System.NullReferenceException for IL hooks in specific places

[Dual-Iron]

Description

In specific cases, a System.NullReferenceException exception will be thrown from an ILHook's generated DMD (and sometimes FastDelegateInvokers.Invoke<...>) when using a custom delegate type for an instance method.

I got this to occur by doing the following:

1. In an assembly A, IL hook an arbitrary method and call ILCursor.EmitDelegate<CustomDelegate>(someInstanceMethod)
2. Call DetourModManager.Unload on A
3. Load a new assembly, B, that shares the same name and version with A
4. Hook the same method in the same way as A

This is when running Unity v4.6.3/Mono, targeting .NET Framework 3.5. Make sure that CustomDelegate is a delegate from your assembly, not e.g. Action or Func. Also make sure that someInstanceMethod is an instance method.

Example

delegate void A();

string Words = "Hi";

void PrintWords() {
    Console.WriteLine(Words);
}

void Enable() {
    IL.Room.Update += MyILHook;
}

void MyILHook(ILContext il) {
    ILCursor cursor = new(il);
    cursor.EmitDelegate<A>(PrintWords);

    // These also bug out because they compile as instance methods:
    // cursor.EmitDelegate<A>(() => Console.WriteLine(Words));
    // cursor.EmitDelegate<A>(() => Console.WriteLine("Hi"));
}

Before step 2, this runs fine. When Room.Update is called, Hi gets printed to the console.

After step 4, however, whenever Room.Update is called, you'll instead get a nasty exception:

System.NullReferenceException: Object reference not set to an instance of an object
at (wrapper dynamic-method) Room.DMD<Room..Update> (Room) <0x0008c>
etc