.net – WCF REST Services – Generic exception handling

netrestwcf

I have a lot of legacy code that is now a backend for a WCF REST service – it used to be a usual WCF service backend before, if that matters. I want to implement a mechanism that would catch any exception in any method and analyze it. If it turns out to be a known error, it will be processed and turned into a friendly-looking fault.

I know that I can throw FaultException or WebProtocolException instead of 'usual' exceptions, but there are lots of places where exceptions are thrown all over the code, and looking for all of them is quite a painful option.

I tried to add an endpoint behavior extension that creates a new behavior which overrides standard WebHttpBehavior.AddServerErrorHandlers method and adds my error handlers (IErrorHandler implementations) to the endpoint dispatcher error handlers collection. Inside the error handlers I analyze the exception and create (or do not create) a desired fault basing on this exception.

I expected this mechanism to return custom data for any known exception, but I was wrong. Good old Microsoft has implemented a wonderful inevitable WebHttpBehavior2, which unconditionally adds an internal Microsoft.ServiceModel.Web.WebErrorHandler into the end of endpoint dispatcher error handlers collection. This handler ignores all previously executed handlers and recognizes only a small set of exceptions, while the majority is interpreted as "Internal Server Error", and nothing more.

The question is whether I am on a right path and there is a way to disable this handler in WCF REST mechanism, or introduce it with a new Exception (e.g., when any exception is caught, it is first processed by my handlers and if they throw/return, for instance, FaultException, then this new exception is supplied to Microsoft.ServiceModel.Web.WebErrorHandler instead of the original one). If all my experiments with IErrorHandler and behavior extensions are worthless, what is an alternative? Again, I really do not want to modify the exception throwing logic, I want one place to catch the exceptions and process them.

Thanks a lot!

Best Answer

When you change the WCF SOAP services to REST, the whole mindset of error reporting and handling changes.

In SOAP, faults are part of your contract. In REST, they simply become codes that you output in the HTTP response code and description.

Here is a catch snippet:

catch (Exception e)
{
    Trace.WriteLine(e.ToString());

    OutgoingWebResponseContext response = WebOperationContext.Current.OutgoingResponse;
    response.StatusCode = System.Net.HttpStatusCode.UnsupportedMediaType; // or anything you want
    response.StatusDescription = e.Message;
    return null; // I was returning a class
}

So I would suggest you create a helper code which creates relevant error codes for you and put in the response.

Related Solutions

C# – Catch multiple exceptions at once

Catch System.Exception and switch on the types

catch (Exception ex)            
{                
    if (ex is FormatException || ex is OverflowException)
    {
        WebId = Guid.Empty;
        return;
    }
    
    throw;
}

REST / SOAP endpoints for a WCF service

You can expose the service in two different endpoints. the SOAP one can use the binding that support SOAP e.g. basicHttpBinding, the RESTful one can use the webHttpBinding. I assume your REST service will be in JSON, in that case, you need to configure the two endpoints with the following behaviour configuration

<endpointBehaviors>
  <behavior name="jsonBehavior">
    <enableWebScript/>
  </behavior>
</endpointBehaviors>

An example of endpoint configuration in your scenario is

<services>
  <service name="TestService">
    <endpoint address="soap" binding="basicHttpBinding" contract="ITestService"/>
    <endpoint address="json" binding="webHttpBinding"  behaviorConfiguration="jsonBehavior" contract="ITestService"/>
  </service>
</services>

so, the service will be available at

Apply [WebGet] to the operation contract to make it RESTful. e.g.

public interface ITestService
{
   [OperationContract]
   [WebGet]
   string HelloWorld(string text)
}

Note, if the REST service is not in JSON, parameters of the operations can not contain complex type.

Reply to the post for SOAP and RESTful POX(XML)

For plain old XML as return format, this is an example that would work both for SOAP and XML.

[ServiceContract(Namespace = "http://test")]
public interface ITestService
{
    [OperationContract]
    [WebGet(UriTemplate = "accounts/{id}")]
    Account[] GetAccount(string id);
}

POX behavior for REST Plain Old XML

<behavior name="poxBehavior">
  <webHttp/>
</behavior>

Endpoints

<services>
  <service name="TestService">
    <endpoint address="soap" binding="basicHttpBinding" contract="ITestService"/>
    <endpoint address="xml" binding="webHttpBinding"  behaviorConfiguration="poxBehavior" contract="ITestService"/>
  </service>
</services>

Service will be available at

REST request try it in browser,

http://www.example.com/xml/accounts/A123

SOAP request client endpoint configuration for SOAP service after adding the service reference,

  <client>
    <endpoint address="http://www.example.com/soap" binding="basicHttpBinding"
      contract="ITestService" name="BasicHttpBinding_ITestService" />
  </client>

in C#

TestServiceClient client = new TestServiceClient();
client.GetAccount("A123");

Another way of doing it is to expose two different service contract and each one with specific configuration. This may generate some duplicates at code level, however at the end of the day, you want to make it working.

Best Answer

Related Solutions

C# – Catch multiple exceptions at once

REST / SOAP endpoints for a WCF service

Reply to the post for SOAP and RESTful POX(XML)

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