Observable collection OnNext not firing - system.reactive

Question

I have this very simple observable collection, and OnNext is not firing.
List<int> intList = new List<int>(){1,2,3};
IObservable<int> observableList = intList.ToObservable();
IDisposable subscription = observableList.Subscribe(
x => Console.WriteLine("Received {0} from source.", x),
ex => Console.WriteLine( "OnError: " + ex.Message ),
( ) => Console.WriteLine( "OnCompleted" )
);
intList.Add(4);
The output I am getting is as follows.
Received 1 from source.
Received 2 from source.
Received 3 from source.
OnCompleted
I am expecting "Received 4 from source." after I add 4 to list.
Can someone please throw some light on where I am doing wrong. I am new Rx

Answer 1

This all depends on your order of operations.
If you structure your code like this:
List<int> intList = new List<int>() { 1, 2, 3 };
IObservable<int> observableList = intList.ToObservable();
intList.Add(4);
IDisposable subscription =
observableList
.Subscribe(
x => Console.WriteLine("Received {0} from source.", x),
ex => Console.WriteLine("OnError: " + ex.Message),
() => Console.WriteLine("OnCompleted"));
...then it works as you expect.
The issue is that the .Subscribe is run on the current thread for .ToObservable(). The actual code run is return (IObservable<TSource>) new ToObservable<TSource>(source, SchedulerDefaults.Iteration);. The SchedulerDefaults.Iteration is the current thread.
You can see this with this code:
List<int> intList = new List<int>() { 1, 2, 3 };
IObservable<int> observableList = intList.ToObservable();
Console.WriteLine("Before Subscription");
IDisposable subscription =
observableList
.Subscribe(
x => Console.WriteLine("Received {0} from source.", x),
ex => Console.WriteLine("OnError: " + ex.Message),
() => Console.WriteLine("OnCompleted"));
Console.WriteLine("After Subscription, Before Add");
intList.Add(4);
Console.WriteLine("After Add");
When I run it I get:
Before Subscription
Received 1 from source.
Received 2 from source.
Received 3 from source.
OnCompleted
After Subscription, Before Add
After Add
So the .Add hasn't even happened until after the subscription is complete.
Now, if I try to get around this by changing the code to intList.ToObservable(Scheduler.Default) then I get a new problem. Running my above code I get this:
Before Subscription
After Subscription, Before Add
After Add
Received 1 from source.
OnError: Collection was modified; enumeration operation may not execute.
Now clearly, we have a concurrency issue. You shouldn't try to manipulate collections and iterate them at the same time.

Answer 2

This is simply because the .ToObservable() on a List will ONLY give you the current items in the list each time you Subscribe to it, without ongoing notifications of added items. The same could be said for a read-only collection which implements IEnumerable.
There are other collections you can use instead which will work as expected.
e.g. ObservableCollection
Alternatively, you could find any collection type which gives change notifications (collection.Added += etc...) and use Observable.FromEvent to hook up subsequent notifications.
It is also worth understanding that IEnumerable.ToObservable is a cold observable and therefore, why the order of subscription would also matter (as per the first answer).