disposal-and-resource-lifetimelisted

# Disposal and Resource Lifetime ## The ownership rule Whoever creates a disposable disposes it; whoever receives one does not. Every review question about disposal reduces to "who owns this instance?" - Created in a method: `using` / `await using`, no exceptions. - Created in a constructor and stored in a field: the class owns it, so the class implements `IDisposable` and disposes the field. - Injected via DI: the container owns it. Disposing an injected `DbContext` or typed `HttpClient` breaks the next consumer of the same scoped instance. A class whose only disposables are injected does not implement `IDisposable` at all. ```csharp // WRONG: disposing what DI owns; second service in the scope gets a disposed context public sealed class OrderService : IDisposable { private readonly AppDbContext _db; public OrderService(AppDbContext db) => _db = db; public void Dispose() => _db.Dispose(); } // RIGHT: no IDisposable; the scope disposes the context public sealed class OrderService { private readonly AppDbContext _db; public OrderService(AppDbContext db) => _db = db; } ``` ## What must never be wrapped in using - `HttpClient` from `IHttpClientFactory`: disposal is a no-op on the handler you care about, but `using` documents a false ownership. Sockets are managed by the factory's handler pool; `new HttpClient()` per call plus `using` is the classic socket-exhaustion bug (TIME_WAIT pileup under load). - `CancellationTokenSource` still referenced by regis