Game Hooks

Introduction

Game Hooks allow you to modify the game's code without replacing any files.
They are done using a patched Assembly-CSharp that contains events for each method in the game. You can get this patched file from here.
Once installed, you can put it in the BepInEx\core directory and reference it in your project.

Creating the hook

To add a new hook you use On.Type.Method in your mod's Awake method. For example:

private void Awake()
{
    On.LevelPlayerMotor.DashComplete += OnDash;
}

private void OnDash(On.LevelPlayerMotor.orig_DashComplete orig, LevelPlayerMotor self)
{
    orig(self);
    //Executed after the player dashes.
}

Here, self is the instance of the class you hooked into, and orig is a delegate for the original method.

INFO

The hook replaces the method completely, that's why we call orig(self); to call the original method. You may execute your own code before or after calling orig, depending on your needs.