Create prepatchers.md

docs/content/pages/guides/prepatchers.md

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+---
+Title: Prepatchers
+Sort_Priority: 33
+---
+
+# Prepatchers
+
+In certain contexts you may need to edit game files before start. This is where
+prepatchers come in. Prepatchers are run by OWML directly before the game starts, allowing you to modify game files you would otherwise not be able to.
+
+To create a prepatcher you'll need a separate project from your mod. This can be done by creating a new project in your solution with the executable type, it should automatically build to your mod folder.
+
+Now in your mod manifest you need to set the `patcher` field to the path of the executable relative to the root of the
+mod folder.
+
+## Creating A Prepatcher
+
+A prepatcher is a simple console app that OWML executes, it's only passed the location of your mod folder.
+However, it is possible to get the game's location by doing `AppDomain.CurrentDomain.BaseDirectory`:
+
+```csharp
+public static void Main(string[] args)
+{
+    var modPath = args.Length > 0 ? args[0] : ".";
+    var gamePath = AppDomain.CurrentDomain.BaseDirectory;
+
+    Console.WriteLine($"Mod Path: {modPath}");
+    Console.WriteLine($"Game Path: {gamePath}");
+
+    // DoStuff(modPath, gamePath);
+}
+```
+
+Keep in mind the `ModHelper` is not available in a prepatcher.
+You'll need to have the prepatcher include libraries like Newtonsoft.Json to read and write JSON files.
+
+### Logging
+
+Due to a lack of `ModHelper`, you'll need to use `Console.WriteLine` to log information.
+This **will not output to the manager window**. To test prepatchers, we recommend you launch `OWML.Launcher.exe` in a
+terminal directly to properly see stdout.
+
+If a prepatcher errors it *should usually* be outputted to the manage window as OWML is setup to catch and
+log any exceptions thrown by the prepatcher.
+
+### Warnings
+
+Due to the nature of prepatchers, the manager cannot undo changes made by them. This means the game will continue to be modified even if the
+mod is uninstalled or disabled.
+
+The manager will try it's best to warn the user of this. If your mod has prepatcher and
+is disabled or uninstalled the manager will show a dialog explaining that
+your mod has modified game files in an irreversible way and encourages them to validate the game files.