C# Language Specification

Yield type

An iterator produces a sequence of values, all of the same type. This type is called the yield type of the iterator.

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  The yield type of an iterator that returns IEnumerator or IEnumerable is object.
  
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  The yield type of an iterator that returns IEnumerator or IEnumerable is T.
  

4. Enumerator objects

When a function member returning an enumerator interface type is implemented using an iterator block, invoking the function member does not immediately execute the code in the iterator block. Instead, an enumerator object is created and returned. This object encapsulates the code specified in the iterator block, and execution of the code in the iterator block occurs when the enumerator object’s MoveNext method is invoked. An enumerator object has the following characteristics:

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  It implements IEnumerator and IEnumerator, where T is the yield type of the iterator.
  
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  It implements System.IDisposable.
  
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  It is initialized with a copy of the argument values (if any) and instance value passed to the function member.
  
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  It has four potential states, before, running, suspended, and after, and is initially in the before state.
  

An enumerator object is typically an instance of a compiler-generated enumerator class that encapsulates the code in the iterator block and implements the enumerator interfaces, but other methods of implementation are possible. If an enumerator class is generated by the compiler, that class will be nested, directly or indirectly, in the class containing the function member, it will have private accessibility, and it will have a name reserved for compiler use (§2.4.2).

An enumerator object may implement more interfaces than those specified above.

The following sections describe the exact behavior of the MoveNext, Current, and Dispose members of the IEnumerable and IEnumerable interface implementations provided by an enumerator object.

Note that enumerator objects do not support the IEnumerator.Reset method. Invoking this method causes a System.NotSupportedException to be thrown.

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