How C# Compiler Transforms yield Into a Hidden State Machine Class

The C# keyword yield return may look simple, but the compiler silently rewrites any method using it into a full state machine class that implements IEnumerable and IEnumerator. Local variables are moved to heap fields, and each yield return becomes a numbered state inside a MoveNext() method, meaning the original method effectively disappears. A key consequence is deferred execution: the method body does not run at all until enumeration actually begins, which can cause unexpected behavior with logging, exceptions, or side effects. On the performance side, yield enables lazy evaluation that keeps memory usage nearly constant when processing large data sets, which is a core reason LINQ operates efficiently. However, developers should weigh trade-offs such as difficulty re-enumerating a sequence and harder debugging before defaulting to yield in every scenario.