EF Core Interview Questions and Answers for .NET Developers

EF Core interview questions at senior level test whether you can keep a clinic or marketplace database correct under concurrent writes, missing tenant filters, and background jobs that share a DbContext — not whether you can define Include.

Junior prompt                    Senior prompt
─────────────                    ─────────────
"What is Include?"               "Two tabs PATCH the same row — what happens?"
"Tracking vs no-tracking?"       "Nightly job emailed clinic B clinic A's data"
                                 "SaveChanges in a foreach on 4,000 rows"
                                 "Another instance with the same key is tracked"

New to this → start with Scenario 1. Merging a PR → rapid-fire table. On-call / interview → all scenarios below · cross-questions.

Terms used here: Lost update = two writers save the same row; last write wins without conflict detection. Captive dependency = a singleton holding a scoped DbContext. Optimistic concurrency = a row token (rowversion) that makes the second save fail with 409.

This URL is interview narration. SQL-shaped performance (N+1, split queries, cartesian explosion, parameter sniffing, tracking vs identity resolution) lives in the EF Core hub. If the prompt is "the dashboard is slow," start at EF Core SQL performance and come back when they ask about correctness.

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Scenario 1: Two clinicians save the same encounter

Prompt: Two Angular tabs load the same encounter. Both PATCH status. Last write wins. How do you make that a conflict the API can show, not silent data loss?

Weak answer

“Add a lock” with no row version, or “use a transaction” as if wrapping two independent HTTP requests in one TransactionScope were possible.

Strong answer

Optimistic concurrency with a RowVersion / xmin / timestamp column that EF maps as a concurrency token. Each GET returns the token. Each PUT sends it back. The second save throws DbUpdateConcurrencyException. Map that to 409 with ProblemDetails, not 500.

modelBuilder.Entity<Encounter>()
    .Property(e => e.RowVersion)
    .IsRowVersion();

try
{
    await _db.SaveChangesAsync(cancellationToken);
}
catch (DbUpdateConcurrencyException)
{
    throw new ConcurrencyConflictException(encounterId);
}

Prove it: two integration tests against a real SQL database (or Testcontainers). Same Id, different RowVersion. One succeeds, one 409s. In-memory provider is a liar for this.

Refuse: pessimistic UPDLOCK on every read for a SPA. Clinicians leave tabs open. You lock the row for the lifetime of a coffee.

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Scenario 2: Global query filter that leaked a tenant

Prompt: HasQueryFilter(e => e.ClinicId == _tenant.ClinicId) is on Encounter. A nightly job emailed clinic B a CSV of clinic A’s encounters. How?

Weak answer

“Filters are always applied.” They are not.

Strong answer

I list the escape hatches out loud:

1. IgnoreQueryFilters() on a report query someone copied from a debug session
2. Raw SQL / FromSqlRaw that never saw the filter
3. A DbContext constructed with a default tenant (Guid.Empty or first clinic in the table) because the hosted service has no HTTP scope
4. Navigation includes from an unfiltered parent (Clinic loaded without a filter, then .Encounters accessed in memory)

Fix the job: create a scope, set tenant from the job payload, or use an explicit Where(e => e.ClinicId == clinicId) on exports and log if IgnoreQueryFilters appears in a PR.

I do not pretend a filter is a security boundary. Authorization still lives in policies. The filter is a seatbelt.

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Scenario 3: SaveChanges inside a foreach

Prompt: Import 4,000 fee-schedule rows. The code does Add + SaveChangesAsync per row. The request times out. What do you change, and what do you not change?

Weak answer

“Use AddRange and one SaveChanges” as the entire answer, or “bulk insert library” without saying when.

Strong answer

One SaveChanges per HTTP request is the default. Per-row save is N round-trips plus N change-tracker snapshots.

For a true bulk import I want ExecuteInsert / a bulk copy path, not 4,000 tracked entities. For a small upsert I batch:

_db.FeeSchedules.AddRange(rows);
await _db.SaveChangesAsync(cancellationToken);

When the rows already exist, I prefer ExecuteUpdate for a set-based column change, not load-mutate-save:

await _db.FeeSchedules
    .Where(f => f.ClinicId == clinicId && f.Year == year)
    .ExecuteUpdateAsync(
        s => s.SetProperty(f => f.IsActive, false),
        cancellationToken);

Refuse: SaveChanges in a loop “so we can return partial success” without a strategy. Either a transaction with a report of failures, or a staging table. Healthcare fee files that half-apply are worse than a 500.

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Scenario 4: Same entity, two tracked instances

Prompt: A PUT loads the encounter, then _db.Update(dtoMappedEntity) and SaveChanges throws: another instance with the same key is already being tracked. What happened?

Weak answer

“Call AsNoTracking on the PUT.” Tracking on a write path is not the bug.

Strong answer

You loaded a tracked instance, then attached a second instance with the same primary key (mapped from the Angular body). EF cannot have two tracked copies.

Pick one:

1. Load, mutate the tracked entity, save
2. Or attach/update without a prior tracked load (ExecuteUpdate, or Update on a disconnected graph with no previous Find)

var encounter = await _db.Encounters.FirstAsync(e => e.Id == id, cancellationToken);
encounter.Status = body.Status;
await _db.SaveChangesAsync(cancellationToken);

Identity resolution on reads (AsNoTrackingWithIdentityResolution) is a different article: AsNoTracking vs identity resolution. Do not mix that story into a PUT.

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Scenario 5: DbContext in a singleton cache

Prompt: A ReportCache singleton injects AppDbContext. Intermittent “cannot access a disposed object” and, once, the wrong clinic’s numbers. Diagnose.

Weak answer

“Make everything singleton” or “make everything scoped.”

Strong answer

This is a captive dependency. DbContext is scoped. A singleton holds the first request’s context (or a disposed one). Tenant data rides along.

Fix: cache is singleton or scoped; context stays scoped. If the cache must be singleton, inject IServiceScopeFactory and open a scope per refresh. Full lifetime rules: DI lifetimes. If the exception is actually “Unable to resolve service for type,” that is the registration post.

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Scenario 6: Migrations in production

Prompt: App Service swap. API 500s: pending model changes or could not apply migration. How do you ship schema?

Weak answer

“Migrate() in Program.cs” as a religion, or “never migrate in prod” without an alternative.

Strong answer

I separate schema from app start:

• Generate SQL (dotnet ef migrations script) and run it in the release pipeline before the new bits take traffic
• Or a dedicated migrator job with a lock so two instances do not race
• Database.Migrate() at startup is acceptable for a single-instance internal tool, not for a slot swap with three instances

Pending model changes at runtime means someone edited entities and skipped dotnet ef migrations add. That is a CI gate, not a try/catch.

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Rapid-fire (30 seconds)

Prompt	Answer I want
Why not the InMemory provider for concurrency tests?	No real transactions, no row versions you can trust
EnsureCreated vs migrations	EnsureCreated skips the migration history; do not mix in one database
Compiled queries	Hot path with a stable shape; not a substitute for indexes
Returning IQueryable from a repository	Callers can still IgnoreQueryFilters and Include the world
AsNoTracking on a write	You will reattach; usually the wrong default for PUT
1-1 vs owned type	Owned type when the child has no independent identity (address on a clinic). Separate entity + FK when you query the child alone
1-n	Required dependent with HasMany / WithOne and a real FK. Optional when the child can exist without the parent
Many-to-many	Skip a hand-rolled join entity until you need payload on the link (assigned-at, role). EF can hide the join table

Entity relationship interview questions

Entity relationship interview questions on EF Core panels are mapping questions, not ERD trivia:

1. One-to-one — optional dependent (HasOne / WithOne + unique FK) vs owned type (OwnsOne) when the child never lives alone.
2. One-to-many — collection navigation + FK. Required vs optional is a nullability decision that leaks into Angular forms (clinicId required on create).
3. Many-to-many — skip an explicit join entity until the link has data (who assigned the clinician, when). A payload-free n-n is a skip-level navigation; a join with AssignedAt is a first-class entity.
4. Delete behavior — Restrict on healthcare FKs until you have an explicit cascade story. Silent Cascade on a clinic wipe is a compliance incident.

If they draw a diagram and ask you to write OnModelCreating, talk FK, required/optional, and indexes — not "I would add a repository." Working mappings, delete behavior, and Angular DTOs: EF Core relationships.

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How this differs from the other EF posts

• This page: correctness under concurrent writes, tenant filters, SaveChanges shape
• EF Core relationships: 1-1 vs owned, 1-n, skip n-n vs join entity with payload
• EF Core SQL performance: N+1, projections, indexes
• N+1 vs Include vs AsSplitQuery: which SQL you meant
• Cartesian explosion: two collection Includes
• Parameter sniffing: one tenant fast, another slow
• RowVersion: how to map the token and return 409
• Query filters: how to wire HasQueryFilter without a leak
• ExecuteUpdate: set-based UPDATE when interceptors must not run

Related reading

• EF Core relationships (1-1 / 1-n / n-n)
• ASP.NET Core interview scenarios
• C# expert-level interview questions
• Interview questions hub
• EF Core topic hub

Cross-questions

Interviewers often chain EF topics. These pairings test depth without repeating the dedicated posts:

If they ask about…	Follow-up they might spring	Point them to
RowVersion / 409	"Does RCSI fix lost updates?"	No — RCSI is blocking; concurrency token is this page, Scenario 1
Query filter leak	"Is the filter a security boundary?"	No — Scenario 2; wiring is query filters
SaveChanges in foreach	"What about ExecuteUpdate?"	Scenario 3; implementation is ExecuteUpdate
Two tracked instances	"What about AsNoTracking on reads?"	Different bug — identity resolution
Singleton + DbContext	"How do you fix the cache?"	IServiceScopeFactory per refresh — Scenario 5
Pending migrations at swap	"Migrate() in Program.cs?"	Scenario 6 — pipeline script, not startup on three instances

Bring a SaveChanges failure or a tenant-leak story, not a definition of DbSet.