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====== IEnumerator ======

Notes on the underlying implementation of C# generators (which are used in Unity for coroutines).

You can use SharpLab to view the decompiled C# from a simple generator, to help understand the transformation that is happening. Both .NET and Mono do seem to use the same compiler transformation when turning Generators into anonymous IEnumerator classes. [[https://sharplab.io/#v2:EYLgtghgzgLgpgJwDQBMQGoA+ABATARgFgAobABgAJt8A6AYQHsAbJuAYxgEsGA7KAbhLYAzFVwU6FAN4kKcigHoFIipx4wKAD0HF5FWft3yVASQCiPAK5hEEGAwQUAsgAoAlNIN651fFQDsFPg63sb4ftiBuCGhVOEBFMIxob4JACzJFAC+XoZ6phbWtvaOACpwsC7YAKwAPMAMzAB8FABmljxsHjJGKfGRbR1s7pk5xFlAA===|example]]

===== States =====

Generators are compiled down into a state machine, with states for each ''yield return'' point. ie:
  * 0: The generator has not yet been run.
  * 1: The generator has paused on the first yield return.
  * 2: The generator has paused on the second yield return.
  * -1: The generator has finished, the generator exited early, or the generator threw an exception.

The state value is set in a field ''private int <>1__state'' on the generated IEnumerator class:
<code c#>
    [CompilerGenerated]
    private sealed class <M>d__0 : IEnumerator<object>, IDisposable, IEnumerator
    {
        private int <>1__state;

private object <>2__current;

public C <>4__this;
        // snipped
</code>

===== Lambda function DisplayClass classes are initialized at the start of the function =====
The following code, in SharpLab decompiles it as:
<code c#>
using System;
using System.Collections;
public class C {
    public IEnumerator M() {
        int x = 5;
        yield return Test(() => x > 5);
        yield return 5;
        yield return 5;
        yield return 5;
    }
    
    public IEnumerator Test(Func<bool> func) {
        yield return func();
    }
}
</code>
<code>
.. snipped
private bool MoveNext()
        {
            switch (<>1__state)
            {
                default:
                    return false;
                case 0:
                    <>1__state = -1;
                    <>8__1 = new <>c__DisplayClass0_0();
                    <>8__1.x = 5;
                    <>2__current = 5;
                    <>1__state = 1;
                    return true;
                case 1:
                    <>1__state = -1;
                    <>2__current = 5;
                    <>1__state = 2;
                    return true;
                case 2:
                    <>1__state = -1;
                    <>2__current = <>4__this.Test(new Func<bool>(<>8__1.<M>b__0));
                    <>1__state = 3;
                    return true;
                case 3:
                    <>1__state = -1;
                    <>2__current = 5;
                    <>1__state = 4;
                    return true;
                case 4:
                    <>1__state = -1;
                    return false;
            }
        }
</code>

The lambda function is stored as a local variable ''<>8__1'' in the generated IEnumerator class. However, the instance is created at the start of the function, not just before usage, as one might expect.

===== All lambdas are piled into a single DisplayClass anonymous type, and share the same instance. =====

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