WrapperViewModel.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace EventHandlerLeakDemo
{
    public class WrapperViewModel : IDisposable
    {
        private readonly WeakEventSubscription<Service, string> _serviceSubscription;
        private bool _disposed;
        public Service Service { get; }
        public string? LastValue { get; private set; }

        public WrapperViewModel(Service service)
        {
            Service = service ?? throw new ArgumentNullException(nameof(service));

            _serviceSubscription = new WeakEventSubscription<Service, string>(
                source: service,
                attach: (s, h) => s.DataChanged += h,
                detach: (s, h) => s.DataChanged -= h,
                target: this,
                // Has to be static to safeguard creating strong references with captured variables in this class
                invoke: static (target, sender, args) =>
                {
                    ((WrapperViewModel)target).OnDataChanged(sender, args);
                });
        }

        private void OnDataChanged(object? sender, string payload)
        {
            LastValue = payload;
            Console.WriteLine($"WrapperViewModel received '{payload}' from {((Service)sender!).Name}");
        }

        public void Dispose()
        {
            if (_disposed) return;
            _serviceSubscription.Dispose();
            _disposed = true;
        }
    }
}