What is the best approach to handle exceptions in WCF service?

Question

I have a WCF service deployed on two or more remote machines and there is a desktop based application that is used by the client to access any wcf service.

The WCF service is connected to SQL server 2005 to read and write data. This is an intranet scenario in which the client should be on same domain.

Now there can be scenarios where the wcf service throws exceptions:

1. Invalid URL
2. WCF service is down
3. SQL server 2005 is not running
4. Client is not on the same domain
5. Authentication fails
6. Authorization fails

and many other exceptions.

For every exception I have to perform some action or update a status bar, depending on the exception. For example if authorization fails I have to prompt the user to re-enter their credentials.

Please suggest the best design approach to handle this.

Answer 1

You can definitely catch and handle all exceptions that happen on your service class and turn them into a FaultException or FaultException exception.

That way, you won't "fault" (or tear down) the communications channel between your client and server.

Even better approach would be to implement the IErrorHandler interface on your service class that provides a way to globally catch all exceptions as they happen and provide a FaultException instead, that's SOAP compliant.

You can even turn your IErrorHandler into a configurable behavior that can be turned on or off in config.

See these articles and blog posts for more details:

• Rory Primrose: Implementing IErrorHandler
• Useful WCF behaviors: IErrorHandler

Answer 2

1. Create a custom fault class that is marked with the DataContract attribute
2. Mark the method on the service contract interface with FaultContract. Ie. [FaultContract(typeof(CustomFault))]
3. In your service method, catch any applicable internal exceptions and throw a FaultException<CustomFault>. Alternatively, as marc_s mentioned, you can use IErrorHandler to map the exception to the fault.

Personally, I create a base Fault class that has a Reason property and I extend all custom faults from this class. When I want to throw the fault, I call:

throw Fault.Create<CustomFault>(new CustomFault("Boo hoo"));

It's also worth noting that I version my fault classes (including the common Fault class) along with all my other services. This is only a concern if service versioning is a concern, though.

Here's the basic Fault class (I've removed argument validation for brevity):

[DataContract(Namespace = XmlVersionNamespace.FaultNamespace)]
public abstract class Fault
{
    internal FaultReason Reason { get; set; }

    protected Fault(string reasonText)
    {
        Reason = new FaultReason(new FaultReasonText(reasonText, CultureInfo.CurrentUICulture));
    }

    public override string ToString()
    {
        return Reason.ToString();
    }

    internal static FaultException<TDetail> Create<TDetail>(TDetail fault) where TDetail : Fault
    {
        return new FaultException<TDetail>(fault, fault.Reason);
    }
}

Answer 3

You can design the specific Fault Data Contracts for each of the exception scenario in your WCF service so that you can handle the fault/exception at client side respectively.

Answer 4

try
{
  // Actions
}
catch (Exception ex)
{
  // Log the exception
  // Throw Fault Exception back to client
  FaultException fe = new FaultException(ex.Message, new FaultCode("Your fault code"));
  //throw fault exception back to WCF client
  throw fe;
}