Test code for Excel-DNA IExcelObservable

ObservableClock.cs

using System;
using System.Diagnostics;
using System.Threading;
using ExcelDna.Integration;

namespace TestDisposable
{
    public static class RtdClock
    {
        [ExcelFunction(Description = "Provides a ticking clock")]
        public static object TestObservable(string param)
        {
            string functionName = nameof(TestObservable);
            object paramInfo = param; // could be one parameter passed in directly, or an object array of all the parameters: new object[] {param1, param2}
            return ExcelAsyncUtil.Observe(functionName, paramInfo, () => new ExcelObservableClock(param));
        }
    }

    class ExcelObservableClock : IExcelObservable
    {
        Timer _timer;
        IExcelObserver _observer;
        string _param;

        public ExcelObservableClock(string param)
        {
            Debug.WriteLine("Created " + param);
            _param = param;
            _timer = new Timer(timer_tick, null, 0, 1000);
        }

        public IDisposable Subscribe(IExcelObserver observer)
        {
            _observer = observer;
            observer.OnNext(DateTime.Now.ToString("HH:mm:ss.fff") + " (Subscribe)");
            return new ActionDisposable(() =>
            {
                _observer = null;
                Debug.WriteLine("Disposed " + _param);
            });
        }

        void timer_tick(object _)
        {
            string now = DateTime.Now.ToString("HH:mm:ss.fff");
            _observer?.OnNext(now);
        }

        class ActionDisposable : IDisposable
        {
            Action _disposeAction;
            public ActionDisposable(Action disposeAction)
            {
                _disposeAction = disposeAction;
            }
            public void Dispose()
            {
                _disposeAction();
            }
        }
    }
}