How to throw an exception from callback in WCF Async using IAsyncResult

Question

I am using WCF Async calls in my project and i am using Client side asynchronous methods. I have a scenario like below -

  //Code in Business Layer and this method is called from Web layer 
    private void GetGeneralNews()
    {
        client.BeginGetGeneralNewsFeed(GeneralNewsCallback, null);
    }

    //Call Back Method
    private static void GeneralNewsCallback(IAsyncResult asyncResult)
    {
       string response = string.Empty;

       try
       {
          response = client.EndGetGeneralNewsFeed(asyncResult);
       }
       catch(Exception ex)
       {
          throw ex; // Here is the problem. It does not throw the exception to the web layer instead it will suppress the   error.
       }
    }

So as shown in the above code snippet it does not throw the exception from business layer to web layer as it will be suppressed here in business layer itself.

I checked in some of the blogs and sites they are suggesting to go for async and await approach, as i have .NET 4.0 framework and i am seeing "Generate task-based Operations" option disabled. So if there are any options using "IAsyncResult" (Begin & End in client side) please let me know. If there are any other approaches also welcome.

Kindly someone help me.

Thanks.

Answer 1

Here is a sample app which shows that WCF doesn't swallow the exception. If you don't receive the exception, it must be swallowed by your server side code.

using System;
using System.ServiceModel;
using System.ServiceModel.Description;
using WcfQ.QServiceReference;

namespace WcfQ
{
[ServiceBehavior(IncludeExceptionDetailInFaults = true)]
public class QService : IQService
{
    public void Foo()
    {
        throw new ApplicationException("Please catch this");
    }
}

[ServiceContract]
public interface IQService
{
    [OperationContract]
    void Foo();
}

class Program
{
    static private QServiceClient client;

    static void Main(string[] args)
    {
        ServiceHost host = new ServiceHost(typeof(QService), new Uri("http://localhost:20001/q"));
        AddWsdlSupport(host);
        host.AddServiceEndpoint(typeof (IQService), new WSHttpBinding(SecurityMode.None), "");
        host.Open();

        client = new QServiceClient();
        client.BeginFoo(FooCallback, null);
        Console.WriteLine("ready");
        Console.ReadKey();
    }

    private static void FooCallback(IAsyncResult asyncResult)
    {
        try
        {
            client.EndFoo(asyncResult);
        }
        catch (Exception ex)
        {
            Console.WriteLine("Got the exception: " + ex.Message);
        }
    }

    static void AddWsdlSupport(ServiceHost svcHost)
    {
        ServiceMetadataBehavior smb = svcHost.Description.Behaviors.Find<ServiceMetadataBehavior>();
        // If not, add one
        if (smb == null)
            smb = new ServiceMetadataBehavior();
        smb.HttpGetEnabled = true;
        smb.MetadataExporter.PolicyVersion = PolicyVersion.Policy15;
        svcHost.Description.Behaviors.Add(smb);
        // Add MEX endpoint
        svcHost.AddServiceEndpoint(
          ServiceMetadataBehavior.MexContractName,
          MetadataExchangeBindings.CreateMexHttpBinding(),
          "mex"
        );

    }
}

}

Here is the output of this program:

ready 
Got the exception: Please catch this