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 List<GCHandle> delegateHandles = new List<GCHandle>();

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

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

    protected static THandler GetSelf(IntPtr target)
    {
        return (THandler)GCHandle.FromIntPtr(target).Target;
    }
}