Exception Handling in Silverlight Application From WCF in VB.NET

Introduction:

This article explains exception handling in a Silverlight application from WCF. I have created the sample Silverlight application, which uses the WCF service to process the data. While testing the application I encoutered the exception message thrown from WCF that it cannot be received at the client side (Silverlight application) even after using the FaultException. I was always getting System.ServiceModel.CommunicationException: The remote server returned an error: NotFound.

 

 

Later I learned that WCF throws a HTTP 500 series fault message but Silverlight can handle only the 200 series. So we need to convert the 500 series to a 200 error message for Silverlight. Here is the sample application for exception handling between WCF and Silverlight.

 

Step 1:

We can customize the Endpoint behavior of the WCF service by inheriting the BehaviorExtensionElement and implementing the IEndpointBehavior. The actual code for converting the 500 error series to 200 series is in the BeforeSendReply method.

Create a ClassLibrary project and name it "Silverlight_WCF_FaultBehavior" and name the class "SilverlightFaultBehavior". Copy and paste the following code inside the SilverlightFaultBehavior class.

 

Imports System.ServiceModel.Configuration

Imports System.ServiceModel.Description

Imports System.ServiceModel.Dispatcher

Imports System.ServiceModel.Channels

Imports System.ServiceModel

 

    PublicClass SilverlightFaultBehavior

        Inherits BehaviorExtensionElement

        Implements IEndpointBehavior

        PublicOverrides ReadOnly Property BehaviorType()As System.Type

            Get

                ReturnGetType(SilverlightFaultBehavior)

            EndGet

        EndProperty

        ProtectedOverrides Function CreateBehavior() AsObject

            ReturnNew SilverlightFaultBehavior

        EndFunction

        PublicSub AddBindingParameters(ByVal endpoint As System.ServiceModel.Description.ServiceEndpoint,ByVal bindingParametersAs System.ServiceModel.Channels.BindingParameterCollection)Implements System.ServiceModel.Description.IEndpointBehavior.AddBindingParameters

        EndSub

        PublicSub ApplyClientBehavior(ByVal endpoint As System.ServiceModel.Description.ServiceEndpoint,ByVal clientRuntimeAs System.ServiceModel.Dispatcher.ClientRuntime) Implements System.ServiceModel.Description.IEndpointBehavior.ApplyClientBehavior

        EndSub

        PublicSub ApplyDispatchBehavior(ByVal endpoint As System.ServiceModel.Description.ServiceEndpoint,ByVal endpointDispatcherAs System.ServiceModel.Dispatcher.EndpointDispatcher)Implements System.ServiceModel.Description.IEndpointBehavior.ApplyDispatchBehavior

            Dim inspector AsNew SilverlightFaultMessageInspector()

            endpointDispatcher.DispatchRuntime.MessageInspectors.Add(inspector)

        EndSub

        PublicSub Validate(ByVal endpoint As System.ServiceModel.Description.ServiceEndpoint)Implementsystem.ServiceModel.Description.IEndpointBehavior.Validate

        EndSub

        PublicClass SilverlightFaultMessageInspector

            Implements IDispatchMessageInspector

            PublicFunction AfterReceiveRequest(ByRef request As System.ServiceModel.Channels.Message, ByVal channel As System.ServiceModel.IClientChannel, ByVal instanceContext As System.ServiceModel.InstanceContext) AsObject Implements System.ServiceModel.Dispatcher.IDispatchMessageInspector.AfterReceiveRequest

                ' Do nothing to the incoming message.

                ReturnNothing

            EndFunction

            PublicSub BeforeSendReply(ByRef reply As System.ServiceModel.Channels.Message, ByVal correlationState AsObject) Implements System.ServiceModel.Dispatcher.IDispatchMessageInspector.BeforeSendReply

                If reply.IsFault Then

                    Dim [property] AsNew HttpResponseMessageProperty() 

                    ' Here the response code is changed to 200.

                    [property].StatusCode = System.Net.HttpStatusCode.OK

                    reply.Properties(HttpResponseMessageProperty.Name) = [property]

                EndIf

            EndSub

        EndClass

    EndClass

 

Note: The highlighted code shows the conversion from the 500 series to 200 series error code.

 

Step 2

Build the project.

 

Step 3: 

 

Create a new WCF service with Interface and implementation class as follows:

 

Interface

 

<ServiceContract()> _

PublicInterface IService

    <OperationContract()> _

    Function Add(ByVal num1 As Integer, ByVal num2As Integer)As Integer

    <OperationContract()> _

    Function Subtract(ByVal num1As Integer,ByVal num2 As Integer)As Integer  

EndInterface

 

Implementation 

 

PublicClass Service

    Implements IService

    PublicSub New()

    EndSub

    PublicFunction Add(ByVal num1 As Integer, ByVal num2As Integer)As IntegerImplements IService.Add

        ThrowNew FaultException("Error thrown by user for Add operation")

        'Return num1 + num2

    EndFunction

    PublicFunction Subtract(ByVal num1 As Integer, ByVal num2As Integer)As IntegerImplements IService.Subtract

        Return num1 - num2

    EndFunction

EndClass

 

Step 4:

 

Add the Silverlight_WCF_FaultBehavior project dll as a reference to the WCF Service.

 

Step 5:

 

In WCF we can extend the binding and behavior by using the <extention> tag. In our case we are also extending the custom endpoint behavior as shown below. In the <behaviorExtensions> tag we need to specify the fully qualified name of the custom behavior assembly.

 

Modify the Web.config file as shown below:

 

<system.serviceModel>

    <services>

      <servicename="Service"behaviorConfiguration="ServiceBehavior">

        <!-- Service Endpoints -->

        <endpointaddress=""binding="basicHttpBinding"contract="IService"behaviorConfiguration="SilverlightFaultBehavior">

          <!--

              Upon deployment, the following identity element should be removed or replaced to reflect the

              identity under which the deployed service runs.  If removed, WCF will infer an appropriate identity

              automatically.

          -->

          <identity>

            <dnsvalue="localhost"/>

          </identity>

        </endpoint>

        <endpointaddress="mex"binding="mexHttpBinding"contract="IMetadataExchange"/>

      </service>

    </services>

    <behaviors>

      <serviceBehaviors>

        <behaviorname="ServiceBehavior">

          <!-- To avoid disclosing metadata information, set the value below to false and remove the metadata endpoint above before deployment -->

          <serviceMetadatahttpGetEnabled="true"/>

          <!-- To receive exception details in faults for debugging purposes, set the value below to true.  Set to false before deployment to avoid disclosing exception information -->

          <serviceDebugincludeExceptionDetailInFaults="false"/>

        </behavior>

      </serviceBehaviors>

      <endpointBehaviors>

        <behaviorname="SilverlightFaultBehavior">

          <silverlightFaults/>

        </behavior>

      </endpointBehaviors>

    </behaviors>

    <extensions>

      <behaviorExtensions>

        <addname="silverlightFaults"type="Silverlight_WCF_FaultBehavior.SilverlightFaultBehavior, Silverlight_WCF_FaultBehavior, Version=1.0.0.0,

Culture=neutral, PublicKeyToken=null"/>

      </behaviorExtensions>

    </extensions>

  </system.serviceModel>

 

Step 6:

 

Create the sample Silverlight application as "Silverlight_WCF_Exception" and add this WCF service as the Service Reference.

 

URL: http://localhost/MathService/Service.svc 

 

Step 7: 

 

Add a button to the MainPage.xaml and call the WCF method as shown below:

 

PrivateSub Button_Click(ByVal sender As System.Object,ByVal e As System.Windows.RoutedEventArgs)

        Dim proxyAs New ServiceProxy.ServiceClient

        AddHandler proxy.AddCompleted, AddressOf AddOperationCompleted

        proxy.AddAsync(5, 6)

End Sub

Private Sub AddOperationCompleted(ByVal senderAs Object,ByVal e As ServiceProxy.AddCompletedEventArgs)

        If e.ErrorIsNot Nothing Then

            MessageBox.Show(e.Error.Message)

        Else

            MessageBox.Show(e.Result)

        EndIf

EndSub

 

Step 8:

The output will look like this:

 

Conclusion:

 

This article explains about exception handling in a Silverlight application from WCF. I will get back to you guys with another article.