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. example

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:

    [CompilerGenerated]
    private sealed class <M>d__0 : IEnumerator<object>, IDisposable, IEnumerator
    {
        private int <>1__state;
 
        private object <>2__current;
 
        public C <>4__this;
        // snipped

The following code, in SharpLab decompiles it as:

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();
    }
}

.. 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;
            }
        }

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.