Wcf – Exception Logging for WCF Services using ELMAH

asp.netelmahexceptionloggingwcf

We are using the excellent ELMAH to deal with unhandled exceptions in an ASP.NET 3.5 web application. This works extremely well for all of the site apart from WCF services which are being consumed using the REST features. When an exception occurs within the operation methods that is not handled by the application code, WCF handles it in various ways depending on the service contracts and configuration settings. This means that the exception does not end up firing the ASP.NET HttpApplication.Error event that ELMAH uses. The two solutions I am aware of to deal with this are:

  • Wrap all method calls in a try { } catch(Exception ex) { Elmah.ErrorSignal.FromCurrentContext().Raise(ex); throw; } to explicitly call Elmah within the catch block.
  • Use IErrorHandler as described in Will Hughes' blog post Making WCF and ELMAH play nice together to factor out the call to ELMAH to a separate ErrorHandler.

The first option is extremely simple but is not exactly DRY. The second option only requires you to decorate each service with the custom attribute after implementing the attribute and the ErrorHandler. I have done this based on Will's work but I want to verify that this is the correct approach before posting the code.

Is there a better way that I have missed?

The MSDN documenation for IErrorHandler says that the HandleError method is the place to do the logging but ELMAH accesses the HttpContext.Current.ApplicationInstance, which is null within this method even though HttpContext.Current is available. Making the call to Elmah within the ProvideFault method is a workaround as ApplicationInstance is set but this does not match the intent described in the API documentation. Am I missing something here? The documentation does state that you should not rely on the HandleError method being called on the operation thread which may be why ApplicationInstance is null in this scope.

Best Answer

The solution from my blog post (referenced in the OP) was based on an existing solution we were/are using to alter HTTP Response Codes during an error state.

So, for us it was a one-line change to pass the Exception to ELMAH. If there's a better solution, I'd love to know about it too.

For Posterity/Reference, and potential improvement - here's the code from the current solution.

HttpErrorHandler and ServiceErrorBehaviourAttribute Classes

using System;
using System.ServiceModel;
using System.ServiceModel.Dispatcher;
using System.ServiceModel.Channels;
using System.ServiceModel.Description;
using System.Collections.ObjectModel;
using System.Net;
using System.Web;
using Elmah;
namespace YourApplication
{
    /// <summary>
    /// Your handler to actually tell ELMAH about the problem.
    /// </summary>
    public class HttpErrorHandler : IErrorHandler
    {
        public bool HandleError(Exception error)
        {
            return false;
        }

        public void ProvideFault(Exception error, MessageVersion version, ref Message fault)
        {
            if (error != null ) // Notify ELMAH of the exception.
            {
                if (System.Web.HttpContext.Current == null)
                    return;
                Elmah.ErrorSignal.FromCurrentContext().Raise(error);
            }
        }
    }
    /// <summary>
    /// So we can decorate Services with the [ServiceErrorBehaviour(typeof(HttpErrorHandler))]
    /// ...and errors reported to ELMAH
    /// </summary>
    public class ServiceErrorBehaviourAttribute : Attribute, IServiceBehavior
    {
        Type errorHandlerType;

        public ServiceErrorBehaviourAttribute(Type errorHandlerType)
        {
            this.errorHandlerType = errorHandlerType;
        }

        public void Validate(ServiceDescription description, ServiceHostBase serviceHostBase)
        {
        }

        public void AddBindingParameters(ServiceDescription description, ServiceHostBase serviceHostBase, Collection<ServiceEndpoint> endpoints, BindingParameterCollection parameters)
        {
        }

        public void ApplyDispatchBehavior(ServiceDescription description, ServiceHostBase serviceHostBase)
        {
            IErrorHandler errorHandler;
            errorHandler = (IErrorHandler)Activator.CreateInstance(errorHandlerType);
            foreach (ChannelDispatcherBase channelDispatcherBase in serviceHostBase.ChannelDispatchers)
            {
                ChannelDispatcher channelDispatcher = channelDispatcherBase as ChannelDispatcher;
                channelDispatcher.ErrorHandlers.Add(errorHandler);
            }
        }
    }
}

Usage Example

Decorate your WCF Services with the ServiceErrorBehaviour Attribute: 

[ServiceContract(Namespace = "http://example.com/api/v1.0/")]
[ServiceErrorBehaviour(typeof(HttpErrorHandler))]
public class MyServiceService
{
  // ...
}
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