Question 1

What is LINQ and what are its advantages?

Accepted Answer

In short: LINQ (Language Integrated Query, C# 3.0/.NET 3.5) gives a uniform, strongly typed query syntax for in-memory collections, databases (EF Core), XML and more. Advantages: compile-time checking and IntelliSense, less boilerplate than loops, readable declarative code, composable queries, deferred execution, and one skill across data sources.

Question 2

What is deferred execution? Which operators execute immediately?

Accepted Answer

In short: A deferred query isn t executed when defined. It runs each time it is enumerated, so it sees the current data. Where , Select , OrderBy , GroupBy , Skip / Take and Join are deferred. ToList , ToArray , ToDictionary , Count , Sum , Max , First , Single , Any , All and Aggregate execute immediately. Pitfalls: enumerating the same query twice runs it twice; captured variables are read at execution time, not definition time.

Question 3

What is the difference between First, FirstOrDefault, Single and SingleOrDefault?

Accepted Answer

In short: First returns the first match and throws if there are none. FirstOrDefault returns default/null if there are none. Single expects exactly one match and throws for none or several. SingleOrDefault returns default for none and throws for several. Use Single when duplicates would indicate a data bug (unique keys). Use First with an OrderBy for top 1 . In EF, Single fetches 2 rows to check uniqueness.

Question 4

What is the difference between Select and SelectMany?

Accepted Answer

In short: Select projects each element to exactly one result; if the result is a collection you get nested collections. SelectMany projects each element to a sequence and flattens them into one sequence. Example: orders.Select(o = o.Lines) gives IEnumerable List Line , while orders.SelectMany(o = o.Lines) gives IEnumerable Line .

Question 5

What is the difference between IEnumerable and IQueryable?

Accepted Answer

In short: IEnumerable T executes in memory with compiled delegates (LINQ to Objects). IQueryable T builds an expression tree that a provider (EF Core) translates into SQL and executes in the database. Filtering an IQueryable reduces the rows transferred; casting a DbSet to IEnumerable before Where loads the whole table into memory. Keep queries as IQueryable until the final materialisation.

Question 6

Query syntax vs method syntax — which is better?

Accepted Answer

In short: They compile to the same method calls. Query syntax reads like SQL and is convenient for joins, let and grouping. Method syntax supports every operator ( Count , Distinct , Take , Any ...) and is more common in codebases. They can be mixed: (from p in products where p.Stock 0 select p).Count() .

Question 7

How do you perform a left outer join in LINQ?

Accepted Answer

In short: Before .NET 10: GroupJoin followed by SelectMany with DefaultIfEmpty() . In query syntax: `join o in orders on c.Id equals o.CustomerId into g from o in g.DefaultIfEmpty() select new { c.Name, Total = o == null ? 0 : o.Total } . .NET 10 adds a LeftJoin operator, and EF Core 10 translates it to LEFT JOIN`. In EF Core you often don t need explicit joins at all: navigation properties generate them.

Question 8

Write a LINQ query to find the second highest salary.

Accepted Answer

csharp Copy var second = employees.Select(e = e.Salary).Distinct() .OrderByDescending(s = s).Skip( 1 ).FirstOrDefault(); // Nth highest: .Skip(n - 1) // with ties (employees having that salary): var who = employees.Where(e = e.Salary == second);

Question 9

How do you group data and get counts or sums per group?

Accepted Answer

In short: GroupBy(key) followed by Select(g = new { g.Key, Count = g.Count(), Total = g.Sum(x = x.Amount) }) . Group by multiple keys with an anonymous type: GroupBy(o = new { o.CustomerId, o.Date.Year }) . Filter groups with Where after grouping (like SQL HAVING ). .NET 9 adds CountBy and AggregateBy for the common cases.

Question 10

What is the difference between Any() and Count() > 0?

Accepted Answer

In short: Any() stops at the first element (O(1) for most sequences); in EF it generates EXISTS , which is efficient. Count() enumerates everything (for non-collections) or runs COUNT(*) over all matching rows. Use Any() to check existence. (For ICollection T , Count() uses the Count property, but Any() is still clearer.)

Question 11

What is the difference between ToList(), ToArray(), ToDictionary() and ToLookup()?

Accepted Answer

In short: All materialise immediately. ToList gives a growable List T , ToArray a fixed array (slightly more compact). ToDictionary(key) maps unique keys to values and throws on duplicate keys . ToLookup(key) groups into a key → many-values structure, allows duplicates, and returns an empty sequence for missing keys.

Question 12

What is the difference between Union and Concat? Distinct with custom objects?

Accepted Answer

In short: Concat appends one sequence to another, keeping duplicates. Union returns distinct elements from both. For classes, Distinct / Union use default equality (reference equality unless Equals / GetHashCode are overridden). Use records, implement IEquatable T , pass an IEqualityComparer T , or use DistinctBy / UnionBy (.NET 6).

Question 13

What does let do in query syntax?

Accepted Answer

In short: It introduces a range variable that stores an intermediate calculation, so you don t repeat it: from p in products let discounted = p.Price * 0.9m where discounted 1000 select new { p.Name, discounted } . It compiles to a Select into an anonymous type.

Question 14

What are expression trees and why does EF Core need them?

Accepted Answer

In short: An expression tree ( Expression Func T,bool ) represents code as an inspectable data structure instead of compiled IL. EF Core walks the tree to understand the lambda ( o = o.Total 1000 ) and translate it into SQL. That s why only translatable constructs work in IQueryable queries. You can build expression trees dynamically for runtime filters, or compile them into delegates.

Question 15

What is the N+1 problem in LINQ/EF and how do you fix it?

Accepted Answer

In short: One query loads N parent rows, then accessing a navigation property per row triggers N extra queries (lazy loading, or querying inside a loop). Fix: eager load with Include / ThenInclude , project with Select into DTOs (one SQL with joins/subqueries), use AsSplitQuery() for multiple collection includes, or batch-load children with Where(x = ids.Contains(x.ParentId)) . Detect it with EF logging, MiniProfiler or Application Insights dependency counts.

Question 16

Why might a LINQ query throw "could not be translated"?

Accepted Answer

In short: The expression contains something the provider can t convert to SQL: a custom C# method, an unsupported .NET method, complex client-side logic, or certain GroupBy shapes. EF Core 3.0+ throws instead of silently evaluating on the client (except in the final projection). Fix: rewrite with translatable operations, move the logic after AsEnumerable() / ToListAsync() (only after filtering), map a DB function ( HasDbFunction ) or use raw SQL.

Question 17

When would you use PLINQ, and when not?

Accepted Answer

In short: For CPU-bound operations on large in-memory collections where each element s work is independent (image processing, heavy calculations): AsParallel() spreads the work across cores. Not for I/O-bound work (use async with Task.WhenAll / Parallel.ForEachAsync ), small collections (overhead dominates), order-sensitive results (unless AsOrdered , which costs), or code with shared mutable state (race conditions).

Question 18

A report screen calls db.Orders.ToList().Where(o => o.CustomerId == id) and is slow with 2 million orders. What's wrong?

Accepted Answer

In short: ToList() executes immediately and loads all 2 million rows into memory, then filters in C#. Move the filter before materialisation: await db.Orders.Where(o = o.CustomerId == id).Select(o = new OrderDto(...)).ToListAsync() . The WHERE then runs in SQL with an index on CustomerId . Add paging and AsNoTracking , and project only the needed columns.

Question 19

Given a list of employees, write LINQ to return each department's highest-paid employee and the department's average salary.

Accepted Answer

csharp Copy var result = employees .GroupBy(e = e.Department) .Select(g = new { Department = g.Key, TopEarner = g.MaxBy(e = e.Salary)!.Name, AverageSalary = g.Average(e = e.Salary), Headcount = g.Count() }) .OrderByDescending(x = x.AverageSalary); For ties at the top, use g.Where(e = e.Salary == g.Max(x = x.Salary)) .

Question 20

Find duplicate emails in a customer list and keep only the first record of each.

Accepted Answer

csharp Copy var duplicates = customers.GroupBy(c = c.Email.ToLowerInvariant()) .Where(g = g.Count() 1 ) .Select(g = new { Email = g.Key, Count = g.Count() }); var deduped = customers.DistinctBy(c = c.Email.ToLowerInvariant()).ToList(); // keeps first occurrence // pre-.NET 6: customers.GroupBy(c = c.Email).Select(g = g.First())

Question 21

A method returns IEnumerable<Order> from a DB query, and two foreach loops over it hit the database twice. Why, and how do you fix it?

Accepted Answer

In short: The method returns a deferred query, so each enumeration executes it again (two SQL round trips, possibly different results). Materialise once with ToList() / ToListAsync() , and return IReadOnlyList Order to make it explicit. Analyzers (CA1851, ReSharper s possible multiple enumeration ) warn about this.

LINQ — Language Integrated Query