wcf rest ierrorhandler webfaultexception

Question

I have a WCF 4 Rest service that is to be consumed by a java client and also an c# mvc3 client. The response content needs to be returned as json.

I want to be able to return any exception/faults back to clients in a consistent way. I read that WebFaultException should be used and indeed it works fine.

However, I do not want to pollute my application logic project with WebFaultExceptions as it may be used by a non web project.

I therefore looked at implementing IErrorHandler in the WCF Rest service to catch ANY exceptions and then for specific ones ie ValidationException, return the exception as a fault wrapped in a WebFaultException. However this doesn't seem to work and I get the following Http 504 response when called form fiddler

[Fiddler] ReadResponse() failed: The server did not return a response for this request.

I've reduced the code to its bare bones to show the essence of what I'm doing.

public void ProvideFault(Exception error, 
                         MessageVersion version, 
                         ref Message fault)
{
    var ex = new WebFaultException<Exception>(
                                      new Exception("you can't do that"), 
                                      HttpStatusCode.Unauthorized);

    var xmf = ex.CreateMessageFault();
    fault = Message.CreateMessage(version,
                                  null,
                                  xmf,
                                 new DataContractJsonSerializer(xmf.GetType()));
}

What am I doing wrong?

Answer 1

Here is a sample code from Carlos's blog . It helps me in the past.

public class ValidationAwareErrorHandler : IErrorHandler
{
    IErrorHandler originalErrorHandler;
    public ValidationAwareErrorHandler(IErrorHandler originalErrorHandler)
    {
        this.originalErrorHandler = originalErrorHandler;
    }

    public bool HandleError(Exception error)
    {
        return error is ValidationException || this.originalErrorHandler.HandleError(error);
    }

    public void ProvideFault(Exception error, MessageVersion version, ref Message fault)
    {
        ValidationException validationException = error as ValidationException;
        if (validationException != null)
        {
            fault = Message.CreateMessage(version, null, new ValidationErrorBodyWriter(validationException));
            HttpResponseMessageProperty prop = new HttpResponseMessageProperty();
            prop.StatusCode = HttpStatusCode.BadRequest;
            prop.Headers[HttpResponseHeader.ContentType] = "application/json; charset=utf-8";
            fault.Properties.Add(HttpResponseMessageProperty.Name, prop);
            fault.Properties.Add(WebBodyFormatMessageProperty.Name, new WebBodyFormatMessageProperty(WebContentFormat.Json));
        }
        else
        {
            this.originalErrorHandler.ProvideFault(error, version, ref fault);
        }
    }

    class ValidationErrorBodyWriter : BodyWriter
    {
        private ValidationException validationException;
        Encoding utf8Encoding = new UTF8Encoding(false);

        public ValidationErrorBodyWriter(ValidationException validationException)
            : base(true)
        {
            this.validationException = validationException;
        }

        protected override void OnWriteBodyContents(XmlDictionaryWriter writer)
        {
            writer.WriteStartElement("root");
            writer.WriteAttributeString("type", "object");

            writer.WriteStartElement("ErrorMessage");
            writer.WriteAttributeString("type", "string");
            writer.WriteString(this.validationException.ValidationResult.ErrorMessage);
            writer.WriteEndElement();

            writer.WriteStartElement("MemberNames");
            writer.WriteAttributeString("type", "array");
            foreach (var member in this.validationException.ValidationResult.MemberNames)
            {
                writer.WriteStartElement("item");
                writer.WriteAttributeString("type", "string");
                writer.WriteString(member);
                writer.WriteEndElement();
            }
            writer.WriteEndElement();

            writer.WriteEndElement();
        }
    }
}