gistfile1.cs

public static Func<T1, IObservable<TRet>> FromRestSharpPattern<T1, TRet>(Action<T1, Action<TRet>> restSharpFunc)
{
    return new Func<T1, IObservable<TRet>>(param => {
        var retObs = new AsyncSubject<TRet>();

        try {
            restSharpFunc(param, returnValue => { 
                retObs.OnNext(returnValue);
                retObs.OnCompleted();
            });
        } catch (Exception ex) {
            retObs.OnError(ex);
        }

        return retObs;
    });
}