.Net WFC/Web service exception handling design pattern

Question

I'm trying to come up with a simple, easy to use design pattern for error handling in a .net wcf service (specifically a silverlight enabled wcf service). If an exception gets thrown in the service method the silverlight application will see a CommunicationException stating "The remote server returned an error: NotFound ---> " and possibly a stack trace depending in your settings, which is entirely not useful because it doesn't tell you the actual error, and usually the real error is not "NotFound".

Reading up on web services and wcf services and exceptions, You need to throw soap/wcf standard exceptions such as FaultException or SoapException. So for a wcf service you need to wrap each method in a try catch, catch each exception, wrap it in a FaultException and throw it. At least that is my understanding, correct me if I am wrong.

So I've created my design pattern:

[ServiceContract(Namespace = "http://MyTest")]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class DataAccess
{
    /// <summary>
    /// Error class, handle converting an exception into a FaultException
    /// </summary>
    [DataContractAttribute]
    public class Error
    {
        private string strMessage_m;
        private string strStackTrace_m;

        public Error(Exception ex)
        {
            this.strMessage_m = ex.Message;
            this.strStackTrace_m = ex.StackTrace;
        }

        [DataMemberAttribute]
        public string Message
        {
            get { return this.strMessage_m; }
            set { this.strMessage_m = value; }
        }

        [DataMemberAttribute]
        public string StackTrace
        {
            get { return this.strStackTrace_m; }
            set { this.strStackTrace_m = value; }
        }

        //Convert an exception into a FaultException
        public static void Throw(Exception ex)
        {
            if (ex is FaultException)
            {
                throw ex;
            }
            else
            {
                throw new FaultException<Error>(new Error(ex));
            }
        }
    }

    [OperationContract]
    [FaultContract(typeof(Error))]
    public void TestException()
    {
        try
        {
            throw new Exception("test");
        }
        catch (Exception ex)
        {
            Error.Throw(ex);
        }
    }
}

So to make a long story short, I'm still not getting the correct error in my silverlight application. I inspect the AsyncCompletedEventArgs.Error object and it still contains a CommunicationException object with the generic error. Help me come up with a nice simple design pattern to allow me to easily throw the correct exception from the service, and easily catch it in the application.

Answer 1

I would suggest you to centralize the error handling of your WCF service instead of putting try/catch on every method. To do this you can implement the IErrorHandler interface:

public class ErrorHandler : IErrorHandler
{
    public bool HandleError(Exception error)
    {
        return true;
    }

    public void ProvideFault(Exception error, MessageVersion version, ref Message msg)
    {
        DataAccessFaultContract dafc = new DataAccessFaultContract(error.Message);
        var fe = new FaultException<DataAccessFaultContract>(dafc);
        Message fault = fe.CreateMessageFault();
        string ns = "http://www.example.com/services/FaultContracts/DataAccessFault";
        msg = Message.CreateMessage(version, fault, ns);
    }
}

The ProvideFault method is called whenever one of your OperationContract throws an exception. It will convert the exception into a custom defined FaultContract and send it to the client. This way you no longer need to put try/catch in every method. You can also send a different FaultContract depending on the exception thrown.

On the client side you need to catch FaultException<DataAccessFaultContract> every time you call a method on your web service.

Answer 2

Ok, I looked into the IErrorHandler idea. I had no idea you could do it this way, and it's perfect because it lets you avoid try catches for every method. Can you do this in standard web services too? I implemented it the following way:

/// <summary>
/// Services can intercept errors, perform processing, and affect how errors are reported using the 
/// IErrorHandler interface. The interface has two methods that can be implemented: ProvideFault and
/// HandleError. The ProvideFault method allows you to add, modify, or suppress a fault message that 
/// is generated in response to an exception. The HandleError method allows error processing to take 
/// place in the event of an error and controls whether additional error handling can run.
/// 
/// To use this class, specify it as the type in the ErrorBehavior attribute constructor.
/// </summary>
public class ServiceErrorHandler : IErrorHandler
{
    /// <summary>
    /// Default constructor
    /// </summary>
    public ServiceErrorHandler()
    {
    }

    /// <summary>
    /// Specifies a url of the service
    /// </summary>
    /// <param name="strUrl"></param>
    public ServiceErrorHandler(string strUrl, bool bHandled)
    {
        this.strUrl_m = strUrl;
        this.bHandled_m = bHandled;
    }

    /// <summary>
    ///HandleError. Log an error, then allow the error to be handled as usual. 
    ///Return true if the error is considered as already handled
    /// </summary>
    /// <param name="error"></param>
    /// <returns></returns>
    public virtual bool HandleError(Exception exError)
    {
        System.Diagnostics.EventLog evt = new System.Diagnostics.EventLog("Application", ".", "My Application");
        evt.WriteEntry("Error at " + this.strUrl_m + ":\n" + exError.Message, System.Diagnostics.EventLogEntryType.Error);

        return this.bHandled_m;
    }

    /// <summary>
    ///Provide a fault. The Message fault parameter can be replaced, or set to
    ///null to suppress reporting a fault.
    /// </summary>
    /// <param name="error"></param>
    /// <param name="version"></param>
    /// <param name="msg"></param>
    public virtual void ProvideFault(Exception exError,
        System.ServiceModel.Channels.MessageVersion version,
        ref System.ServiceModel.Channels.Message msg)
    {
        //Any custom message here
        /*
        DataAccessFaultContract dafc = new DataAccessFaultContract(exError.Message);

        System.ServiceModel.FaultException fe = new System.ServiceModel.FaultException<DataAccessFaultContract>(dafc);
        System.ServiceModel.Channels.MessageFault fault = fe.CreateMessageFault();

        string ns = "http://www.example.com/services/FaultContracts/DataAccessFault";
        msg = System.ServiceModel.Channels.Message.CreateMessage(version, fault, ns);
        */
    }

    private string strUrl_m;
    /// <summary>
    /// Specifies a url of the service, displayed in the error log
    /// </summary>
    public string Url
    {
        get
        {
            return this.strUrl_m;
        }
    }

    private bool bHandled_m;
    /// <summary>
    /// Determines if the exception should be considered handled
    /// </summary>
    public bool Handled
    {
        get
        {
            return this.bHandled_m;
        }
    }
}

/// <summary>
/// The ErrorBehaviorAttribute exists as a mechanism to register an error handler with a service. 
/// This attribute takes a single type parameter. That type should implement the IErrorHandler 
/// interface and should have a public, empty constructor. The attribute then instantiates an 
/// instance of that error handler type and installs it into the service. It does this by 
/// implementing the IServiceBehavior interface and then using the ApplyDispatchBehavior 
/// method to add instances of the error handler to the service.
/// 
/// To use this class specify the attribute on your service class.
/// </summary>
public class ErrorBehaviorAttribute : Attribute, IServiceBehavior
{
    private Type typeErrorHandler_m;

    public ErrorBehaviorAttribute(Type typeErrorHandler)
    {
        this.typeErrorHandler_m = typeErrorHandler;
    }

    public ErrorBehaviorAttribute(Type typeErrorHandler, string strUrl, bool bHandled)
        : this(typeErrorHandler)
    {
        this.strUrl_m = strUrl;
        this.bHandled_m = bHandled;
    }

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

    public virtual void AddBindingParameters(ServiceDescription description, ServiceHostBase serviceHostBase, System.Collections.ObjectModel.Collection<ServiceEndpoint> endpoints, BindingParameterCollection parameters)
    {
        return;
    }

    protected virtual IErrorHandler CreateTypeHandler()
    {
        IErrorHandler typeErrorHandler;

        try
        {
            typeErrorHandler = (IErrorHandler)Activator.CreateInstance(this.typeErrorHandler_m, this.strUrl_m, bHandled_m);
        }
        catch (MissingMethodException e)
        {
            throw new ArgumentException("The ErrorHandler type specified in the ErrorBehaviorAttribute constructor must have a public constructor with string parameter and bool parameter.", e);
        }
        catch (InvalidCastException e)
        {
            throw new ArgumentException("The ErrorHandler type specified in the ErrorBehaviorAttribute constructor must implement System.ServiceModel.Dispatcher.IErrorHandler.", e);
        }

        return typeErrorHandler;
    }

    public virtual void ApplyDispatchBehavior(ServiceDescription description, ServiceHostBase serviceHostBase)
    {
        IErrorHandler typeErrorHandler = this.CreateTypeHandler();            

        foreach (ChannelDispatcherBase channelDispatcherBase in serviceHostBase.ChannelDispatchers)
        {
            ChannelDispatcher channelDispatcher = channelDispatcherBase as ChannelDispatcher;
            channelDispatcher.ErrorHandlers.Add(typeErrorHandler);
        }
    }

    private string strUrl_m;
    /// <summary>
    /// Specifies a url of the service, displayed in the error log
    /// </summary>
    public string Url
    {
        get
        {
            return this.strUrl_m;
        }
    }

    private bool bHandled_m;
    /// <summary>
    /// Determines if the ServiceErrorHandler will consider the exception handled
    /// </summary>
    public bool Handled
    {
        get
        {
            return this.bHandled_m;
        }
    }
}

Service:

[ServiceContract(Namespace = "http://example.come/test")]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
[ErrorBehavior(typeof(ServiceErrorHandler),"ExceptonTest.svc",false)]
public class ExceptonTest
{
    [OperationContract]
    public void TestException()
    {   
        throw new Exception("this is a test!");
    }
}

Answer 3

hi darin/Jeremy, Nice find on Exception handling. I tried to implement this on vb.net using vs 2008, but i keep getting a compile time error "Type 'ErrorBehavior' is not defined". This happens when i try to decorate my WCF service with the "ErrorBehavior" attribute. What am i missing?

Any Help is appreciated thanx -indy

Answer 4

Excellent article covering this topic with good example code: http://codeidol.com/csharp/wcf/Faults/Error-Handling-Extensions/

Answer 5

For the lazy (like me):

using System.ServiceModel;
using System.ServiceModel.Dispatcher;
using System.ServiceModel.Description;

Thanks Jeremy!