using System; 

using HarmonyLib;

​

namespace PatchTarget {

  public abstract class QSingleton<T> where T : QSingleton<T>, new() {

    public static T instance = null; protected QSingleton() { } 

​

    public static T Instance { get {

      if (instance == null) {

        instance = new T();

        Console.WriteLine($"{typeof(T).Name}.Instance: impl=QSingleton");

      }

      return instance; 

    } }

  }

​

  public class SampleA : QSingleton<SampleA> {

    public SampleA() { Console.WriteLine("SampleA ctor"); }

  }

​

  public class SampleB : QSingleton<SampleB> {

    public SampleB() { Console.WriteLine("SampleB ctor"); }

  }

}

​

namespace Patch {

  public class TypeHelper<T> where T : PatchTarget.QSingleton<T>, new() {

    public static T InstanceHack() {

      // var instance = PatchTarget.QSingleton<T>.instance;  // Crash, why?

      // Still crash in Unity real-world usecase?

      var instance = AccessTools.Field(typeof(T), "instance").GetValue(null) as T;

      // It seems that anything more than typeof(T).Name may crash.

      // Things like PatchTarget.QSingleton<T>.instance or FieldInfo.GetValue() definitely dooms, while FieldInfo sometimes seems ok.

      Console.WriteLine($"{typeof(T).Name}.Instance: impl=InstanceHack, instance={instance?.ToString() ?? "null"}");

      return null;

    }

  }

​

  public static class HarmonyPatch {

    public static Harmony harmony = new Harmony("Try");

​

    public static void init() {

      // Attribute does not work.

      // Transpiler does not work because the lack of TypeSpec to setup generic parameters.

      var miOriginal = AccessTools.Property(typeof(PatchTarget.QSingleton<PatchTarget.SampleB>), "Instance").GetMethod;

      var miHack = AccessTools.Method(typeof(TypeHelper<PatchTarget.SampleB>), "InstanceHackPrefix");

      harmony.Patch(miOriginal, prefix: new HarmonyMethod(miHack));

    }

  }

​

  public static class MonoModPatch {

    public static MonoMod.RuntimeDetour.Detour sHook;

​

    public static void init() {

      var miOriginal = AccessTools.Property(typeof(PatchTarget.QSingleton<PatchTarget.SampleB>), "Instance").GetMethod;

      var miHack = AccessTools.Method(typeof(TypeHelper<PatchTarget.SampleB>), "InstanceHack");

      sHook = new MonoMod.RuntimeDetour.Detour(miOriginal, miHack);

    }

  }

}

​

class Program {

  public static void Main() {

    _ = PatchTarget.SampleA.Instance;

    // Both Harmony and MonoMod works.

    // try { Patch.HarmonyPatch.init(); } catch (Exception e) { Console.WriteLine($"Harmony error: {e.ToString()}"); }

    try { Patch.MonoModPatch.init(); } catch (Exception e) { Console.WriteLine($"MonoMod error: {e.ToString()}"); }

    _ = PatchTarget.SampleB.Instance;

    _ = PatchTarget.SampleA.Instance;

    Console.WriteLine($"done");

  }

}