#562 – What an Iterator Looks Like Under the Covers

When you use an iterator to return an enumerable sequence of items, the C# compiler converts your iterator block (containing one or more yield return statements) into a new class that performs the actual enumeration.  The new class actually implements all four interfaces that an iterator can return–IEnumerableIEnumerable<T>IEnumerator and IEnumerator<T>.

Below is a simple iterator block that returns a sequence of two elements.

        private static IEnumerable<int> MyIterator()
            yield return 100;
            yield return 200;

If you use a disassembler to look at the IL that the compiler generates for this code, you’ll see a new class that implements the four interfaces.

You’ll notice that the class implements GetEnumerator, which just returns the current instance of the class.  (Code shown is from Reflector).

The MoveNext method returns the next element in the sequence.


About Sean
Software developer in the Twin Cities area, passionate about software development and sailing.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: