When should you call ToList or ToArray in a LINQ pipeline, and when should you keep the result lazy?

In production I make materialization visible at the layer that owns its cost. A repository returning a lazy sequence over a disposed resource is broken, while a repository that always loads millions of rows is also broken. For remote data I project and filter before materializing and enforce pagination or limits. For in-memory data I consider whether a snapshot or a live view is the promised contract. I avoid multiple ToList calls used only to silence type mismatches, and I do not return a mutable list when consumers need read-only access. Metrics and profiling reveal allocation-heavy paths, but the semantic boundary comes first: execute where lifetime, consistency, and failure ownership are clear.