generator.cs

using System;

interface Generator<A,Y,R>
    where Y : struct
    where R : struct
{
    unsafe bool Resume(A a, System.Span<Y> y, System.Span<R> r);
}

struct None {
    public static None Value = new None();
}

class FibTo : Generator<None, int, None> {
    public int n;
    public int pos;
    public int n1;
    public int n2;
    
    public unsafe bool Resume(None _none,  System.Span<int> y, System.Span<None> _r) {
        if (this.pos == this.n) {
            return true;
        }
        
        var next = n1 + n2;
        
        this.n2 = n1;
        this.n1 = next;
        this.pos++;
        y[0] = next;
        return false;
    }
    
    public static FibTo To(int n) {
        return new FibTo(){n = n, pos = 0, n1 = 1, n2 = 0};
    }
}



static class Generator {
    public static void Run<Y, R,G>(G gen) 
        where G : class, Generator<None, Y, R> 
        where R : struct
        where Y : struct
    {
        unsafe {
            Span<byte> yb = stackalloc byte[System.Runtime.InteropServices.Marshal.SizeOf(typeof(Y))];
            Span<byte> yr = stackalloc byte[System.Runtime.InteropServices.Marshal.SizeOf(typeof(R))];
            
            Span<Y> y = System.Runtime.InteropServices.MemoryMarshal.Cast<byte, Y>(yb);
            Span<R> r = System.Runtime.InteropServices.MemoryMarshal.Cast<byte, R>(yr);


            bool finished;
            do
            {
                finished = gen.Resume(None.Value, y, r);
                if (!finished){
                    Console.WriteLine($"yield: {y[0]}");
                }
            }
            while (!finished);
            Console.WriteLine($"return: {r[0]}");
        }        
    }
}

static class Program {
  public static int Main(string[] args) {
    Generator.Run<int, None, FibTo>(FibTo.To(10));
  }
}

yield: 1
yield: 2
yield: 3
yield: 5
yield: 8
yield: 13
yield: 21
yield: 34
yield: 55
yield: 89
return: Submission#2+None