PassThroughPostfixExample.cs

using System;
using System.Collections;
					
public class Program
{
	// simulating a Harmony Passthrough Postfix
	// (run it here: https://dotnetfiddle.net/qMPu6m)
	//
	// A passthrough postfix can be used to execute code between yields
	// of an enumerator method. Normally, you cannot patch those methods
	// because the compiler turns them into hidden state machine classes
	// and patching the original method will just influence the small stub
	// method that creates an instance of that compiler generated class
	//
	// This example will visualize how you can use the fact that enumerators
	// are in fact "generator functions" that get their results "pulled" out
	// of them one-by-one instead of running like normal methods.
	//
	// Harmony uses this to wrap the original with the enumerator that is
	// generated when you create a PASSTHROUGH POSTFIX:
	
	// --------------------------------------------------------
	// static T Postfix(T varname, ...normal postfix arguments)
	// --------------------------------------------------------
	
	// This works for any T but is especially useful for T = IEnumerator
	
	public static void Main()
	{
		Console.WriteLine("\n# Original");
		var it1 = Original();
		while(it1.MoveNext())
			Console.WriteLine(it1.Current);
		
		Console.WriteLine("\n# With Passthrough Postfix)");
		var it2 = Postfix(Original());
		while(it2.MoveNext())
			Console.WriteLine(it2.Current);
	}
	
	static IEnumerator Original()
	{
		Console.WriteLine("Apple");
		yield return 10;
		Console.WriteLine("Banana");
		yield return 20;
		Console.WriteLine("Cherry");
		yield return 30;
		Console.WriteLine("Melon");
		yield return 40;
	}
	
	static IEnumerator Postfix(IEnumerator e)
	{
		var n = 0;
		while(e.MoveNext())
		{
			var element = e.Current;
			if (n == 2)
				Console.WriteLine("[PRE]");
			yield return element;
			if (n == 2)
				Console.WriteLine("[POST]");
			n += 1;
		}
	}
}

// # Original
// Apple
// 10
// Banana
// 20
// Cherry
// 30
// Melon
// 40
// 
// # With Passthrough Postfix)
// Apple
// 10
// Banana
// 20
// Cherry
// [PRE]
// 30
// [POST]
// Melon
// 40