Interview Q&A

• Optimistic concurrency: Assumes that conflicts are rare. EF checks for changes

before saving using concurrency tokens.

• Pessimistic concurrency: Locks records in the DB to prevent other updates (not

supported out-of-the-box in EF Core).

EF Core supports optimistic concurrency using [ConcurrencyCheck] or

[Timestamp].

concurrency token is a property used to detect concurrency conflicts.

Example with [Timestamp]:

[Timestamp]

public byte[] RowVersion { get; set; }

• EF Core includes this column in the WHERE clause of updates.
• If the row has changed since it was loaded, EF will throw a

DbUpdateConcurrencyException.

Entity Framework Core – Relationships

nnotations & Fluent API?

• Primary Key (PK): Unique identifier of a record.
• Foreign Key (FK): A field that references a PK in another table.

Data Annotations:

[Key]

public int Id { get; set; }

[ForeignKey("Blog")]

public int BlogId { get; set; }

Fluent API:

modelBuilder.Entity<Post>()

.HasKey(p => p.Id);

modelBuilder.Entity<Post>()

.HasOne(p => p.Blog)

.WithMany(b => b.Posts)

.HasForeignKey(p => p.BlogId);

Answer: composite key is a primary key made of multiple columns. EF Core does not support composite keys via data annotations, so you must use Fluent PI: modelBuilder.Entity&lt;OrderDetail&gt;() .HasKey(od =&gt; new { od.OrderId, od.ProductId });

What interviewers expect

• A clear definition tied to EF Core in Entity Framework Core projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Entity Framework Core application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Entity Framework Core architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Cascade delete ensures that related entities are deleted when the parent entity is deleted.

Configure with Fluent API:

modelBuilder.Entity<Blog>()

.HasMany(b => b.Posts)

.WithOne(p => p.Blog)

.OnDelete(DeleteBehavior.Cascade);

Delete behaviors:

• Cascade
• Restrict
• SetNull
• NoAction

EF Core defaults to Cascade for required relationships.

Eager loading loads related data as part of the initial query, reducing round-trips to the

database.

✅ Use .Include() to load related entities:

var blogs = context.Blogs

.Include(b => b.Posts)

.ToList();

Use .ThenInclude() for deeper nesting:

context.Blogs

.Include(b => b.Posts)

.ThenInclude(p => p.Comments);

Eager loading prevents lazy load performance issues and the N+1 problem.

How is .Include() / .ThenInclude()

used?

Eager loading loads related data as part of the initial query, reducing round-trips to the

database.

✅ Use .Include() to load related entities:

var blogs = context.Blogs

.Include(b => b.Posts)

.ToList();

Use .ThenInclude() for deeper nesting:

context.Blogs

.Include(b => b.Posts)

.ThenInclude(p => p.Comments);

Eager loading prevents lazy load performance issues and the N+1 problem.

re proxies?

Lazy loading delays the loading of related data until it's accessed for the first time.

EF Core requires proxies for lazy loading:

Install NuGet:

Microsoft.EntityFrameworkCore.Proxies

• Enable in OnConfiguring or AddDbContext:

options.UseLazyLoadingProxies();

• Mark navigation properties as virtual:

public virtual ICollection<Post> Posts { get; set; }

• EF creates runtime proxies to override navigation properties and load them when

ccessed.

How to enable lazy loading in EF Core? What

are proxies?

Lazy loading delays the loading of related data until it's accessed for the first time.

EF Core requires proxies for lazy loading:

Install NuGet:

Microsoft.EntityFrameworkCore.Proxies

Enable in OnConfiguring or AddDbContext:

options.UseLazyLoadingProxies();

Mark navigation properties as virtual:

public virtual ICollection<Post> Posts { get; set; }

EF creates runtime proxies to override navigation properties and load them when

accessed.

Explicit loading means loading related data manually, after the main entity is loaded.

Use when:

• You need full control over what and when to load
• You don’t want automatic lazy loading

Example:

var blog = context.Blogs.First();

context.Entry(blog)

.Collection(b => b.Posts)

.Load();

context.Entry(blog)

.Reference(b => b.Owner)

.Load();

✅ Use .Reference().Load() for single navigation

✅ Use .Collection().Load() for collections

When & how to use it?

Explicit loading means loading related data manually, after the main entity is loaded.

Use when:

• You need full control over what and when to load
• You don’t want automatic lazy loading

Example:

var blog = context.Blogs.First();

context.Entry(blog)

.Collection(b => b.Posts)

.Load();

context.Entry(blog)

.Reference(b => b.Owner)

.Load();

✅ Use .Reference().Load() for single navigation

✅ Use .Collection().Load() for collections

The N+1 problem occurs when:

• 1 query loads N parent entities
• N additional queries load related entities (1 per parent)

Example (lazy loading):

foreach (var blog in context.Blogs)
{

Console.WriteLine(blog.Owner.Name); // triggers a query for each

blog

}

This causes N+1 queries, which can hurt performance significantly.

✅ Solution: Use eager loading with .Include() to fetch everything in one query.

How can lazy loading lead to it?

The N+1 problem occurs when:

• 1 query loads N parent entities
• N additional queries load related entities (1 per parent)

Example (lazy loading):

foreach (var blog in context.Blogs)

Console.WriteLine(blog.Owner.Name); // triggers a query for each

blog

This causes N+1 queries, which can hurt performance significantly.

✅ Solution: Use eager loading with .Include() to fetch everything in one query.

By default, EF Core uses no automatic loading — it doesn’t lazy-load or eager-load

relationships unless you:

• Use .Include() for eager loading
• Enable lazy loading proxies
• Use .Load() for explicit loading

This default avoids unintended queries and promotes performance control.

EF Core LINQ Queries & Querying

Answer: Projection transforms entities into custom shapes, e.g., DTOs. var projected = context.Products .Select(p =&gt; new { p.Name, p.Price }); Useful for: Reducing data load. Returning only needed fields. Mapping to custom types.

What interviewers expect

• A clear definition tied to EF Core in Entity Framework Core projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Entity Framework Core application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Entity Framework Core architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

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.

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.

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()).

What interviewers expect

• A clear definition tied to EF Core in Entity Framework Core projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Entity Framework Core application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Entity Framework Core architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: A filter automatically applied to all queries on an entity. Useful for soft deletes, multi-tenancy (filtering by tenant ID), or data partitioning.

What interviewers expect

• A clear definition tied to EF Core in Entity Framework Core projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Entity Framework Core application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Entity Framework Core architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: When/why to use it? A filter automatically applied to all queries on an entity. Useful for soft deletes, multi-tenancy (filtering by tenant ID), or data partitioning.

What interviewers expect

• A clear definition tied to EF Core in Entity Framework Core projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Entity Framework Core application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Entity Framework Core architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: EF Core batches multiple commands into a single database round trip when possible. Improves performance by reducing network overhead.

What interviewers expect

• A clear definition tied to EF Core in Entity Framework Core projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Entity Framework Core application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Entity Framework Core architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: How does EF Core handle batch operations? EF Core batches multiple commands into a single database round trip when possible. Improves performance by reducing network overhead.

What interviewers expect

• A clear definition tied to EF Core in Entity Framework Core projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Entity Framework Core application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Entity Framework Core architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Mark entities as deleted without physically removing them. Implemented using a boolean flag and global query filters to exclude “deleted” records.

What interviewers expect

• A clear definition tied to EF Core in Entity Framework Core projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Entity Framework Core application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Entity Framework Core architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Implementation strategies in EF Core? Mark entities as deleted without physically removing them. Implemented using a boolean flag and global query filters to exclude “deleted” records.

What interviewers expect

• A clear definition tied to EF Core in Entity Framework Core projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Entity Framework Core application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Entity Framework Core architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Assumes conflicts are rare; detects conflicts when saving. Uses concurrency tokens (e.g., timestamp or row version) to detect if data was changed by another process. EF Core throws DbUpdateConcurrencyException if a conflict occurs.

What interviewers expect

• A clear definition tied to EF Core in Entity Framework Core projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Entity Framework Core application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Entity Framework Core architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.