public sealed class ReactiveStubObserver : IStubObserver
{

    readonly ISubject<Tuple<Type, Delegate>> _subject1;
    readonly ISubject<Tuple<Type, Delegate, object>> _subject2;
    readonly ISubject<Tuple<Type, Delegate, object, object>> _subject3;
    readonly ISubject<Tuple<Type, Delegate, object, object, object>> _subject4;
    readonly ISubject<Tuple<Type, Delegate, object[]>> _subject5;

    public ReactiveStubObserver(IObserver<Tuple<Type, Delegate, object[]>> observer) : this(CreateSubject(observer))
    {
    }

    public ReactiveStubObserver(ISubject<Tuple<Type, Delegate, object[]>> subject) : this(ToSubject1(subject), ToSubject2(subject), ToSubject3(subject), ToSubject4(subject), subject)
    {
    }

    public ReactiveStubObserver(ISubject<Tuple<Type, Delegate>> subject1, ISubject<Tuple<Type, Delegate, object>> subject2, ISubject<Tuple<Type, Delegate, object, object>> subject3, ISubject<Tuple<Type, Delegate, object, object, object>> subject4, ISubject<Tuple<Type, Delegate, object[]>> subject5)
    {
        _subject1 = subject1;
        _subject2 = subject2;
        _subject3 = subject3;
        _subject4 = subject4;
        _subject5 = subject5;
    }

    public void Enter(Type stubbedType, Delegate stubCall)
    {
        _subject1.OnNext(new Tuple<Type, Delegate>(stubbedType, stubCall));
    }

    public void Enter(Type stubbedType, Delegate stubCall, object arg1)
    {
        _subject2.OnNext(new Tuple<Type, Delegate, object>(stubbedType, stubCall, arg1));
    }

    public void Enter(Type stubbedType, Delegate stubCall, object arg1, object arg2)
    {
        _subject3.OnNext(new Tuple<Type, Delegate, object, object>(stubbedType, stubCall, arg1, arg2));
    }

    public void Enter(Type stubbedType, Delegate stubCall, object arg1, object arg2, object arg3)
    {
        _subject4.OnNext(new Tuple<Type, Delegate, object, object, object>(stubbedType, stubCall, arg1, arg2, arg3));
    }

    public void Enter(Type stubbedType, Delegate stubCall, params object[] args)
    {
        _subject5.OnNext(new Tuple<Type, Delegate, object[]>(stubbedType, stubCall, args));
    }

    static ISubject<Tuple<Type, Delegate, object[]>> CreateSubject(IObserver<Tuple<Type, Delegate, object[]>> observer)
    {
        var subject = new Subject<Tuple<Type, Delegate, object[]>>();
        subject.Subscribe(observer);
        return subject;
    }

    static ISubject<Tuple<Type, Delegate>> ToSubject1(ISubject<Tuple<Type, Delegate, object[]>> subject)
    {
        var observer = Observer.Create<Tuple<Type, Delegate>>(value => subject.OnNext(new Tuple<Type, Delegate, object[]>(value.Item1, value.Item2, new object[0])));
        var observable = subject.Select(value => new Tuple<Type, Delegate>(value.Item1, value.Item2));
        return Subject.Create<Tuple<Type, Delegate>>(observer, observable);
    }

    static ISubject<Tuple<Type, Delegate, object>> ToSubject2(ISubject<Tuple<Type, Delegate, object[]>> subject)
    {
        var observer = Observer.Create<Tuple<Type, Delegate, object>>(value => subject.OnNext(new Tuple<Type, Delegate, object[]>(value.Item1, value.Item2, new[] { value.Item3 })));
        var observable = subject.Select(value => new Tuple<Type, Delegate, object>(value.Item1, value.Item2, value.Item3.FirstOrDefault()));
        return Subject.Create<Tuple<Type, Delegate, object>>(observer, observable);
    }

    static ISubject<Tuple<Type, Delegate, object, object>> ToSubject3(ISubject<Tuple<Type, Delegate, object[]>> subject)
    {
        var observer = Observer.Create<Tuple<Type, Delegate, object, object>>(value => subject.OnNext(new Tuple<Type, Delegate, object[]>(value.Item1, value.Item2, new[] { value.Item3, value.Item4 })));
        var observable = subject.Select(value => new Tuple<Type, Delegate, object, object>(value.Item1, value.Item2, value.Item3.FirstOrDefault(), value.Item3.Skip(1).FirstOrDefault()));
        return Subject.Create<Tuple<Type, Delegate, object, object>>(observer, observable);
    }

    static ISubject<Tuple<Type, Delegate, object, object, object>> ToSubject4(ISubject<Tuple<Type, Delegate, object[]>> subject)
    {
        var observer = Observer.Create<Tuple<Type, Delegate, object, object, object>>(value => subject.OnNext(new Tuple<Type, Delegate, object[]>(value.Item1, value.Item2, new[] { value.Item3, value.Item4, value.Item5 })));
        var observable = subject.Select(value => new Tuple<Type, Delegate, object, object, object>(value.Item1, value.Item2, value.Item3.FirstOrDefault(), value.Item3.Skip(1).FirstOrDefault(), value.Item3.Skip(2).FirstOrDefault()));
        return Subject.Create<Tuple<Type, Delegate, object, object, object>>(observer, observable);
    }

}