Sending and Receiving Faults
SOAP faults convey error condition information from a service to a client and in the duplex case from a client to a service in an interoperable way. Typically a service defines custom fault content and specifies which operations can return them. (For more information, see Defining and Specifying Faults.) This topic discusses how a service or duplex client can send those faults when the corresponding error condition has occurred and how a client or service application handles these faults. For an overview of error handling in Windows Communication Foundation (WCF) applications, see Specifying and Handling Faults in Contracts and Services.
Sending SOAP Faults
Declared SOAP faults are those in which an operation has a System.ServiceModel.FaultContractAttribute that specifies a custom SOAP fault type. Undeclared SOAP faults are those that are not specified in the contract for an operation.
Sending Declared Faults
To send a declared SOAP fault, detect the error condition for which the SOAP fault is appropriate and throw a new System.ServiceModel.FaultException where the type parameter is a new object of the type specified in the FaultContractAttribute for that operation. The following code example shows the use of FaultContractAttribute to specify that the
SampleMethod operation can return a SOAP fault with the detail type of
<OperationContract, FaultContractAttribute(GetType(GreetingFault), Action:="http://www.contoso.com/GreetingFault", ProtectionLevel:=ProtectionLevel.EncryptAndSign)> _ Function SampleMethod(ByVal msg As String) As String
To convey the
GreetingFault error information to the client, catch the appropriate error condition and throw a new System.ServiceModel.FaultException of type
GreetingFault with a new
GreetingFault object as the argument, as in the following code example. If the client is an WCF client application, it experiences this as a managed exception where the type is System.ServiceModel.FaultException of type
Sending Undeclared Faults
Sending undeclared faults can be very useful to quickly diagnose and debug problems in WCF applications, but its usefulness as a debugging tool is limited. More generally, when debugging it is recommended that you use the System.ServiceModel.Description.ServiceDebugBehavior.IncludeExceptionDetailInFaults property. When you set this value to true, clients experience such faults as FaultException exceptions of type ExceptionDetail.
|Because managed exceptions can expose internal application information, setting System.ServiceModel.ServiceBehaviorAttribute.IncludeExceptionDetailInFaults or System.ServiceModel.Description.ServiceDebugBehavior.IncludeExceptionDetailInFaults to true can permit WCF clients to obtain information about internal service operation exceptions, including personally identifiable or other sensitive information. Therefore, setting System.ServiceModel.ServiceBehaviorAttribute.IncludeExceptionDetailInFaults or System.ServiceModel.Description.ServiceDebugBehavior.IncludeExceptionDetailInFaults to true is only recommended as a way of temporarily debugging a service application. In addition, the WSDL for a method that returns unhandled managed exceptions in this way does not contain the contract for the FaultException of type ExceptionDetail. Clients must expect the possibility of an unknown SOAP fault (returned to WCF clients as System.ServiceModel.FaultException objects) to obtain the debugging information properly.|
To send an undeclared SOAP fault, throw a System.ServiceModel.FaultException object (that is, not the generic type FaultException) and pass the string to the constructor. This is exposed to the WCF client applications as a thrown System.ServiceModel.FaultException exception where the string is available by calling the System.ServiceModel.FaultException.ToString method.
|If you declare a SOAP fault of type string, and then throw this in your service as a FaultException where the type parameter is a System.String the string value is assigned to the System.ServiceModel.FaultException.Detail property, and is not available from System.ServiceModel.FaultException.ToString.|
In WCF clients, SOAP faults that occur during communication that are of interest to client applications are raised as managed exceptions. While there are many exceptions that can occur during the execution of any program, applications using the WCF client programming model can expect to handle exceptions of the following two types as a result of communication.
TimeoutException objects are thrown when an operation exceeds the specified timeout period.
CommunicationException objects are thrown when there is some recoverable communication error condition on either the service or the client.
The CommunicationException class has two important derived types, FaultException and the generic FaultException type.
FaultException exceptions are thrown when a listener receives a fault that is not expected or specified in the operation contract; usually this occurs when the application is being debugged and the service has the System.ServiceModel.Description.ServiceDebugBehavior.IncludeExceptionDetailInFaults property set to true.
FaultException exceptions are thrown on the client when a fault that is specified in the operation contract is received in response to a two-way operation (that is, a method with an OperationContractAttribute attribute with IsOneWay set to false).
|When an WCF service has the System.ServiceModel.ServiceBehaviorAttribute.IncludeExceptionDetailInFaults or System.ServiceModel.Description.ServiceDebugBehavior.IncludeExceptionDetailInFaults property set to true the client experiences this as an undeclared FaultException of type ExceptionDetail. Clients can either catch this specific fault or handle the fault in a catch block for FaultException.|
Typically, only FaultException, TimeoutException, and CommunicationException exceptions are of interest to clients and services.
|Other exceptions, of course, do occur. Unexpected exceptions include catastrophic failures like System.OutOfMemoryException; typically applications should not catch such methods.|
Catch Fault Exceptions in the Correct Order
Because FaultException derives from FaultException, and FaultException derives from CommunicationException, it is important to catch these exceptions in the proper order. If, for example, you have a try/catch block in which you first catch CommunicationException, all specified and unspecified SOAP faults are handled there; any subsequent catch blocks to handle a custom FaultException exception are never invoked.
Remember that one operation can return any number of specified faults. Each fault is a unique type and must be handled separately.
Handle Exceptions When Closing the Channel
Most of the preceding discussion has to do with faults sent in the course of processing application messages, that is, messages explicitly sent by the client when the client application calls operations on the WCF client object.
Even with local objects disposing the object can either raise or mask exceptions that occur during the recycling process. Something similar can occur when you use WCF client objects. When you call operations you are sending messages over an established connection. Closing the channel can throw exceptions if the connection cannot be cleanly closed or is already closed, even if all the operations returned properly.
Typically, client object channels are closed in one of the following ways:
When the WCF client object is recycled.
When the client application calls System.ServiceModel.ClientBase.Close.
When the client application calls System.ServiceModel.ICommunicationObject.Close.
When the client application calls an operation that is a terminating operation for a session.
In all cases, closing the channel instructs the channel to begin closing any underlying channels that may be sending messages to support complex functionality at the application level. For example, when a contract requires sessions a binding attempts to establish a session by exchanging messages with the service channel until a session is established. When the channel is closed, the underlying session channel notifies the service that the session is terminated. In this case, if the channel has already aborted, closed, or is otherwise unusable (for example, when a network cable is unplugged), the client channel cannot inform the service channel that the session is terminated and an exception can result.
Abort the Channel If Necessary
Because closing the channel can also throw exceptions, then, it is recommended that in addition to catching fault exceptions in the correct order, it is important to abort the channel that was used in making the call in the catch block.
If the fault conveys error information specific to an operation and it remains possible that others can use it, there is no need to abort the channel (although these cases are rare). In all other cases, it is recommended that you abort the channel. For a sample that demonstrates all of these points, see Expected Exceptions.
The following code example shows how to handle SOAP fault exceptions in a basic client application, including a declared fault and an undeclared fault.
|This sample code does not use the using construct. Because closing channels can throw exceptions, it is recommended that applications create a WCF client first, and then open, use, and close the WCF client in the same try block. For details, see WCF Client Overview and Avoiding Problems with the Using Statement.|
Imports System Imports System.ServiceModel Imports System.ServiceModel.Channels Imports Microsoft.WCF.Documentation Public Class Client Public Shared Sub Main() ' Picks up configuration from the config file. Dim wcfClient As New SampleServiceClient() Try ' Making calls. Console.WriteLine("Enter the greeting to send: ") Dim greeting As String = Console.ReadLine() Console.WriteLine("The service responded: " & wcfClient.SampleMethod(greeting)) Console.WriteLine("Press ENTER to exit:") Console.ReadLine() ' Done with service. wcfClient.Close() Console.WriteLine("Done!") Catch timeProblem As TimeoutException Console.WriteLine("The service operation timed out. " & timeProblem.Message) Console.ReadLine() wcfClient.Abort() Catch greetingFault As FaultException(Of GreetingFault) Console.WriteLine(greetingFault.Detail.Message) Console.ReadLine() wcfClient.Abort() Catch unknownFault As FaultException Console.WriteLine("An unknown exception was received. " & unknownFault.Message) Console.ReadLine() wcfClient.Abort() Catch commProblem As CommunicationException Console.WriteLine("There was a communication problem. " & commProblem.Message + commProblem.StackTrace) Console.ReadLine() wcfClient.Abort() End Try End Sub End Class