exp-mock-usage-analysis

Mock Usage Analysis

Trace each mock setup through the production code's execution path to determine which setups are actually exercised at runtime and which are dead, unreachable, redundant, or replaceable with real implementations.

When to Use

• User asks to audit, review, or analyze mock usage in .NET tests
• User wants to find unused, unnecessary, or redundant mock setups
• User wants to simplify test setup or reduce over-mocking
• User asks whether mocks of ILogger, IOptions, or similar types are needed

When Not to Use

• User wants to write new mocks or tests (general testing guidance)
• User wants to detect non-mock test anti-patterns (use test-anti-patterns)
• User wants to migrate between mock frameworks (out of scope)

Inputs

Workflow

Step 1: Read all provided code

Read the test files and always read the production code. You cannot determine whether a mock setup is necessary without understanding the production method's control flow.

Identify the mock framework by scanning for its patterns:

• Moq: new Mock<T>(), .Setup(...), .Verify(...)
• NSubstitute: Substitute.For<T>(), .Returns(...), .Received(...)
• FakeItEasy: A.Fake<T>(), A.CallTo(...), .MustHaveHappened()

Use the correct framework's terminology throughout your analysis.

Step 2: Trace each mock setup through the production code

For each test method, do the following:

1. Identify every mock setup line (.Setup, .Returns, A.CallTo, etc.)
2. Read the production method being tested and trace its execution path for the specific inputs used in that test
3. Determine which mock setups are actually reached during execution
4. Classify each setup:

Pay special attention to:

• Early returns and guard clauses — setups for mocks called after a guard clause are unreachable when the guard triggers
• Exception throws — if the method throws before using dependencies, all setups for those dependencies are unnecessary
• Branch-specific logic — if a method dispatches by channel/type, setups for other channels are unused
• Verify-only tests — tests that only call .Verify/.Received/.MustHaveHappened without asserting on the method's return value

Step 3: Check for replaceable mocks

Flag mocks of stable framework types that should use real implementations:

• Mock<ILogger<T>> → NullLogger<T>.Instance (unless log output is asserted)
• Mock<IOptions<T>> → Options.Create(new T { ... })
• Mocks of DTOs, records, or value objects → use new T { ... } directly

Explicitly confirm which mocks are correctly placed — external boundaries (databases, HTTP clients, message queues, third-party APIs) and security-sensitive types should remain mocked.

Step 4: Report findings

For each finding, state:

1. The specific test method and mock setup line
2. Why the setup is unnecessary (trace the production code path to explain)
3. A concrete fix — which lines to remove, what to replace them with, or how to extract shared setup

When multiple tests duplicate mock configurations, provide a before/after example showing how to extract shared setup into a fixture or helper method.

Validation

• Production code was read and execution paths were traced (not just test code reviewed)
• Every finding references a specific test method and setup line
• Unreachable setups include an explanation of which production code path makes them unreachable
• Correctly-placed mocks (external boundaries) are explicitly noted as appropriate
• Correct framework terminology is used throughout (not mixing Moq/NSubstitute/FakeItEasy terms)

Common Pitfalls