uniquake/Assets/Scripts/CallbackHandler.cs

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

public abstract class CallbackHandler<THandler>
    where THandler: CallbackHandler<THandler>
{
    private GCHandle selfHandle;
    private GCHandle callbacksHandle;

    public IntPtr ToIntPtr => callbacksHandle.AddrOfPinnedObject();
    
    protected IntPtr SelfPtr => GCHandle.ToIntPtr(selfHandle);

    protected CallbackHandler()
    {
        selfHandle = GCHandle.Alloc(this);
    }
    
    public virtual void Destroy()
    {
        if (callbacksHandle.IsAllocated)
            callbacksHandle.Free();
        
        selfHandle.Free();
    }
    
    protected IntPtr CreateCallback<TDelegate>(TDelegate callback)
    {
        return Marshal.GetFunctionPointerForDelegate(callback);
    }

    protected void RegisterCallbacks<TCallbacks>(TCallbacks callbacks)
    {
        // Pin the callbacks struct so that native code can safely keep a pointer to it and re-use it on subsequent calls
        callbacksHandle = GCHandle.Alloc(callbacks, GCHandleType.Pinned);
    }
    
    protected static THandler GetSelf(IntPtr target)
    {
        return (THandler)GCHandle.FromIntPtr(target).Target;
    }
}