C# Language Specification

Iterators

A function member (§7.5) implemented using an iterator block (§8.2) is called an iterator.

An iterator block may be used as the body of a function member as long as the return type of the corresponding function member is one of the enumerator interfaces (§10.14.1) or one of the enumerable interfaces (§10.14.2). It can occur as a method-body, operator-body or accessor-body, whereas events, instance constructors, static constructors and destructors cannot be implemented as iterators.

When a function member is implemented using an iterator block, it is a compile-time error for the formal parameter list of the function member to specify any ref or out parameters.

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