Why coroutines inherit from IEnumerator?

How does inheriting from IEnumerator make the method to behave like a coroutine? This has to be somewhat related to the compiler, right?

Code (CSharp):

1.     IEnumerator Fade()
2.     {
3.         for (float ft = 1f; ft >= 0; ft -= 0.1f)
4.         {
5.             Color c = renderer.material.color;
6.             c.a = ft;
7.             renderer.material.color = c;
8.             yield return null;
9.         }
10.     }

 

IEnumerator is the return type, it's not inheriting from it.

 

Unity uses IEnumerators in a different/highly specialized way from their intended use, which is more along the lines of running through a list without fully blocking execution of all other code. It's a bit of a hack that is used this way because they're the only C# concept that allows you to stop code execution in the middle of a function, then resume it later.

It's really not IEnumerator that carries out coroutines' magic, but rather StartCoroutine. The actual call to Fade will immediately run the code up to the first yield return, then stop. StartCoroutine then takes that "yield returned" function, and sort of shuffles it onto a pile of "things to worry about later". In the next frame, the MonoBehaviour will go to this pile and, once again, run all the code in it until it comes to the next "yield return" (which is why, in a stacktrace, you'll see "MoveNext" at the top of any coroutine code). It's Unity doing all this work, not the IEnumerator type itself; the IEnumerator is just the foot in the door to allow this kind of interrupted code.

 

Aha! So it's the StartCoroutine who decides whether to call a method or not, and the "IEnumerator / yield" is used just because there is no other way to stop the method execution! That's smart

For anyone in the future, here is a quick example that may explain it even further:

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class CorSample : MonoBehaviour
6. {
7.     // Start is called before the first frame update
8.     void Start()
9.     {
10.         Numbers numbers = new Numbers();
11.         foreach (int n in numbers)
12.         {
13.             Debug.Log("n: " + n);
14.             Debug.Log("and i am here: " + n);
15.         }
16.         Debug.Log("End");
17.     }
18. }
19.  
20. class Numbers
21. {
22.     public IEnumerator GetEnumerator()
23.     {
24.         for (int i = 0; i < 6; i++)
25.         {
26.             yield return i * i;
27.             Debug.Log("so i am here: " + i);
28.         }
29.     }
30. }
31.