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WCF Tutorial | WCF Exception Handling in Silverlight

Handling Exception in Silverlight application from WCF

This article explains about handling the exception in Silverlight application from WCF. I have created the sample Silverlight application, which uses the WCF service for process the data. While testing the application I came to know that exception message thrown from WCF 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 came to know that WCF throws the HTTP 500 series Fault message but Silverlight can handle only 200 series. So we need to convert the 500 series to 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 Beha and implementing the IEndpointBehavior. Actual code for converting the 500 error serice to 200 serivce in BeforeSendReply method.

Create a ClassLibrary project and name it as “Silverlight_WCF_FaultBehavior” and name the class as “SilverlightFaultBehavior”. Copy and paste the follwing code inside the SilverlightFaultBehavior class.

Imports System.ServiceModel.ConfigurationrviceModel.Configuration
Imports System.ServiceModel.Description
Imports System.ServiceModel.Dispatcher
Imports System.ServiceModel.Channels
Imports System.ServiceModel

    Public Class SilverlightFaultBehavior
        Inherits BehaviorExtensionElement
        Implements IEndpointBehavior


        Public Overrides ReadOnly Property BehaviorType() As System.Type
            Get
                Return GetType(SilverlightFaultBehavior)
            End Get
        End Property

        Protected Overrides Function CreateBehavior() As Object
            Return New SilverlightFaultBehavior
        End Function

        Public Sub AddBindingParameters(ByVal endpoint As ServiceEndpoint,
                                     ByVal bindingParameters As BindingParameterCollection)
                                     Implements IEndpointBehavior.AddBindingParameters

        End Sub

        Public Sub ApplyClientBehavior(ByVal endpoint As ServiceEndpoint, 
                                       ByVal clientRuntime As ClientRuntime) 
                                       Implements IEndpointBehavior.ApplyClientBehavior

        End Sub

        Public Sub ApplyDispatchBehavior(ByVal endpoint As ServiceEndpoint, 
                          ByVal endpointDispatcher As EndpointDispatcher) 
                          Implements IEndpointBehavior.ApplyDispatchBehavior
            Dim inspector As New SilverlightFaultMessageInspector()
            endpointDispatcher.DispatchRuntime.MessageInspectors.Add(inspector)
        End Sub

        Public Sub Validate(ByVal endpoint As ServiceEndpoint)
                                         Implements IEndpointBehavior.Validate

        End Sub

        Public Class SilverlightFaultMessageInspector
            Implements IDispatchMessageInspector

            Public Function AfterReceiveRequest(ByRef request As Message, 
                                     ByVal channel As IClientChannel,
                                     ByVal instanceContext As InstanceContext) As Object 
                                     Implements IDispatchMessageInspector.AfterReceiveRequest
                ' Do nothing to the incoming message. 
                Return Nothing
            End Function

            Public Sub BeforeSendReply(ByRef reply As System.ServiceModel.Channels.Message,
                                       ByVal correlationState As Object) 
                                       Implements IDispatchMessageInspector.BeforeSendReply
                If reply.IsFault Then
                    Dim [property] As New HttpResponseMessageProperty()

                    ' Here the response code is changed to 200. 
                    [property].StatusCode = System.Net.HttpStatusCode.OK
                    reply.Properties(HttpResponseMessageProperty.Name) = [property]
                End If
            End Sub
        End Class

    End Class


Note: Highlighted code shows the conversion for 500 serices 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()> _
Public Interface IService
    <OperationContract()> _
    Function Add(ByVal num1 As Integer, ByVal num2 As Integer) As Integer
    <OperationContract()> _
    Function Subtract(ByVal num1 As Integer, ByVal num2 As Integer) As Integer

   
End Interface

Implementation

Public Class Service
    Implements IService

    Public Sub New()
    End Sub

    Public Function Add(ByVal num1 As Integer, ByVal num2 As Integer)
                                         As Integer Implements IService.Add
        Throw New FaultException("Error thrown by user for Add operation")
        'Return num1 + num2
    End Function

    Public Function Subtract(ByVal num1 As Integer, 
                            ByVal num2 As Integer) As Integer Implements IService.Subtract
        Return num1 - num2
    End Function
End Class


< Add the Silverlight_WCF_FaultBehavior project dll as reference to WCF Service

Step 5:

Step 5: In WCF we can extend the binding and behavior by using <extention> tag. In our case also we are extending the custom endpoint behavior as shown below. In the <behaviorExtensions> tag we need specify the fully qualified name of the cutom behaviour assembly.

Modify the Web.config file as shown bellow

<system.serviceModel>
    <services>
      <service name="Service" behaviorConfiguration="ServiceBehavior">
        <!-- Service Endpoints -->
        <endpoint address="" 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>
            <dns value="localhost"/>
          </identity>
        </endpoint>
        <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange"/>
      </service>
    </services>
    <behaviors>
      <serviceBehaviors>
        <behavior name="ServiceBehavior">
          <!-- To avoid disclosing metadata information, set the value below to false and 
          remove the metadata endpoint above before deployment -->
          <serviceMetadata httpGetEnabled="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 -->
          <serviceDebug includeExceptionDetailInFaults="false"/>
        </behavior>
      </serviceBehaviors>
      <endpointBehaviors>
        <behavior name="SilverlightFaultBehavior">
          <silverlightFaults/>
        </behavior>
      </endpointBehaviors>
    </behaviorss=“BlueCode”>>
    <extensions>
      <behaviorExtensions>
        <add name="silverlightFaults" 
        type="Silverlight_WCF_FaultBehavior.SilverlightFaultBehavior, 
        Silverlight_WCF_FaultBehavior, Version=1.0.0.0, Culture=neutral, 
        PublicKeyToken=null"/>
      </behaviorExtensions>
    </extensionss=“BlueCode”>>
  </system.serviceModel>

Step 6: Create the any sample silverlight application as “Silverlight_WCF_Exception” and add this WCF service as 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

    Private Sub Button_Click(ByVal sender As System.Object,
                             ByVal e As System.Windows.RoutedEventArgs)
        Dim proxy As New ServiceProxy.ServiceClient
        AddHandler proxy.AddCompleted, AddressOf AddOperationCompleted
        proxy.AddAsync(5, 6)
    End Sub

    Private Sub AddOperationCompleted(ByVal sender As Object, 
                                    ByVal e As ServiceProxy.AddCompletedEventArgs)
        If e.Error IsNot Nothing Then
            MessageBox.Show(e.Error.Message)
        Else
            MessageBox.Show(e.Result)
        End If
    End Sub

Step 8: Output will look like this

Conclution: This article explains about handling the exception in Silverlight application from WCF

Tips!

  • Always create the service with Interface->Implementation format, mention the contract in Interface.
  • Define the service in Class library and refer the class library in Host project. Don’t use service class in host project.
  • Change the instance mode to per call as default.
  • Always catch exception using try/catch block and throw exception using FaultException < T >.
  • Logging and Include exception should be enable while compiling the project in debug mode. While in production deployment disable the logging and Include exception details.