377 questions for senior .NET engineer interviews, each with a 30-second answer, the full answer, a deep dive, the follow-ups interviewers ask next, and real-world production problems.
How .NET runs your code: the CLR, assemblies, JIT and AOT, runtime type layout and dispatch, the Roslyn compilation pipeline, and garbage collection.
Vtable slots in the MethodTable, the instruction sequence of a virtual call, override versus new, sealed and devirtualisation.
Interface maps, virtual stub dispatch (indirection cells, dispatch and resolve stubs), default interface methods, constrained calls on structs.
RuntimeType and member caches, Invoke costs and the .NET 7/8 invoker improvements, delegates, compiled expressions, UnsafeAccessor and source generators.
Roslyn's phases, immutable syntax trees, Compilation and SemanticModel, IOperation, emit, the compiler server and using the APIs to build tools.
Source generator authoring: IIncrementalGenerator, project setup (netstandard2.0, analyser references), post-initialisation output, debugging, packaging and pitfalls.
Incremental pipelines, caching and equality, ForAttributeWithMetadataName, avoiding symbols and syntax in models, and measuring generator performance.
Authoring DiagnosticAnalyzer: descriptors, registration actions, concurrency, generated code, testing with the Roslyn testing packages, severity configuration and packaging.
CodeFixProvider authoring: FixableDiagnosticIds, RegisterCodeFixesAsync, CodeAction with equivalence keys, syntax editing with trivia, Fix All and testing.
Comparing source generators, MSBuild-generated files (gRPC, OpenAPI), T4, IL weaving and interceptors for build-time code generation, with trade-offs.
The generational hypothesis, Gen 0/1/2, promotion, card tables and write barriers, regions, and mark-sweep-compact.
Allocation contexts and bump-pointer allocation, the Gen 0 budget, what triggers a Gen 0 GC, allocation rate metrics and reducing allocations.
Gen 1 as a buffer between short- and long-lived objects, Gen 1 GCs, objects that die soon after reaching Gen 2, and how to fix mid-life lifetimes.
Full GCs, triggers (budget, LOH allocation, induced, low memory), blocking versus background, compaction, and a method to diagnose frequent Gen 2 collections.
The 85,000-byte threshold, LOH collection with Gen 2, no compaction by default, fragmentation, churn, GCLOHThreshold and fixes such as pooling and streaming.
Pinning, why it fragments the heap, the POH (.NET 5), GC.AllocateArray with pinned: true, its restrictions and typical uses such as I/O buffer pools.
Workstation GC: single heap, background GC on by default, non-concurrent option, defaults per app type, and choosing it for desktop, tools and dense containers.
Server GC heaps and threads, throughput versus memory, DATAS dynamic heap count (default with Server GC since .NET 9), and container settings such as GCHeapHardLimit and GCHeapCount.
Concurrent versus background Gen 2 collection, foreground GCs during background GC, configuration naming, latency modes and no-GC regions.
Finalisation queue and f-reachable queue, extra GC survival, the single finaliser thread, ordering and exit behaviour, SafeHandle, and debugging finaliser problems.
The dispose pattern, sealed simple Dispose, idempotency, ObjectDisposedException, ownership rules, DI container disposal and analyser support.