Dependency Injection Interview Questions
Service lifetimes, scopes, and IoC patterns.
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Questions (20)
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How would you test code that uses Dependency Injection in .NET?
Use unit tests with mocks or fakes for external dependencies, integration tests against a real database or message broker when appropriate, and contract tests for APIs. Dependency Injection should have clear inputs/outputs so tests remain fast and deterministic.
What logging or monitoring would you add around Dependency Injection?
Log structured events with correlation IDs, track latency and error rates, and alert on SLO breaches. For Dependency Injection, capture enough context to reproduce failures without logging secrets such as passwords or tokens.
How does Dependency Injection interact with authentication in .NET applications?
Auth often gates access to endpoints or resources that rely on Dependency Injection. Apply least privilege, validate tokens or sessions at the boundary, and never trust client-side checks alone. Mention OAuth, JWT, or session cookies as appropriate to the stack.
What environment variables or config files typically control Dependency Injection?
Separate config from code using environment-specific settings, secrets managers, and twelve-factor practices. Document required variables for Dependency Injection so deployments to staging and production remain repeatable and auditable.
Describe a REST or HTTP endpoint design concern related to Dependency Injection.
Consider idempotency, status codes, pagination, versioning, and error payloads. Dependency Injection should not leak internal exceptions to clients; return consistent error shapes and document them in OpenAPI or similar specs.
What is a simple way to handle errors when Dependency Injection fails in .NET?
Catch exceptions at appropriate layers, map them to user-safe messages, retry transient failures with backoff where suitable, and record failures for operators. Avoid swallowing errors silently—failed Dependency Injection operations should be visible in logs and metrics.
What documentation would you consult when working with Dependency Injection in .NET?
Use the official .NET docs for Dependency Injection, language or framework references, and reputable community guides. Bookmark release notes and migration guides when upgrading versions, since Dependency Injection behavior can change between releases.
What is a common beginner mistake when learning Dependency Injection?
Copying snippets without understanding why Dependency Injection works leads to fragile code. Beginners often skip error handling, tests, or edge cases. Slow down, trace execution step by step, and validate assumptions with small experiments.
How does Dependency Injection fit into an ASP.NET Core request pipeline?
Describe where Dependency Injection runs relative to middleware, endpoints, and DI. Mention hosting (Kestrel) and how configuration binds for Dependency Injection.
Which C# language features commonly appear with Dependency Injection?
async/await, records, nullable reference types, and LINQ. Show how they improve correctness for Dependency Injection.
How do you unit test code that uses Dependency Injection?
xUnit + Moq/NSubstitute, WebApplicationFactory for integration, and in-memory providers when appropriate. Keep Dependency Injection tests deterministic.
What DI lifetime would you choose for a service that implements Dependency Injection?
Singleton vs scoped vs transient depends on state and DbContext rules. Justify the lifetime for Dependency Injection with a concrete example.
How does configuration (appsettings/env) control Dependency Injection?
Options pattern, validation on start, and secrets outside source control. Document required settings for Dependency Injection.
What logging practices apply when Dependency Injection fails?
ILogger with structured properties, correlation IDs, and no secrets in logs. Map Dependency Injection exceptions to problem details for APIs.
When should you use middleware vs filters vs endpoint filters for Dependency Injection?
Middleware for cross-cutting pipeline concerns; filters for MVC/minimal APIs specifics. Pick the narrowest layer that fits Dependency Injection.
How would you implement Dependency Injection in a production .NET codebase?
Follow team conventions, split concerns into testable units, handle edge cases, and document assumptions. Review similar modules in the codebase, add observability, and ship incrementally with feature flags if Dependency Injection is risky.
What are common pitfalls when scaling Dependency Injection in .NET?
Watch for bottlenecks, shared state races, config drift, and unbounded resource usage. Load-test Dependency Injection paths, set limits, and plan horizontal scaling or caching before traffic spikes.
Compare two approaches to Dependency Injection in .NET and when to use each.
One approach optimizes simplicity and time-to-market; the other optimizes performance, flexibility, or compliance. Choose based on team skill, traffic, and maintenance horizon—there is rarely a single best answer for Dependency Injection.
How do you debug a production issue involving Dependency Injection?
Reproduce in staging, check logs/metrics/traces, narrow scope with binary search deploys, and write a postmortem. Fix Dependency Injection root cause, add regression tests, and improve alerts so similar failures are caught earlier.
What code review feedback would you give on a Dependency Injection pull request in .NET?
Check correctness, tests, naming, error handling, security, and performance. Ask whether Dependency Injection belongs in this layer, if docs updated, and if rollback is safe.
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