meet it

hands out items one at a time.

foreach does not need a list. It needs an : anything that can hand out items one after another. Lists, arrays and LINQ queries all are. Return IEnumerable<T> from a method and callers can loop over it without caring how the items are made.

The neatest way to write one is an : a method that uses yield return. Each yield return hands one item to the loop and pauses the method right there. When the loop asks for the next item, the method carries on from where it paused.

yield break ends the iterator early, like return in a normal method.

using System;
using System.Collections.Generic;

foreach (string slot in Slots(9, 12))
{
    Console.WriteLine(slot);
}

IEnumerable<string> Slots(int from, int to)
{
    for (int hour = from; hour < to; hour++)
    {
        yield return $"{hour}:00";
        yield return $"{hour}:30";
    }
}

Nothing is stored: each time slot is made at the moment the loop asks for it.

guess first

Who prints first? What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

using System;
using System.Collections.Generic;

foreach (int n in Numbers())
{
    Console.WriteLine($"got {n}");
}

IEnumerable<int> Numbers()
{
    Console.WriteLine("start");
    yield return 1;
    Console.WriteLine("middle");
    yield return 2;
}

your turn

Your turn. Count down to lift off.

Choose where to count down from on line 4. Then, on line 10, hand each number to the caller from inside the loop.

Your code

output

press run to see what happens

fix it

The page list will not compile. Can you fix it?

Broken code to fix

output

press run to see what happens

C# refuses return null inside an iterator on line 14. How does an iterator say there is nothing more?
An iterator cannot return a value. It ends early with its own pair of words.
Change return null; to yield break; on line 14, then run.

nailed it

you can make sequences of your own. :D

foreach walks anything that is an IEnumerable<T>.
yield return hands out one item, then pauses.
yield break ends an iterator early.

new words in your collection

IEnumerable
iterator
yield return

This lesson isn’t saved. A free account remembers every lesson you finish.

yield return hands out items one at a time.

Who prints first? What will this show?

Your turn. Count down to lift off.

The page list will not compile. Can you fix it?

you can make sequences of your own. :D

hands out items one at a time.