How to Handle WCF Service Error in Silverlight When Authenticated Session Times Out

Question

I have a WCF Service that I'm accessing in Silverlight. The Silverlight application requires authentication (authentication happens through a call to the service.)

I've noticed that when the authenticated session times out and the user does something application-side that would make a call to the service, the application just hangs waiting for the service to respond. My service is designed to output an exception in these cases, however nothing on the Silverlight side is assigned, and instead an exception is thrown:

CommunicationException was unhandled by user code

The remote server returned an error: NotFound.

In my service methods, I check to see if the user is still authenticated, and if so I continue with the operation. If not, I assign a value to an out parameter (of type Exception). On the Silverlight side, I check to see if this exception is null, and otherwise take the appropriate action. When I execute, the out parameter is given the proper exception, but then Silverlight throws the above exception (in Reference.cs) before it even gets to the point where I can handle it...

Has anyone else seen this? Any suggestions?

Edit: It seems that having any kind of Exception, including inheriting and containing them, causes the above exception. The simplest way I've found to get around this is just by passing the exception information inside my own custom Exception object (which again, cannot include the System.Exception object)

Answer 1

Thoughts and requests for more info:

You've checked your WCF service against a console app and everything works as expected?

You're able to make calls against your WCF service from your Silverlight app, and this behavior only occurs after the authenticated session times out?

Are you keeping your connection to the WCF open throughout the life of your Silverlight app? If so, I'd recommend only opening and closing it as needed. This way, you should only be calling it when the session is authenticated.

Answer 2

Enable WCF logging in your service and see what the output says. It is usually much more informative than just "NotFound". Here is some info on how to configure WCF logging:
http://msdn.microsoft.com/en-us/library/ms730064.aspx

Answer 3

Unless there's a special endpoint behavior defined for WCF services you are accessing (see http://msdn.microsoft.com/en-us/library/dd470096(VS.95).aspx) browser networking stack will convert ALL of the web service exceptions to 404 - and that's what you are observing.

So you have 2 choices:

• if you can modify the behavior of WCF services you can add the endpoint as explained at the link above.

• if you can't modify the behavior of WCF services you can switch to client networking stack by adding these lines to your App.xaml.cs file:

  HttpWebRequest.RegisterPrefix("https://", WebRequestCreator.ClientHttp); HttpWebRequest.RegisterPrefix("http://", WebRequestCreator.ClientHttp);`

This will ensure that you get actual faults in Silverlight rather than 404.

Obviously switching to client networking stack has its own drawbacks (e.g. you will have to handle authentication, caching, cookies yourself)

Answer 4

This website seems to explain what I'm experiencing: Understanding WCF Services in Silverlight

Apparently Silverlight will get a 400 error for any exception, but will not have much clue what this exception is.

The author provides a solution, but I'm a little confused because the exceptions I've got aren't being thrown, merely assigned. I would think my implementation would work... can anyone provide any insight?

Answer 5

Take a look at my answer here:

http://stackoverflow.com/questions/2548076/random-not-found-error-with-silverlight-accessing-asp-net-web-services/

That is for an asmx service but the same applies to a WCF service. WCF faults are not passed through to the silverlight application. It just gets a 404 status code from the response.