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The 20 things to be sure of

If you remember nothing else, remember these.

1	Value types copy the data, reference types copy the pointer. "Stack vs heap" is an implementation detail.
2	Override Equals → you must override GetHashCode, and hash only immutable fields.
3	throw; preserves the stack trace. throw ex; destroys it.
4	await on I/O creates no thread — it frees one. That's why it scales.
5	I/O-bound → async. CPU-bound → Parallel. Never Task.Run around async I/O.
6	No fire-and-forget background work: exceptions vanish, and the scoped DbContext is already disposed.
7	.Result/.Wait() = deadlock in classic ASP.NET, thread-pool starvation in Core.
8	Always bound concurrency — size it to the bottleneck (connection pool, rate limit), not the CPU.
9	IQueryable runs in SQL, IEnumerable runs in memory. Don't downgrade too early.
10	LINQ is deferred — enumerating twice runs the query twice.
11	Any() not Count() > 0. And All() on an empty list is true.
12	Read-only EF query → AsNoTracking() + project to a DTO.
13	N+1 is the most common EF bug — find it by logging the SQL, fix it with Include or a projection.
14	Concurrency conflicts: a rowversion token → DbUpdateConcurrencyException → 409, don't overwrite silently.
15	Middleware order is behaviour: exception handler first, UseAuthentication before UseAuthorization.
16	A Singleton must never hold a Scoped service — captive dependency. Catch it with ValidateOnBuild.
17	Never expose entities: data leaks, over-posting, circular references. Use DTOs.
18	Status codes are the contract: 400 malformed · 401 unauthenticated · 403 not permitted · 409 conflict. Never 200 with an error inside.
19	CORS is a browser rule — that's why Postman works. It protects users, not your API.
20	A timeout ≠ a failure. Retry only transient errors, with backoff + jitter, and only if the operation is idempotent.

Words that signal seniority

idempotent · captive dependency · thread-pool starvation · optimistic concurrency · deferred execution · N+1 · backpressure · exponential backoff with jitter · circuit breaker · trust boundary · set-based · separation of concerns · measure first · trade-off