ObservableCancellationTokenSource.cs

 public class ObservableCancellationTokenSource<T> : CancellationTokenSource,IDisposable
    {
        private readonly IDisposable _subscription;

        public ObservableCancellationTokenSource(IObservable<T> observable)
        {
            _subscription = observable.Subscribe(token=>this.Cancel());
            
        }

  
        public new void Dispose()
        {
            _subscription.Dispose();
            base.Dispose();
        }
    }
     public static class Extension
    {
        public static ObservableCancellationTokenSource<T> ToCancellationTokenSource<T>(this IObservable<T> source)
        {
            return new ObservableCancellationTokenSource<T>(source);
        }
    }