Interview Q&A

Short answer: IQueryable<T>: Represents a query against the data source. Query is translated to SQL and executed on the database. Supports deferred execution. IEnumerable<T>: Represents an in-memory collection. LINQ operators execute in memory. Usually results after the query has executed and data is loaded.

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: Inner join

Example code

var query = from c in context.Customers join o in context.Orders on c.Id equals o.CustomerId select new { c.Name, o.OrderDate }; Left join (using DefaultIfEmpty()): var query = from c in context.Customers join o in context.Orders on c.Id equals o.CustomerId into orders from o in orders.DefaultIfEmpty() select new { c.Name, OrderDate = o != null ? o.OrderDate : (DateTime?)null };

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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 groupedData = context.Orders .GroupBy(o => o.CustomerId) .Select(g => new { CustomerId = g.Key, TotalOrders = g.Count(), TotalAmount = g.Sum(o => o.Amount), MaxAmount = g.Max(o => o.Amount), MinAmount = g.Min(o => o.Amount) });

Example code

var groupedData = context.Orders .GroupBy(o => o.CustomerId) .Select(g => new { CustomerId = g.Key, TotalOrders = g.Count(), TotalAmount = g.Sum(o => o.Amount), MaxAmount = g.Max(o => o.Amount), MinAmount = g.Min(o => o.Amount) });

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: Filtering: var filtered = context.Products.Where(p => p.Price > 100); Ordering: var ordered = context.Products .OrderBy(p => p.Category) .ThenByDescending(p => p.Price);

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: Projection transforms entities into custom shapes, e.g., DTOs. var projected = context.Products

Example code

.Select(p => new { p.Name, p.Price }); Useful for: Reducing data load. Returning only needed fields. Mapping to custom types.

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: First(): Returns the first element, throws if none found. FirstOrDefault(): Returns first element or default (null) if none. Single(): Returns the single element, throws if zero or more than one. SingleOrDefault(): Returns the single element or default if none; throws if more than one.

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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 .Skip() and .Take() for pagination: var page2 = context.Products .OrderBy(p => p.Id) .Skip(10) // Skip first 10 .Take(10) // Take next 10 .ToList(); Use .Skip() and .Take() for pagination: var page2 = context.Products .OrderBy(p => p.Id) .Skip(10) // Skip first 10 .Take(10) // Take next 10 .ToList(); Real-world example… (ShopNest) ShopNest’s order…… service uses EF Core to save an Order and its line items in one…

Explain a bit more

SaveChangesAsync() call inside a transaction. Use .Skip() and .Take() for pagination: var page2 = context.Products .OrderBy(p => p.Id) .Skip(10) // Skip first 10 .Take(10) // Take next 10 .ToList(); Use .Skip() and .Take() for pagination: var page2 = context.Products .OrderBy(p => p.Id) .Skip(10) // Skip first 10 .Take(10) // Take next 10 .ToList(); Real-world example… (ShopNest) ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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, Take) Use .Skip() and .Take() for pagination: var page2 = context.Products .OrderBy(p => p.Id) .Skip(10) // Skip first 10 .Take(10) // Take next 10 .ToList();

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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 FromSqlRaw() or FromSqlInterpolated(): var products = context.Products .FromSqlRaw("SELECT * FROM Products WHERE Price > {0}", 100) .ToList(); Use FromSqlRaw() or FromSqlInterpolated(): var products = context.Products .FromSqlRaw("SELECT * FROM Products WHERE Price > {0}", 100) .ToList(); Use FromSqlRaw() or FromSqlInterpolated(): var products = context.

Explain a bit more

Products .FromSqlRaw("SELECT * FROM Products WHERE Price > {0}", 100) .ToList(); Use FromSqlRaw() or FromSqlInterpolated(): var products = context.Products .FromSqlRaw("SELECT * FROM Products WHERE Price > {0}", 100) .ToList();

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: (FromSqlRaw, etc.) Use FromSqlRaw() or FromSqlInterpolated(): var products = context.Products .FromSqlRaw("SELECT * FROM Products WHERE Price > {0}", 100) .ToList();

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: Stored procedures can be called via raw SQL or FromSqlRaw: var orders = context.Orders .FromSqlRaw("EXEC GetOrdersByCustomer @CustomerId = {0}", customerId) .ToList(); For non-query SPs, use Database.ExecuteSqlRaw().

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: Precompile frequently used queries to improve performance: private static readonly Func<MyDbContext, int, Product> _getProductById = EF.CompileQuery((MyDbContext ctx, int id) => ctx.Products.First(p => p.Id == id)); // Usage: var product = _getProductById(context, 5);

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: Enable logging in DbContext options: optionsBuilder .UseSqlServer(connectionString) .LogTo(Console.WriteLine, LogLevel.Information); Or configure logging in ASP.NET Core logging pipeline. Repository Pattern & Unit of Work

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: pplication. Pros: Decouples data access logic from business logic. Easier to mock/test. Encapsulates queries in a single place. Cons: EF Core’s DbContext already acts like a repository and unit of work. Can add unnecessary abstraction and boilerplate. May limit EF Core’s powerful querying features.

Real-world example (ShopNest)

ShopNest registers AppDbContext as scoped. One HTTP request = one context. Never inject it as a singleton.

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 use it with EF Core (pros and cons)? Repository Pattern abstracts data access logic, exposing CRUD methods to the application. Pros: Decouples data access logic from business logic. Easier to mock/test. Encapsulates queries in a single place. Cons: EF Core’s DbContext already acts like a repository and unit of work. Can add unnecessary abstraction and boilerplate. May limit EF Core’s powerful querying features.

Real-world example (ShopNest)

ShopNest registers AppDbContext as scoped. One HTTP request = one context. Never inject it as a singleton.

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: Unit of Work maintains a list of operations to be committed as a single transaction. EF Core’s DbContext implements Unit of Work, tracking changes and coordinating commits (SaveChanges()).

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: Generic repository provides CRUD for all entity types. Advantages: Reusable and reduces boilerplate. Simple for basic CRUD. Disadvantages: Can become too generic, losing flexibility for complex queries. May leak EF Core specifics or cause over-abstraction. Sometimes custom repositories per aggregate are better.

Real-world example (ShopNest)

ShopNest registers AppDbContext as scoped. One HTTP request = one context. Never inject it as a singleton.

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: What are the trade-offs? Generic repository provides CRUD for all entity types. Advantages: Reusable and reduces boilerplate. Simple for basic CRUD. Disadvantages: Can become too generic, losing flexibility for complex queries. May leak EF Core specifics or cause over-abstraction. Sometimes custom repositories per aggregate are better.

Real-world example (ShopNest)

ShopNest registers AppDbContext as scoped. One HTTP request = one context. Never inject it as a singleton.

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: public interface IRepository<T> where T : class { Task<T> GetByIdAsync(int id); Task<IEnumerable<T>> GetAllAsync(); Task AddAsync(T entity); void Update(T entity); void Delete(T entity); Task SaveChangesAsync(); } public interface IRepository<T> where T : class { Task<T> GetByIdAsync(int id); Task<IEnumerable<T>> GetAllAsync();… Task AddAsync(T entity);…… void Update(T entity); void Delete(T entity); Task…

Explain a bit more

SaveChangesAsync(); } public interface IRepository<T> where T : class { Task<T> GetByIdAsync(int id); Task<IEnumerable<T>> GetAllAsync(); Task AddAsync(T entity); void Update(T entity); void Delete(T entity); Task SaveChangesAsync(); } public interface IRepository<T> where T : class { Task<T> GetByIdAsync(int id); Task<IEnumerable<T>> GetAllAsync();… Task AddAsync(T entity); void Update(T entity); void Delete(T entity); Task SaveChangesAsync(); }

Real-world example (ShopNest)

ShopNest registers AppDbContext as scoped. One HTTP request = one context. Never inject it as a singleton.

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: Sample signature / interface? public interface IRepository<T> where T : class { Task<T> GetByIdAsync(int id); Task<IEnumerable<T>> GetAllAsync(); Task AddAsync(T entity); void Update(T entity); void Delete(T entity); Task SaveChangesAsync(); }

Real-world example (ShopNest)

ShopNest registers AppDbContext as scoped. One HTTP request = one context. Never inject it as a singleton.

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: public interface IUnitOfWork : IDisposable { IRepository<Customer> Customers { get; } IRepository<Order> Orders { get; } Task<int> CommitAsync(); } Implementation typically injects a single DbContext instance shared across repositories. CommitAsync() calls DbContext.SaveChangesAsync().

Example code

public interface IUnitOfWork : IDisposable
{ IRepository<Customer> Customers { get; } IRepository<Order> Orders { get; } Task<int> CommitAsync(); } Implementation typically injects a single DbContext instance shared across repositories. CommitAsync() calls DbContext.SaveChangesAsync().

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: wait _unitOfWork.CommitAsync(); wait transaction.CommitAsync(); } catch { wait transaction.RollbackAsync(); throw; } wait _unitOfWork.CommitAsync(); wait transaction.CommitAsync(); } catch { wait transaction.RollbackAsync(); throw; } wait _unitOfWork.CommitAsync(); wait transaction.CommitAsync(); } catch { wait transaction.RollbackAsync(); throw; } wait… _unitOfWork.CommitAsync(); wait transaction.CommitAsync(); }…

Explain a bit more

catch { wait transaction.RollbackAsync(); throw; }

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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 DbContext transaction or IDbContextTransaction: using var transaction = await _context.Database.BeginTransactionAsync(); try { // multiple repository operations await _unitOfWork.CommitAsync();

Example code

await transaction.CommitAsync();
} catch {
await transaction.RollbackAsync(); throw; }

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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: Mocking DbContext using frameworks like Moq (complex). Use InMemoryDatabase provider from EF Core for integration-like tests. Abstract dependencies and inject mocked interfaces for isolation. Performance, Best Practices, & Advanced Features

Real-world example (ShopNest)

ShopNest registers AppDbContext as scoped. One HTTP request = one context. Never inject it as a singleton.

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: Properties not defined in your CLR classes but tracked by EF Core. Useful for storing metadata like CreatedBy, LastModified, or foreign keys without cluttering domain models.

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

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.