September 18, 20263 min readBy Muhammad Shahid

LINQ Interview Questions for C# and EF Core

LINQ interview questions with scenario answers for C# and EF Core: deferred execution, IQueryable SQL, client evaluation, and queries that look fine in memory and fail in production.

Part of Interview Questions

Quick answers

What LINQ interview questions show up for .NET roles?: Deferred execution, the difference between LINQ to Objects and LINQ to Entities, multiple enumeration, and a Where clause that silently runs in memory. They want the SQL consequence, not the method name.
Why does my LINQ query hit the database twice?: The query is deferred. Each foreach, Count, or ToList executes it. Materialize once with ToListAsync when you need the same rows twice.
What is wrong with calling a local method inside an EF Core Where?: EF Core cannot translate an arbitrary C# method to SQL. It either throws or, on older versions, client-evaluates and loads the whole table. Keep the predicate translatable.

This page is for LINQ interview questions that follow the "what does Where do" warm-up. The interviewer wants to know whether your query runs in SQL or in the process.

EF Core context: EF Core interview questions and SQL performance. Hub: interview questions.

Real-world analogy

LINQ is a waiter writing down an order. The pad is the query. The kitchen has not cooked anything until the waiter tears off the sheet and hands it in. That tear is ToList or First. Adding "no onions" after the kitchen has plated is a second pass on the plate, not a change to the ticket. In EF Core the kitchen is SQL Server. In a List the kitchen is the process you are already standing in.

Worked example

A reviewer sees orders.Where(o => o.IsOpen).Select(o => o.Total) and asks why the page is slow. The variable orders is already a List<Order> from an earlier ToList. The Where is C#, and the totals are summed after every open order was loaded, including the columns the page never shows. The fix is to filter and project before the materializing call, then Sum in SQL if the page only needs the number. Deferred execution is the reason this compiled and still did the wrong thing: nothing ran at the Where line.

Short answers

Question	Crisp answer
Deferred execution	The query runs when you iterate or call ToList, not when you write Where
LINQ to Objects	`IEnumerable`. Runs in memory
LINQ to Entities	`IQueryable`. Builds SQL until you materialize
Multiple enumeration	Two loops, two executions
Client evaluation	A predicate EF cannot translate, so it pulls rows into the app

The questions they ask

Why customers.Where(...) does not hit SQL yet
Why the same query runs twice
Why DateTime.Now inside a query is a bug
Why a local IsVip method blows up or scans the table
Select before Where and what that does to SQL
First versus FirstOrDefault versus Single

Scenario 1: deferred execution

Prompt: This code logs one SQL. Then the method counts and then loops. Production shows two queries. Why?

IQueryable<Order> open = db.Orders.Where(o => o.Status == OrderStatus.Open);
var count = await open.CountAsync(ct);
var page = await open.OrderBy(o => o.Id).Take(20).ToListAsync(ct);

Answer: CountAsync and ToListAsync each execute. That is correct if you need both a count and a page, and you should say so. The bug is assuming Where already loaded rows. If you only need the page, do not count. If you need both, two round-trips are honest. Do not .ToList() the whole table to count in memory.

Scenario 2: client evaluation

Prompt: Where(o => IsOverdue(o)) worked in a unit test with a List<Order> and timed out in production.

Answer: The test was LINQ to Objects. EF Core needs a translatable expression. IsOverdue is an opaque method. Write the comparison inline (o.DueOn < DateTime.UtcNow) or use a mapped computed column. Do not "fix" it by ToList() then filtering. That downloads the table.

Scenario 3: First, FirstOrDefault, Single

Prompt: SingleAsync on a lookup by email starts throwing in a migration.

Answer: Single means zero or one, and it throws if there are two. After a bad import there are two rows. First hides the data bug. Say which one you want: Single when the invariant is one row, FirstOrDefault when missing is normal. Do not use Single as a synonym for First.

Related: IEnumerable vs IQueryable.

Quick answers

What LINQ interview questions show up for .NET roles?

Deferred execution, the difference between LINQ to Objects and LINQ to Entities, multiple enumeration, and a Where clause that silently runs in memory. They want the SQL consequence, not the method name.

Why does my LINQ query hit the database twice?

The query is deferred. Each foreach, Count, or ToList executes it. Materialize once with ToListAsync when you need the same rows twice.

What is wrong with calling a local method inside an EF Core Where?

EF Core cannot translate an arbitrary C# method to SQL. It either throws or, on older versions, client-evaluates and loads the whole table. Keep the predicate translatable.

Real-world analogy

Worked example

Short answers

Question	Crisp answer
Deferred execution	The query runs when you iterate or call ToList, not when you write Where
LINQ to Objects	`IEnumerable`. Runs in memory
LINQ to Entities	`IQueryable`. Builds SQL until you materialize
Multiple enumeration	Two loops, two executions
Client evaluation	A predicate EF cannot translate, so it pulls rows into the app

Scenario 1: deferred execution

Prompt: This code logs one SQL. Then the method counts and then loops. Production shows two queries. Why?

IQueryable<Order> open = db.Orders.Where(o => o.Status == OrderStatus.Open); var count = await open.CountAsync(ct); var page = await open.OrderBy(o => o.Id).Take(20).ToListAsync(ct);

Scenario 2: client evaluation

Prompt: Where(o => IsOverdue(o)) worked in a unit test with a List<Order> and timed out in production.

Scenario 3: First, FirstOrDefault, Single

Prompt: SingleAsync on a lookup by email starts throwing in a migration.