pardeike-Harmony/Documentation/examples/patching-transpiler.cs

namespace Patching_Transpiler
{
	using HarmonyLib;
	using System.Collections.Generic;
	using System.Linq;
	using System.Reflection;
	using System.Reflection.Emit;

	class TypicalExample
	{
		// <typical>
		static FieldInfo f_someField = AccessTools.Field(typeof(SomeType), nameof(SomeType.someField));
		static MethodInfo m_MyExtraMethod = SymbolExtensions.GetMethodInfo(() => Tools.MyExtraMethod());

		// looks for STDFLD someField and inserts CALL MyExtraMethod before it
		static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions)
		{
			var found = false;
			foreach (var instruction in instructions)
			{
				if (instruction.StoresField(f_someField))
				{
					yield return new CodeInstruction(OpCodes.Call, m_MyExtraMethod);
					found = true;
				}
				yield return instruction;
			}
			if (found is false)
				ReportError("Cannot find <Stdfld someField> in OriginalType.OriginalMethod");
		}
		// </typical>

#pragma warning disable CS0649
		class SomeType { public string someField; }
#pragma warning restore CS0649

		class Tools
		{
			public static MethodInfo MyExtraMethod() => null;
		}

		static void ReportError(string s) { }
	}

	class CaravanExample
	{
		// <caravan>
		[HarmonyPatch(typeof(Dialog_FormCaravan))]
		[HarmonyPatch(nameof(Dialog_FormCaravan.CheckForErrors))]
		public static class Dialog_FormCaravan_CheckForErrors_Patch
		{
			static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions)
			{
				var foundMassUsageMethod = false;
				var startIndex = -1;
				var endIndex = -1;

				var codes = new List<CodeInstruction>(instructions);
				for (var i = 0; i < codes.Count; i++)
				{
					if (codes[i].opcode == OpCodes.Ret)
					{
						if (foundMassUsageMethod)
						{
							Log.Error("END " + i);

							endIndex = i; // include current 'ret'
							break;
						}
						else
						{
							Log.Error("START " + (i + 1));

							startIndex = i + 1; // exclude current 'ret'

							for (var j = startIndex; j < codes.Count; j++)
							{
								if (codes[j].opcode == OpCodes.Ret)
									break;
								var strOperand = codes[j].operand as string;
								if (strOperand == "TooBigCaravanMassUsage")
								{
									foundMassUsageMethod = true;
									break;
								}
							}
						}
					}
				}
				if (startIndex > -1 && endIndex > -1)
				{
					// we cannot remove the first code of our range since some jump actually jumps to
					// it, so we replace it with a no-op instead of fixing that jump (easier).
					codes[startIndex].opcode = OpCodes.Nop;
					codes.RemoveRange(startIndex + 1, endIndex - startIndex - 1);
				}

				return codes.AsEnumerable();
			}
		}
		// </caravan>

		class Dialog_FormCaravan { public void CheckForErrors() { } }

		class Log
		{
			public static void Error(string s) { }
		}
	}
}