Is there a C# System.Reactive version of RxJava's doOnSubscribe?

Question

In RxJava I tend to use Observable.doOnSubscribe to log when an observable is subscribed (to know when some work to create\fetch data is happening) to and found it useful to catch mistakes on when certain heavy work is invoked.

The Do() operator does seem to provide doOnNext(), doOnError(), doOnCompleted() RxJava functionality however uniess I'm missing it, it doesn't seem to provide functionality similar to doOnSubscribe().

I could add logging to the create\fetch data code however often this could be an Observable sourced via a 3rd party library and thus not as convenient vs having an operator such as RxJava's doOnSubscribe() it seems.

Am I missing the C# version of doOnSubscribe() or is there an alternative that would solve my needs?

Answer 1

Just use Observable.Defer():

var someObservable = ...;
var newObservable = Observable.Defer(() =>
{
    Console.WriteLine("subscribed!");
    return someObservable;
});

You can make your own extension if you wish:

public static IObservable<T> DoOnSubscribe(this IObservable<T> source, Action action)
{
    return Observable.Defer(() =>
    {
        action();
        return source;
    });
}