Iterators & yield in C#

Implement IEnumerable<T> with yield return and yield break, create lazy sequences, and understand how the C# iterator state machine works.

Why Iterators Matter

1. What is an Iterator?

An iterator produces values one at a time — only when the consumer asks.

The Iteration Protocol

1. IEnumerable<T> and IEnumerator<T>

Any type implementing `IEnumerable<T>` can be used in a `foreach` loop.

2. Built-in Iterables

Arrays, List, Dictionary, string, Range — all implement IEnumerable.

yield return

1. Basic yield return

A method with `yield return` is a generator — it produces values lazily.

2. Generator State Control

The generator remembers local variables across yield returns.

yield break

1. Early Termination with yield break

`yield break` stops the generator — equivalent to `return` in a regular method.

Custom Iterable Class

1. Implementing IEnumerable<T> with yield

Return `IEnumerable<T>` from `GetEnumerator()` using yield — no manual enumerator class needed.

Lazy Evaluation

1. Infinite Sequence with yield

Generators can be infinite — they only compute values when consumed.

Backend Use Cases

1. Batch Processing

Stream large datasets in batches using yield — avoids out-of-memory errors.

Pitfalls & Best Practices

1. Recommended Guidelines

Key rules for writing safe, efficient iterator methods.