What are boxing and unboxing in C#, and where can boxing appear unexpectedly when working with structs?

In production I treat boxing as a measured hot-path concern, not a reason to distort ordinary code. A few boxes in administrative logic are rarely important, while boxing per item in a high-volume parser, telemetry loop, or serializer can create visible allocation and GC pressure. I would use a profiler or BenchmarkDotNet to confirm the source, inspect interface and delegate conversions, and prefer constrained generic APIs where the call is performance-sensitive. Nullable value types have special boxing behavior: a null nullable boxes to null, while a populated nullable boxes the underlying value. Ref structs cannot be boxed at all because their lifetime cannot escape to the heap.