Unit Testing & TDD Interview Questions and Answers

Unit testing interview questions for .NET developers — xUnit, NUnit and MSTest, mocking with Moq, the AAA pattern, test doubles, TDD, and integration testing in ASP.NET Core.

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Unit Testing & TDD

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Fresher to 5+ YrsExperience

All Unit Testing & TDD Questions (19) Sort by

01 What is the difference between an integration test and an end-to-end test? An integration test exercises a slice of the system with some real components (the API + a real database, but no… Integration Testing Intermediate 2–3 Yrs
02 What is a unit test, and how does it differ from an integration test? A unit test verifies one small piece of logic (a method/class) in isolation, with dependencies replaced by test… Fundamentals Basic Fresher
03 What is the Arrange-Act-Assert (AAA) pattern? AAA structures a test into three clear sections: Arrange — set up the object under test and its inputs; Act — call the… Fundamentals Basic Fresher
04 What is the difference between a mock, a stub, a fake and a spy? All are test doubles. A stub returns canned answers to calls. A mock also has expectations you verify (“was Send… Test Doubles Intermediate 2–3 Yrs
05 What is Test-Driven Development (TDD)? TDD is writing a failing test before the code that makes it pass, in a tight loop: Red (write a failing test), Green… Practices Intermediate 2–3 Yrs
06 What makes a good unit test (FIRST principles)? Good tests are FIRST: Fast (milliseconds), Isolated (no shared state, no order dependence), Repeatable (same result… Practices Intermediate 1–2 Yrs
07 What are the differences between xUnit, NUnit and MSTest? All three are mature .NET test frameworks. xUnit is the modern default for new projects — no [SetUp]/[TearDown]… Frameworks Basic 1–2 Yrs
08 How do you write a parameterized (data-driven) test? In xUnit, use [Theory] with [InlineData(...)] for simple constant cases or [MemberData(nameof(Cases))] / [ClassData]… Frameworks Basic 1–2 Yrs
09 How do you use Moq to fake a dependency? Create a mock of the interface, configure its behaviour with Setup(...).Returns(...) (or ReturnsAsync), pass… Test Doubles Intermediate 2–3 Yrs
10 How do you test code that depends on DateTime.Now or randomness? Don't call DateTime.Now / Random / Guid.NewGuid() directly — inject them. Use an IClock abstraction (or .NET 8's… Practices Intermediate 2–3 Yrs
11 What is WebApplicationFactory, and how do you write an integration test for a Web API? WebApplicationFactory<TEntryPoint> spins up your whole ASP.NET Core app in memory (real pipeline, real DI) and gives… Integration Testing Advanced 3–5 Yrs
12 What is code coverage, and is 100% a good target? Code coverage measures which lines/branches run during the test suite. It tells you what's definitely not tested, but… Practices Intermediate 2–3 Yrs
13 How do you test a private method? You generally don't test private methods directly — you test them through the public method that calls them. If a… Practices Intermediate 2–3 Yrs
14 What is a flaky test, and how do you deal with it? A flaky test passes and fails without the code changing. Common causes: dependence on timing/Thread.Sleep, shared… Practices Advanced 3–5 Yrs
15 What is the difference between black-box and white-box testing? Black-box testing checks behaviour against the spec without looking at the code — you pick inputs from the… Fundamentals Basic Fresher
16 What is a smoke test, and what is a regression test? A smoke test is a quick check that the critical paths work at all — “does the app start, can I log in, does the… Fundamentals Basic 1–2 Yrs
17 How do you keep integration tests isolated when they share a database? Options: wrap each test in a transaction and roll it back; use a library like Respawn to delete all data between… Integration Testing Advanced 3–5 Yrs
18 What is Behaviour-Driven Development (BDD)? BDD describes behaviour in a structured natural language (Gherkin: Given… When… Then…) that both business and… Practices Intermediate 2–3 Yrs
19 Should you use the EF Core in-memory provider for tests? The EF Core in-memory provider is not a relational database — it doesn't enforce constraints, doesn't do referential… Integration Testing Advanced 3–5 Yrs

About these Unit Testing & TDD questions

Almost every mid-level .NET interview asks about testing, because it reveals how you actually write code. Interviewers want to hear that you know the difference between a unit test and an integration test, when to use a mock versus a stub versus a real object, how to structure a test (Arrange-Act-Assert), and how dependency injection makes code testable in the first place.

The .NET-specific parts are the frameworks (xUnit is the default for new projects, NUnit and MSTest are common in existing ones), Moq / NSubstitute for test doubles, and WebApplicationFactory for spinning up the app in-memory for integration tests.

Unit Testing & TDD Interview Questions and Answers

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