OutWithYieldReturn.cs

// LINQPad script showing a "use" for out parameters on iterators.
// It's a mockup of a discrete event simulator process (similar to SimPy - http://simpy.readthedocs.io).
// Since it's not possible to use 'out' with iterators, this uses an Action<T> instead.
// I'm not suggesting it's valid or in good taste :)

void Main()
{
	foreach (var ie in Simulate().Take(100)) ie.Dump();
}

Container someContainer = new Container();
Container anotherContainer = new Container();

IEnumerable<IEvent> Simulate()
{
	while (true)
	{
		yield return someContainer.Get(1);
		yield return new WaitEvent(0.2);

		int quantity = 0;

		foreach (var ie in GetSomeNestedEvents(q => quantity = q)) yield return ie;

		yield return anotherContainer.Put(quantity);
	}
}

IEnumerable<IEvent> GetSomeNestedEvents(Action<int> setQuantity)
{
	setQuantity(0);
	yield return new WaitEvent(0.1);
	setQuantity(42);
}

interface IEvent
{
}

class Event : IEvent
{
}

class WaitEvent : Event
{
	public double Duration { get; }

	public WaitEvent(double duration) => Duration = duration;
}

class GetEvent : Event
{
	public int Quantity { get; }

	public GetEvent(int quantity) => Quantity = quantity;
}

class PutEvent : Event
{
	public int Quantity { get; }

	public PutEvent(int quantity) => Quantity = quantity;
}

class Container
{
	public GetEvent Get(int quantity) => new GetEvent(quantity);
	public PutEvent Put(int quantity) => new PutEvent(quantity);
}