What is the hidden cost of multiple LINQ enumerations on the same query?

Each enumeration re-executes the entire LINQ pipeline, potentially repeating expensive operations.

LINQ queries are not cached by default. Every time an IEnumerable or IQueryable is enumerated, the full pipeline is executed again. For database-backed queries, this means repeated SQL execution. For in-memory sequences, it means repeated iteration and transformation. This can lead to performance degradation, inconsistent results if data changes, and unnecessary CPU or I/O usage.