Harmony Patching Transpilers - Identifying a Switch Statement and it's cases

switch (i)
{
    case 1:
        thing;
        break;
    case 2:
        thing2;
        break;
    case 3:
        thing3;
        break;
    default:
        other;
        break;
}

switch (i)
{
    case 1:
        thing;
        break;
    case 2:
        thing2;
        break;
    case 3:
        thing3;
        break;
    default:
        other;
        break;
}

ldarg i
ldc.i4.1
sub // subtract 1 to make it start at 0
switch (case1Label, case2Label, case3Label)

// default case is here, its when you get when the value is > N, it falls through to here
// other
br end

case1Label:
// thing1
br end

case2Label:
// thing2
br end

case3Label:
// thing3
br end

end:

ldarg i
ldc.i4.1
sub // subtract 1 to make it start at 0
switch (case1Label, case2Label, case3Label)

// default case is here, its when you get when the value is > N, it falls through to here
// other
br end

case1Label:
// thing1
br end

case2Label:
// thing2
br end

case3Label:
// thing3
br end

end:

C#
        static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions, ILGenerator generator)
        {
            var codes = new List<CodeInstruction>(instructions);

            var fileLogMethod = AccessTools.Method(typeof(System.IO.File), nameof(System.IO.File.AppendAllText), new[] { typeof(string), typeof(string) });

            codes.Insert(321, new CodeInstruction(OpCodes.Ldstr, @"C:\temp\harmony_log.txt")); //I couldn't get console logging to work, file logging instead.
            codes.Insert(322, new CodeInstruction(OpCodes.Ldstr, "Case 1\n"));
            codes.Insert(323, new CodeInstruction(OpCodes.Call, fileLogMethod));

            return codes;
        }

C#
        static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions, ILGenerator generator)
        {
            var codes = new List<CodeInstruction>(instructions);

            var fileLogMethod = AccessTools.Method(typeof(System.IO.File), nameof(System.IO.File.AppendAllText), new[] { typeof(string), typeof(string) });

            codes.Insert(321, new CodeInstruction(OpCodes.Ldstr, @"C:\temp\harmony_log.txt")); //I couldn't get console logging to work, file logging instead.
            codes.Insert(322, new CodeInstruction(OpCodes.Ldstr, "Case 1\n"));
            codes.Insert(323, new CodeInstruction(OpCodes.Call, fileLogMethod));

            return codes;
        }

C#
        static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions, ILGenerator generator)
        {
            var codes = new List<CodeInstruction>(instructions);

            var fileLogMethod = AccessTools.Method(typeof(System.IO.File), nameof(System.IO.File.AppendAllText), new[] { typeof(string), typeof(string) });

            var broadcastMethod = AccessTools.Method(typeof(PlayerControlsHandlerPatch), nameof(PlayerControlsHandlerPatch.BroadcastModdedItemUsed));

            int index = 0;

            for(int i = 0; i < codes.Count; i++)
            {
                if (codes[i].opcode == OpCodes.Switch)
                {
                    index = i; break; //Find the switch opcode
                }
            }

            codes.Insert(index + 1, new CodeInstruction(OpCodes.Ldloca_S, 1));
            codes.Insert(index + 2, new CodeInstruction(OpCodes.Call, broadcastMethod));

            return codes;
        }

        public static void BroadcastModdedItemUsed(ref byte ID)
        {
            if (ID == 0) return;
            Console.WriteLine("Default Case");
            GetService<IItemService>()?.UseModdedItem();
            GetService<ModdingAPI>()?.Notify(new ModdedItemUsedEventArgs());
            ID = 1;
        }

C#
        static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions, ILGenerator generator)
        {
            var codes = new List<CodeInstruction>(instructions);

            var fileLogMethod = AccessTools.Method(typeof(System.IO.File), nameof(System.IO.File.AppendAllText), new[] { typeof(string), typeof(string) });

            var broadcastMethod = AccessTools.Method(typeof(PlayerControlsHandlerPatch), nameof(PlayerControlsHandlerPatch.BroadcastModdedItemUsed));

            int index = 0;

            for(int i = 0; i < codes.Count; i++)
            {
                if (codes[i].opcode == OpCodes.Switch)
                {
                    index = i; break; //Find the switch opcode
                }
            }

            codes.Insert(index + 1, new CodeInstruction(OpCodes.Ldloca_S, 1));
            codes.Insert(index + 2, new CodeInstruction(OpCodes.Call, broadcastMethod));

            return codes;
        }

        public static void BroadcastModdedItemUsed(ref byte ID)
        {
            if (ID == 0) return;
            Console.WriteLine("Default Case");
            GetService<IItemService>()?.UseModdedItem();
            GetService<ModdingAPI>()?.Notify(new ModdedItemUsedEventArgs());
            ID = 1;
        }

ldarg.0 // the class instance to store into
ldloc.1
call uint8 YourHelper(uint8)
stfld theField 

ldarg.0 // the class instance to store into
ldloc.1
call uint8 YourHelper(uint8)
stfld theField 

C#
    int index = 0;
    int fldIndex = 0;

    for(int i = 0; i < codes.Count; i++)
    {
        if (codes[i].opcode == OpCodes.Switch)
        {
            index = i; 
            for(int j = index; j > 1; j--)
            {
                if (codes[j].opcode == OpCodes.Ldfld)
                {
                    fldIndex = j;
                    break;
                }
            }
            break; //Find the switch opcode
        }
    }

    var fldOperand = codes[fldIndex].operand;

    codes.Insert(index + 1, new CodeInstruction(OpCodes.Ldloc_0));
    codes.Insert(index + 2, new CodeInstruction(OpCodes.Ldloca_S, (byte)1));
    codes.Insert(index + 3, new CodeInstruction(OpCodes.Call, broadcastMethod));
    codes.Insert(index + 4, new CodeInstruction(OpCodes.Stfld, fldOperand));

    return codes;
}

public static byte BroadcastModdedItemUsed(ref byte ID)
{
    if (ID == 0) return 0;
    Console.WriteLine("Default Case");
    GetService<IItemService>()?.UseModdedItem();
    GetService<ModdingAPI>()?.Notify(new ModdedItemUsedEventArgs());
    return 1;
}

C#
    int index = 0;
    int fldIndex = 0;

    for(int i = 0; i < codes.Count; i++)
    {
        if (codes[i].opcode == OpCodes.Switch)
        {
            index = i; 
            for(int j = index; j > 1; j--)
            {
                if (codes[j].opcode == OpCodes.Ldfld)
                {
                    fldIndex = j;
                    break;
                }
            }
            break; //Find the switch opcode
        }
    }

    var fldOperand = codes[fldIndex].operand;

    codes.Insert(index + 1, new CodeInstruction(OpCodes.Ldloc_0));
    codes.Insert(index + 2, new CodeInstruction(OpCodes.Ldloca_S, (byte)1));
    codes.Insert(index + 3, new CodeInstruction(OpCodes.Call, broadcastMethod));
    codes.Insert(index + 4, new CodeInstruction(OpCodes.Stfld, fldOperand));

    return codes;
}

public static byte BroadcastModdedItemUsed(ref byte ID)
{
    if (ID == 0) return 0;
    Console.WriteLine("Default Case");
    GetService<IItemService>()?.UseModdedItem();
    GetService<ModdingAPI>()?.Notify(new ModdedItemUsedEventArgs());
    return 1;
}