观察者不能捕获动态扩展方法中抛出的异常

Question

I created an extension to another class. I invoked the extension method, which throws exception. Unfortunately, my observer CANNOT catch that exception.

Anybody knows why?

public static class MyExtensions {
    public static async void myextfunc(this SomeClass some, dynamic data) {
        throw new Exception("Atrápame si puedes.");
    } // fin myextfunc()
} // fin class



SomeClass some = new SomeClass();
IObserver<dynamic> obsv = Observer.Create<dynamic>(async (d) => {
    some.myextfunc((object)d); // NO PUEDO atrapar la exceptión
}, (exception0) => {
    Console.WriteLine(e1.Message);
});
try { obsv.OnNext(123); } catch { /* no funciona */ }

Answer 1

Actually, this behavior doesn't depend on using extension method. It depends on using async function when a method expects synchronous delegate ( Observer.Create takes only Action< T > as an input for onNext).

First of all, you should never use async void because there is no way for caller to await it correctly (meaning that there will be no exception handling by try / catch for example, more details here ). But even if we try to change your code (adding Task instead of void return to your extension method and awaiting it in Observer's onNext) it won't work.

It happens exactly because of Observer.Create expecting Action< T >, not Func< Task >.

It's hard to understand what exactly you wanted to achieve, so I played with your code assuming you want to catch exception thrown from your async call. That's what I've got:

async static Task Main(string[] args)
{
    SomeClass some = new SomeClass();

    var subscription = Observable.Interval(TimeSpan.FromSeconds(1))
        .SelectMany(number => Observable.FromAsync(async () => await some.myextfunc(number)))
        .Subscribe(_ => Console.WriteLine("onNext"),
            ex => Console.WriteLine(ex.Message));

    await Task.Delay(TimeSpan.FromSeconds(10));
}

You can find out more from this discussion on GitHub and by searching SO questions on async / await with RX.