Modding Tutorials/Troubleshooting/Finding Exceptions

Preamble:

I try to use proper C# terminology where possible, if a term confuses you, I urge you to just google it to further your understanding of the language. This approach can be used to track down any type of exception, but the one you are most likely to encounter in RW modding will be a NRE (NullReferenceException), meaning some code is trying to access a member of a NULL instance, this tutorial will elaborate on this exact scenario.

What you need:

1. An exception
2. A decompiler of your choice

Example

This example adds a debug action for my convenience to call another method and an actual static method that spawns potatoes on the same cell as the pawn is standing on

using RimWorld;
using Verse;

namespace ExceptionShowcase
{

    public static class ExceptionShowcase
    {
        /// <summary>
        /// This is a debug action, it shows up in the debug actions menu of your game. This specific one is a ToolMapForPawns, which means the method is passed the Pawn p that was targeted by the user
        /// You don't need this, it's just an easy way for me to make this showcase execute a method on-demand
        /// </summary>
        /// <param name="p">The automatically assigned pawn that you select with the mouse</param>
        [DebugAction("Showcase", "Spawn Potatoes", actionType = DebugActionType.ToolMapForPawns, allowedGameStates = AllowedGameStates.PlayingOnMap)]
        public static void MyDebugAction(Pawn p)
        {
            SpawnPotatoesOnPawn(p);
        }

        /// <summary>
        /// This is the actual method that we are working on
        /// </summary>
        /// <param name="pawn">The pawn to spawn potatoes on</param>
        public static void SpawnPotatoesOnPawn(Pawn pawn)
        {
            // only spawn the potatoes if the pawn can eat them
            bool willEat = FoodUtility.WillEat(pawn, ThingDefOf.RawPotatoes);

            if(!willEat)
            {
                // if the pawn would not eat the potatoes, don't spawn them and just exit the method
                return;
            }

            // get the position and map of the pawn
            IntVec3 position = pawn.Position;
            Map map = pawn.Map;

            // GenSpawn will make a new item of the def and put it on the map and position that we specify
            GenSpawn.Spawn(ThingDefOf.RawPotatoes, position, map);
        }
    }
}

It executes as expected, I am presented with a DebugAction in the menu, it spawns a Targeter, I select a pawn and it spawns potatoes on that pawns position.