By using ActionResult, you can yield an inline IResult by constructing it with a delegate (Action) via Lambda Expression. You will receive an action as parameter, which you must invoke in order to do the completion of the IResult.

ActionResult.cs

//Usage:

        public IEnumerable<IResult> Logout()
        {
            yield return new ActionResult(completed =>
            {
                var operation = WebContext.Current.Authentication.Logout(false);
                operation.Completed += (sender, args) => completed();
            });
        }


//usage with unsubscription of the event

        public IEnumerable<IResult> Logout()
        {
            yield return new ActionResult(completed =>
            {
                var operation = WebContext.Current.Authentication.Logout(false);
                EventHandler operationOnCompleted = null;
                operationOnCompleted = (sender, args) =>
                {
                    operation.Completed -= operationOnCompleted;
                    completed();
                };
                operation.Completed += operationOnCompleted;
            });
        }


    public class ActionResult : IResult
    {
        private readonly Action<System.Action> _completed;

        public ActionResult(Action<System.Action> completed)
        {
            _completed = completed;
        }

        #region Implementation of IResult

        public void Execute(ActionExecutionContext context)
        {
            _completed(Invoke);
        }

        void Invoke()
        {
            Completed(this, new ResultCompletionEventArgs());
        }

        public event EventHandler<ResultCompletionEventArgs> Completed = delegate { };

        #endregion
    }