Convert Exceptions in WCF Services to Fault Exceptions a Client Can Use

This article will show you how to trap an exception that occurs within a WCF service but also inform the client application of the exception so it can be handled.  To do this we will make use of a custom faultException.

1. You need to create a fault contract for the type of fault that you want to be returned to the client via WCF.  Below I have created a ‘DBConcurrencyFault’ fault that tells the client when a database concurrency violation has occurred (e.g. 2 people tried to save the same record).

namespace Common.Services.Faults {     [DataContract]     public class DBConcurrencyFault     {         [DataMember]         public String Message { get; set; }         [DataMember]         public String ExceptionType { get; set; }     } }

2. Now we are going to create a new attribute that we can use to decorate our service classes with.  This attribute will trap any exception occurring in a WCF service and convert them into a specific fault exception.

namespace Common.Services {     public class WcfErrorHandler : Attribute, IErrorHandler, IServiceBehavior     {         public bool HandleError(Exception error)         {             return false;         }                 public void ProvideFault(Exception error, System.ServiceModel.Channels.MessageVersion version, ref System.ServiceModel.Channels.Message fault)         {             if (fault != null)                 return;             if (error.GetType() == typeof(System.Data.DBConcurrencyException))              {                 // Handle Database record being written to by different users (Conncurrency Fault)                 DBConcurrencyFault databaseConcurrencyFault = new DBConcurrencyFault();                 databaseConcurrencyFault.Message = error.Message;                 databaseConcurrencyFault.ExceptionType = error.GetType().ToString();                 String faultReason = "Someone else has already saved this record.";                 FaultException<DBConcurrencyFault> faultException = new FaultException<DBConcurrencyFault>(databaseConcurrencyFault, faultReason);                 MessageFault messageFault = faultException.CreateMessageFault();                 fault = Message.CreateMessage(version, messageFault, faultException.Action);             }             else             {                 // Handle all other errors as standard FaultExceptions                 FaultException faultException = new FaultException(error.Message);                 MessageFault messageFault = faultException.CreateMessageFault();                 fault = Message.CreateMessage(version, messageFault, faultException.Action);             }         }         #region IServiceBehavior Members         public void AddBindingParameters(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase, System.Collections.ObjectModel.Collection<ServiceEndpoint> endpoints, BindingParameterCollection bindingParameters)         {                    }         public void ApplyDispatchBehavior(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)         {             foreach (ChannelDispatcher disp in serviceHostBase.ChannelDispatchers)             {                 disp.ErrorHandlers.Add(this);             }         }         public void Validate(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)         {                    }         #endregion     } }

Note: the ‘ProvideFault’ method contains code that determines if an exception of type ‘System.Data.DBConcurrencyException’ has occurred.  If one has it is converted into a ‘DBConcurrencyFault’ which is then returned to the calling client by the WCF service. 

3. Now you can either add the WcfErrorHandler attribute to all your wcf services, or you can as I have below define a base service class that all your service classes will inherit from.

namespace Common.Services {    [WcfErrorHandler()]    public class BaseService    {        // Common service methods go here    } }

4. Have your service class inherit from the base service class. 

public class MileageClaimService : BaseService, IMileageClaimService {       // Service methods go here }

5. Trap the fault exception from a web client and redirect the user to a tailored error page.
   5a.  For an Asp.Net web application you can add the exception handling code to the global.asax Application_Error method.

protected void Application_Error(object sender, EventArgs e)  {       Exception ex = Server.GetLastError().GetBaseException();       if (ex.GetType() == typeof(System.ServiceModel.FaultException<Common.Services.Faults.DBConcurrencyFault>) )       {          Server.ClearError();          Response.Redirect("~/Errors/ErrorDbConcurrency.aspx");       }  }

  5b. From an Asp.Net MVC web application you just add a HandleError attribute to the controller with the type of DBConcurrencyFault and redirect to a specific view.

[HandleError(ExceptionType = typeof(System.ServiceModel.FaultException<Common.Services.Faults.DBConcurrencyFault>), View = "ErrorDbConcurrency")] [HandleError] public class ClaimController : Controller {     // Controller methods go here }

Note: There are 2 HandleError attributes, the first handles the DBConcurrencyFault and redirects to a custom error page.  The 2nd traps all other exceptions and redirects to the default error page.  The order of these 2 HandleError attributes is important.