Interview Q&A

Short answer: var listA = employees.Take(3); var listB = employees.Skip(2); var common = listA.Select(e => e.Id) .Intersect(listB.Select(e => e.Id));

Example code

var listA = employees.Take(3);
var listB = employees.Skip(2);
var common = listA.Select(e => e.Id)
.Intersect(listB.Select(e => e.Id));

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id)…

Explain a bit more

Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id));

Example code

var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id)); var onlyInA = listA.Select(e => e.Id) .Except(listB.Select(e => e.Id));

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Skip top 2 highest salaries and take next 2 var result = employees .OrderByDescending(e => e.Salary) .Skip(2) .Take(2); Skip top 2 highest salaries and take next 2 var result = employees .OrderByDescending(e => e.Salary) .Skip(2) .Take(2); Skip top 2 highest salaries and take next 2 var result = employees .OrderByDescending(e => e.Salary) .Skip(2) .Take(2);… Skip top 2 highest salaries and take next 2 var result =…

Explain a bit more

employees .OrderByDescending(e => e.Salary) .Skip(2) .Take(2);

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary >…

Example code

bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000); bool allHigh = employees.All(e => e.Salary > 50000);

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department ==…

Explain a bit more

"Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal");

Example code

bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal"); bool anyLegal = employees.Any(e => e.Department == "Legal");

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: var hrList = employees This forces query execution (immediate execution). var hrList = employees This forces query execution (immediate execution).

Real-world example (ShopNest)

ShopNest filters active products with products.Where(p => p.IsActive && p.Stock > 0) before showing the catalog.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: This forces query execution (immediate execution).

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: var taxResult = from e in employees select new { e.Name, Tax = tax }; let stores computed value for reuse.

Example code

var taxResult = from e in employees
select new { e.Name, Tax = tax }; let stores computed value for reuse.

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse.

Example code

let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse. let stores computed value for reuse.

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: foreach (var emp in query) Console.WriteLine(emp.Name); ✅ Output includes “Frank” — because query is executed at enumeration time. var query = employees.Where(e => e.Salary > 70000); employees.Add(new Employee { Id = 6, Name = "Frank", Department =

Example code

"IT", Salary = 90000 });

Real-world example (ShopNest)

ShopNest builds a LINQ query for search filters, then calls ToListAsync() once in the repository—so the SQL runs one time, not inside a loop.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Why is LINQ deferred? var query = employees.Where(e => e.Salary > 70000); employees.Add(new Employee { Id = 6, Name = "Frank", Department = "IT", Salary = 90000 }); foreach (var emp in query) Console.WriteLine(emp.Name); ✅ Output includes “Frank” — because query is executed at enumeration time. Why is LINQ deferred? var query =… employees.Where(e =>…… e.Salary > 70000); employees.Add(new Employee { Id = 6, Name =…

Explain a bit more

"Frank", Department = "IT", Salary = 90000 }); foreach (var emp in query) Console.WriteLine(emp.Name); ✅ Output includes “Frank” — because query is executed at enumeration time.

Real-world example (ShopNest)

ShopNest builds a LINQ query for search filters, then calls ToListAsync() once in the repository—so the SQL runs one time, not inside a loop.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Get top 3 highest-paid employees var top3 = employees .OrderByDescending(e => e.Salary) .Take(3); Get top 3 highest-paid employees var top3 = employees .OrderByDescending(e => e.Salary) .Take(3); Get top 3 highest-paid employees var top3 = employees .OrderByDescending(e => e.Salary) .Take(3); Get top 3 highest-paid employees var top3 = employees… OrderByDescending(e => e.Salary) .Take(3);

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Get bottom 2 lowest-paid employees var bottom2 = employees .OrderBy(e => e.Salary) .Take(2); Get bottom 2 lowest-paid employees var bottom2 = employees .OrderBy(e => e.Salary) .Take(2); Get bottom 2 lowest-paid employees var bottom2 = employees .OrderBy(e => e.Salary) .Take(2); Get bottom 2 lowest-paid employees var bottom2 = employees .OrderBy(e =>…

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e =>…

Example code

var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name); var namesStartingA = employees .Select(e => e.Name);

Real-world example (ShopNest)

ShopNest filters active products with products.Where(p => p.IsActive && p.Stock > 0) before showing the catalog.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Get names of employees whose names start with 'A' var namesStartingA = employees .Where(e => e.Name.StartsWith("A")) .Select(e => e.Name); Follow : Get names of employees whose names start with 'A' var namesStartingA = employees .Where(e => e.Name.StartsWith("A")) .Select(e => e.Name); Follow : Get names of employees whose names start with 'A' var… namesStartingA = employees .Where(e => e.Name.StartsWith("A"))…

Explain a bit more

Select(e => e.Name); Follow : Get names of employees whose names start with 'A' var namesStartingA = employees .Where(e => e.Name.StartsWith("A")) .Select(e => e.Name); Follow :

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Get total salary by department var totalByDept = employees .GroupBy(e => e.Department) .Select(g => new { Department = g.Key, Total = g.Sum(e => e.Salary) }); Get total salary by department var totalByDept = employees .GroupBy(e => e.Department) .Select(g => new { Department = g.Key, Total = g.Sum(e => e.Salary) }); Real-world… example (ShopNest) ShopNest…… uses LINQ to query orders: filter by date, project to a…

Explain a bit more

summary DTO, and order by total—readable C# that becomes SQL under EF Core. Get total salary by department var totalByDept = employees .GroupBy(e => e.Department) .Select(g => new { Department = g.Key, Total = g.Sum(e => e.Salary) }); Get total salary by department var totalByDept = employees .GroupBy(e => e.Department) .Select(g => new { Department = g.Key, Total = g.Sum(e => e.Salary) }); Real-world… example (ShopNest) ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n);

Example code

var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n);

Real-world example (ShopNest)

ShopNest maps entities to DTOs with .Select(o => new OrderSummaryDto { Id = o.Id, Total = o.Total }) so the API does not leak internal fields.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Flatten list of lists using SelectMany List<List<int>> numbers = new() { new() { 1, 2 }, new() { 3, 4 }, new() { 5 } var flat = numbers.SelectMany(n => n); Flatten list of lists using SelectMany List<List<int>> numbers = new() { new() { 1, 2 }, new() { 3, 4 }, new() { 5 } var flat = numbers.SelectMany(n => n); Real-world example… (ShopNest) ShopNest maps…… entities to DTOs with .Select(o => new OrderSummaryDto { Id…

Explain a bit more

= o.Id, Total = o.Total }) so the API does not leak internal fields. Flatten list of lists using SelectMany List<List<int>> numbers = new() { new() { 1, 2 }, new() { 3, 4 }, new() { 5 } var flat = numbers.SelectMany(n => n); Flatten list of lists using SelectMany List<List<int>> numbers = new() { new() { 1, 2 }, new() { 3, 4 }, new() { 5 } var flat = numbers.SelectMany(n => n); Real-world example… (ShopNest) ShopNest maps entities to DTOs with .Select(o => new OrderSummaryDto { Id = o.Id, Total = o.Total }) so the API does not leak internal fields.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Use Distinct() on names var distinctNames = employees .Select(e => e.Name) .Distinct(); Use Distinct() on names var distinctNames = employees .Select(e => e.Name) .Distinct(); Use Distinct() on names var distinctNames = employees .Select(e => e.Name) .Distinct(); Use Distinct() on names var distinctNames = employees .Select(e => e.Name) .Distinct();

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: double[] salaryArray = employees

Real-world example (ShopNest)

ShopNest maps entities to DTOs with .Select(o => new OrderSummaryDto { Id = o.Id, Total = o.Total }) so the API does not leak internal fields.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Convert list of salaries to array double[] salaryArray = employees .Select(e => e.Salary) .ToArray(); Convert list of salaries to array double[] salaryArray = employees .Select(e => e.Salary) .ToArray(); Convert list of salaries to array double[] salaryArray = employees .Select(e => e.Salary) .ToArray(); Convert list of salaries to array double[]… salaryArray = employees .Select(e => e.Salary) .ToArray();

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key);

Example code

var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key); var duplicateDepts = employees .Select(g => g.Key);

Real-world example (ShopNest)

ShopNest filters active products with products.Where(p => p.IsActive && p.Stock > 0) before showing the catalog.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Find duplicate departments var duplicateDepts = employees Follow : .GroupBy(e => e.Department) .Where(g => g.Count() > 1) .Select(g => g.Key); Find duplicate departments var duplicateDepts = employees Follow : .GroupBy(e => e.Department) .Where(g => g.Count() > 1) .Select(g => g.Key); Find duplicate departments var duplicateDepts = employees Follow :… GroupBy(e => e.Department) .Where(g => g.Count() > 1) .Select(g…

Explain a bit more

=> g.Key); Find duplicate departments var duplicateDepts = employees Follow : .GroupBy(e => e.Department) .Where(g => g.Count() > 1) .Select(g => g.Key);

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Compare LINQ to SQL vs LINQ to Objects? Answer: LINQ to Objects runs in memory on collections (List<T>, arrays). LINQ to SQL/EF builds expression trees and translates to SQL queries.

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup

Example code

var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup var allDepts = from d in departments into empGroup

Real-world example (ShopNest)

ShopNest maps entities to DTOs with .Select(o => new OrderSummaryDto { Id = o.Id, Total = o.Total }) so the API does not leak internal fields.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.