Question

Short Version: Is there a/what is the suggested way to return error details to the client when an exception is thrown in an AJAX-Enabled WCF Service (aside from just throwing the gates open and sending back all of the exception details)?

Long Version:

I’ve got a relatively simple AJAX-enabled WCF service that I’m calling from the client using the default service proxy. I’ve provided code snippets below, but I do not believe there is anything wrong with the code per se.

My problem is that if I throw an exception in the service, the error object returned to the client is always generic:

{
    "ExceptionDetail":null,
    "ExceptionType":null,
    "Message":"The server was unable to process the request..."
    "StackTrace":null
}

Ideally I would like to display different error messages on the client depending on what went wrong.

One option is to allow exceptions in WCF faults, which would provide me with the full stack trace and everything, but I appreciate the security concerns with this, and that’s actually a lot more information than I need. I could make do with just being able to send back a string describing the problem or something, but I don’t see a way to do this.

My Service Code:

[ServiceContract(Namespace = "MyNamespace")]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class MyService
{
    [OperationContract]
    public void DoStuff(string param1, string etc)
    {
        //Do some stuff that maybe causes an exception
    }
}

On the client:

MyNamespace.MyService.DoStuff(
    param1,
    etc,
    function() { alert("success"); },
    HandleError);

where “HandleError” is just a generic error handling method that would display details about the error.

Answer 1

EDIT: Updated the post with a proper custom json error handler

The quick but non-preffered way.

<serviceDebug includeExceptionDetailInFaults="true"/>

In Your service behavior will give you all the details you need.

The Nice way

All exceptions from the application are converted to an JsonError and serialized using a DataContractJsonSerializer. The Exception.Message is used directly. FaultExceptions provide the FaultCode and other exception are threated as unknown with faultcode -1.

FaultException are sent with HTTP status code 400 and other exceptions are HTTP code 500 – internal server error. This not nescessary as the faultcode can be used to decide if it is and unknown error. It was however convenient in my app.

The error handler

internal class CustomErrorHandler : IErrorHandler
{
    public bool HandleError(Exception error)
    {
        //Tell the system that we handle all errors here.
        return true;
    }

    public void ProvideFault(Exception error, System.ServiceModel.Channels.MessageVersion version, ref System.ServiceModel.Channels.Message fault)
    {
        if (error is FaultException<int>)
        {
            FaultException<int> fe = (FaultException<int>)error;

            //Detail for the returned value
            int faultCode = fe.Detail;
            string cause = fe.Message;

            //The json serializable object
            JsonError msErrObject = new JsonError { Message = cause, FaultCode = faultCode };

            //The fault to be returned
            fault = Message.CreateMessage(version, "", msErrObject, new DataContractJsonSerializer(msErrObject.GetType()));

            // tell WCF to use JSON encoding rather than default XML
            WebBodyFormatMessageProperty wbf = new WebBodyFormatMessageProperty(WebContentFormat.Json);

            // Add the formatter to the fault
            fault.Properties.Add(WebBodyFormatMessageProperty.Name, wbf);

            //Modify response
            HttpResponseMessageProperty rmp = new HttpResponseMessageProperty();

            // return custom error code, 400.
            rmp.StatusCode = System.Net.HttpStatusCode.BadRequest;
            rmp.StatusDescription = "Bad request";

            //Mark the jsonerror and json content
            rmp.Headers[HttpResponseHeader.ContentType] = "application/json";
            rmp.Headers["jsonerror"] = "true";

            //Add to fault
            fault.Properties.Add(HttpResponseMessageProperty.Name, rmp);
        }
        else
        {
            //Arbitraty error
            JsonError msErrObject = new JsonError { Message = error.Message, FaultCode = -1};

            // create a fault message containing our FaultContract object
            fault = Message.CreateMessage(version, "", msErrObject, new DataContractJsonSerializer(msErrObject.GetType()));

            // tell WCF to use JSON encoding rather than default XML
            var wbf = new WebBodyFormatMessageProperty(WebContentFormat.Json);
            fault.Properties.Add(WebBodyFormatMessageProperty.Name, wbf);

            //Modify response
            var rmp = new HttpResponseMessageProperty();

            rmp.Headers[HttpResponseHeader.ContentType] = "application/json";
            rmp.Headers["jsonerror"] = "true";

            //Internal server error with exception mesasage as status.
            rmp.StatusCode = System.Net.HttpStatusCode.InternalServerError;
            rmp.StatusDescription = error.Message;

            fault.Properties.Add(HttpResponseMessageProperty.Name, rmp);
        }
    }

    #endregion
}

Webbehaviour used to install the above error handler

internal class AddErrorHandlerBehavior : WebHttpBehavior
{
    protected override void AddServerErrorHandlers(ServiceEndpoint endpoint, EndpointDispatcher endpointDispatcher)
    {
        base.AddServerErrorHandlers(endpoint, endpointDispatcher);

        //Remove all other error handlers
        endpointDispatcher.ChannelDispatcher.ErrorHandlers.Clear();
        //Add our own
        endpointDispatcher.ChannelDispatcher.ErrorHandlers.Add(new CustomErrorHandler());
    }
}

The json error data contract

Specifies the json error format.
Add properties here to change the error format.

[DataContractFormat]
public class JsonError
{
    [DataMember]
    public string Message { get; set; }

    [DataMember]
    public int FaultCode { get; set; }
}

Using the error handler

Self-hosted

ServiceHost wsHost = new ServiceHost(new Webservice1(), new Uri("http://localhost/json")); 

ServiceEndpoint wsEndpoint = wsHost.AddServiceEndpoint(typeof(IWebservice1), new WebHttpBinding(), string.Empty);

wsEndpoint.Behaviors.Add(new AddErrorHandlerBehavior());

App.config

<extensions>  
  <behaviorExtensions>  
    <add name="errorHandler"  
        type="WcfServiceLibrary1.ErrorHandlerElement, WcfServiceLibrary1" />  
  </behaviorExtensions>  
</extensions>