    What it tests
    Inner join semantics, keeping unmatched rows with LeftJoin, and counting distinct customers rather than joined rows.
    First step
    An inner join keeps only customers with at least one matching order, so customers without orders in the period vanish from it. Then notice that the joined rows are orders, not customers, so counting them answers another question.
    Follow-ups

    From the lesson LINQ joins: Join, GroupJoin, LeftJoin and RightJoin

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  10. Mid to senior

    A nightly job matches 200,000 payments to 200,000 invoices with payments.Select(p => (p, invoices.First(i => i.Number == p.InvoiceNumber))). It now takes hours and sometimes throws. Explain both, estimate the work, and rewrite it.

    What it tests
    The cost of a search inside a loop, estimating comparisons, Join or a dictionary as the fix, and what First does with a missing match.
    First step
    Count the comparisons. First scans the invoices from the start for every payment: about half the list on average when the invoice exists, all of it when it does not. That is billions of comparisons, and a payment whose invoice is missing makes First throw.
    Follow-ups

    From the lesson LINQ joins: Join, GroupJoin, LeftJoin and RightJoin

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  11. Mid to senior

    Two exports, train runs and platform allotments, are joined on new { Station, Day }. Half the runs find no platform, although every pair looks present when you read the two files. What do you check, in what order, and how do you fix the join?

    What it tests
    Composite key equality, case and spaces in keys, normalising keys, and using a left join to find the rows that fail.
    First step
    Find the failing rows first: LeftJoin on the same key and keep the runs whose platform is null. Then compare their keys with the platforms' keys character by character: case, spaces at the ends, and spellings such as "Mon" against "Monday".
    Follow-ups

    From the lesson LINQ joins: Join, GroupJoin, LeftJoin and RightJoin

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  12. Fresher to mid

    An endpoint returns db.Customers.AsEnumerable().Where(c => c.City == city).Take(20).ToList() through EF Core. It was fast in testing; in production each request takes seconds, and the database sends every customer. Explain what runs where, and fix it without changing the result.

    What it tests
    IQueryable against IEnumerable operators, what AsEnumerable does to the rest of a query, and reading the SQL that a provider sends.
    First step
    Find where the query leaves the tree. Everything before AsEnumerable() is translated, and that is only the table, so the database sends every customer; Where and Take then run in the web server, on rows it had to receive and build first.
    Follow-ups

    From the lesson IQueryable and expression trees

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  13. Mid to senior

    An admin search page builds an Expression<Func<Order, bool>> from query-string fields such as field=Total, op=gt and value=500, passing the field name straight to Expression.Property. Review the design: what can go wrong, how do you build the tree safely, and how do you test it?

    What it tests
    Treating names from a request as input, allowlists, constants of the field's own type, refusing bad criteria when the filter is built, and testing a builder rule by rule.
    First step
    List what the request controls: a property name, an operator and a value. Each can be wrong or hostile, so each needs a rule: an allowlist of fields, a fixed set of operators, and a value parsed into the field's own type.
    Follow-ups

    From the lesson IQueryable and expression trees

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

  14. Mid to senior

    A repository's unit tests pass against a list wrapped with AsQueryable(), but in production the same query throws when it is read, because the database provider cannot translate one predicate. Why do the tests miss it, and how do you test queries so they would not?

    What it tests
    That AsQueryable() over a list runs any tree in memory while a real provider translates only what it knows, and testing queries against the real provider.
    First step
    Compare what runs the tree in each place. Over a list, the predicate runs as compiled C#, so a call to any method works. The database provider must turn every node into SQL, and it refuses a method it does not know, when the query is read.
    Follow-ups

    From the lesson IQueryable and expression trees

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 2 minutes

All C# & .NET (C# 14, .NET 10 LTS) interview questions · The C# & .NET (C# 14, .NET 10 LTS) track