Question about Catch and exceptions

Question

I am using Observable.FromAsyncPattern in a Silverlight app to wrap some old IAsyncResult code.  Everything works fine in Subscribe when it succeeds, but when the async End function throws an exception, I don't seem to know where to catch it.

I'm guessing the Observable<T>.Catch() method might be what I want, but the only reference I've seen is a video talking about it and I can't seem to find any example code of how to use it.  Does anyone have an example of how I might write a method/lambda expression that catches an exception in this scenario?

Thanks!

Answer 1

Hi Andy,

there's several ways to deal with exceptions. the most simple one is to pass a second argument to your Subscribe call.

e.g.:

IObservable<string> x = Observable.FromAsyncPattern(...);  
x.Subscribe(value=>HtmlPage.Window.Alert(value), error=>HtmlPage.Window.Alert(error.Message));

if you want to do more operations on the stream, you can use the Catch operator.

One way is to silence the catch: e.g. x.Catch(Observable.Empty<string>()).Subscibe(...)

another way is to write a catch block, in Rx, the catch block returns a new observable that "repairs" the original source:

x.Catch<ArgumentNullException>(error=> Observable.Return("The operation failed.")).Subscribe(...);

this sample will send the string "The operation failed" to the OnNext handler if an exception happens.

Hope this helps.

Jeffrey

Answer 2

Thanks Jeffrey, that's exactly what I was looking for!

Answer 3

Jeff,

Is there a way to resume processing with the next element in the original observable sequence  in the Catch operator instead of generating a new sequence?

 

Thx.

-Abhijeet

Answer 4

Hi Abhijeet,

This is not possible. The OnError notification represents a critical error, that terminates the stream. Similarly, enumerables can't yield values after an exception has been raised. Consider the following.

void Main()
{
	var e = Get().GetEnumerator();
	e.MoveNext();
	e.MoveNext();
	try{ e.MoveNext(); }catch{}
	e.MoveNext();
	Console.WriteLine(e.Current); // will output 2 (not 3)
}

public IEnumerable Get()
{
	yield return 1;
	yield return 2;
	throw new Exception("test");
	yield return 3; // you can never read this value
}

One option is for the "source" to use Notification<T> (or your own custom envelope type) instead.

public IObservable<Notification<int>> MyObservable()
{
	return Observable.Create<Notification<int>>(o =>
	{
		o.OnNext(new Notification<int>.OnNext(42));
		o.OnNext(new Notification<int>.OnError(new Exception("error1")));
		o.OnNext(new Notification<int>.OnNext(43));
		o.OnNext(new Notification<int>.OnError(new Exception("error2")));
		o.OnNext(new Notification<int>.OnNext(44));
		o.OnNext(new Notification<int>.OnError(new Exception("error3")));
		return () => {};
	});
}

Hope that helps.
James

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James Miles http://enumeratethis.com