Example of IEnumerable and IEnumerator. Show what 'yiled' statement dose.

IEnumerableTest.cs

using UnityEngine;
using System.Collections;

/// <summary>
/// Try to learn IEnumerable and IEnumerator.
/// Find out what 'yiled' statement dose. Ref: https://msdn.microsoft.com/en-us/library/9k7k7cf0.aspx
/// </summary>
namespace Lab.IEnumerableTest {
	public class Car
	{
		public string name;
		public int speed;
		public string inGarageName;

		public Car(string n, int s, string inGarageName) { name = n; speed = s; this.inGarageName = inGarageName; }

		public string Print() {
			return name + " " + speed + ". in garage: " + inGarageName;
		}
	}

	public class Garage : IEnumerable {
		public string name;

		// Can go to next garage from this one.
		public Garage gotoNext;			

		public Car[] cars = new Car[4];

		public Garage (string name) {
			this.name = name;
			cars [0] = new Car ("Mazada", 100, name);
			cars [1] = new Car ("Toyota", 200, name);
			cars [2] = new Car ("SuperMazada", 300, name);
			cars [3] = new Car ("SuperToyota", 400, name);
		}

		public void SetNextGarage(Garage gotoNext) { 
			this.gotoNext = gotoNext; 
		}

		// Do foreach on a garage instance will loop all cars.
		public IEnumerator GetEnumerator() {
			return cars.GetEnumerator ();
		}

		public string PrintCar(Car car) {
			return car.name + " " + car.speed + ". in garage name: " + car.inGarageName;
		}
	}

	/// <summary>
	/// Contains several garages. They are connected one by one like a single path link.
	/// Try to use foreach to loop all garages by IEnumerable interface.
	/// </summary>
	public class GarageChain : IEnumerable {
		public Garage garageA = new Garage ("A");
		public Garage garageB = new Garage ("B");
		public Garage garageC = new Garage ("C");

		public GarageChain () {
			// Connect them.
			garageA.SetNextGarage (garageB);
			garageB.SetNextGarage (garageC);
			garageC.SetNextGarage (null);		// Don't need this line. Just make it clear.
		}

		// Do foreach on GarageChian instance will loop cars in all garages.
		public IEnumerator GetEnumerator() {
			Garage current = garageA;

			// NOTE: If don't use 'yield' statement, following code only loop cars in first garage.
			// 'When a yield return statement is reached in the iterator method, expression is returned, and the current location in code is retained. 
			// Execution is restarted from that location the next time that the iterator function is called.'
			while (current != null) {
				foreach (Car car  in current) {
					yield return car;
				}

				current = current.gotoNext;
			}

			// End
			yield break;
		}
	}

	/// <summary>
	/// Test script works on Unity3D.
	/// </summary>
	public class CheckCars : MonoBehaviour {
		GarageChain chain = new GarageChain();

		void Start() {
			// This is internal logic.
			Debug.Log ("- Manully Loop - ");
			IEnumerator carEnumerator = chain.GetEnumerator ();
			while (carEnumerator.MoveNext ()) {
				// This is where 'foreach' performance bug happens in Unity3D. 
				// There are boxing and unboxing operations as carEnumerator.Current is object.
				Debug.Log ("cars: " + ((Car)carEnumerator.Current).Print ());
			}

			// Wrap to foreach statement.
			Debug.Log ("- Foreach Loop - ");
			foreach (Car c in chain) {
				Debug.Log ("cars: " + c.Print ());
			}
		}
	}
}