CancellationTokenExtensions.cs

public static class CancellationTokenExtensions {

    /// <summary>
    /// Creates an observable sequence from a cancellationToken
    /// </summary>
    /// <param name="cancellationToken">token to observe</param>
    /// <param name="throwOperationCanceledException">true to terminate the sequence with an exception instead of emitting a value</param>
    /// <returns>an observable sequence that emits a value when the token is fired</returns>
    public static IObservable<Unit> ToObservable (this CancellationToken cancellationToken, bool throwOperationCanceledException = false) {
        if (!cancellationToken.CanBeCanceled) {
            return Observable.Never<Unit> ();
        }

        Func<IObserver<Unit>, IDisposable> subscribe;

        if (throwOperationCanceledException) {
            subscribe = observer => cancellationToken.Register (() => observer.OnError (new OperationCanceledException (cancellationToken)));
        }
        else {
            subscribe = observer => cancellationToken.Register (() => {
                                                                    observer.OnNext (Unit.Default);
                                                                    observer.OnCompleted ();
                                                                });
        }

        return Observable.Create (subscribe);
    }

}