Passing WCF Exception to Silverlight

Since it is not possible to catch regular exception from a WCF service in a silverlight application, there is another way to do it by using BehaviorExtensionElement.

1. On Server-side First create a behavior like this:

public class MyFaultBehavior : BehaviorExtensionElement, IEndpointBehavior
    {
        public void ApplyDispatchBehavior(ServiceEndpoint endpoint, EndpointDispatcher endpointDispatcher)
        {
            SilverlightFaultMessageInspector inspector = new SilverlightFaultMessageInspector();
            endpointDispatcher.DispatchRuntime.MessageInspectors.Add(inspector);
        }
        public class SilverlightFaultMessageInspector : IDispatchMessageInspector
        {
            public void BeforeSendReply(ref Message reply, object correlationState)
            {
                if (reply.IsFault)
                {
                    HttpResponseMessageProperty property = new HttpResponseMessageProperty();

                    // Here the response code is changed to 200.
                    property.StatusCode = System.Net.HttpStatusCode.OK;
                    reply.Properties[HttpResponseMessageProperty.Name] = property;
                }
            }
            public object AfterReceiveRequest(ref Message request, IClientChannel channel, InstanceContext instanceContext)
            {
                // Do nothing to the incoming message.
                return null;
            }
        }
        // The following methods are stubs and not relevant.
        public void AddBindingParameters(ServiceEndpoint endpoint, BindingParameterCollection bindingParameters)
        {
        }
        public void ApplyClientBehavior(ServiceEndpoint endpoint, ClientRuntime clientRuntime)
        {
        }
        public void Validate(ServiceEndpoint endpoint)
        {
        }
        public override System.Type BehaviorType
        {
            get { return typeof(MyFaultBehavior); }
        }
        protected override object CreateBehavior()
        {
            return new MyFaultBehavior();
        }
    }

2. On Server-side Modify you web.config:

	<system.serviceModel>
		<!--Add a behavior extension within the service model-->
		<extensions>
			<behaviorExtensions>
				<add name="myFault" type="SilverlightWCF.Web.MyFaultBehavior, SilverlightWCF.Web, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"/>
			</behaviorExtensions>
		</extensions>
		<behaviors>
			<!--Add a endpointBehavior below the behaviors-->
			<endpointBehaviors>
				<behavior name="myFaultBehavior">
					<myFault/>
				</behavior>
			</endpointBehaviors>
			<serviceBehaviors>
				<behavior name="SilverlightWCF.Web.MyCoolServiceBehavior">
					<serviceMetadata httpGetEnabled="true"/>
					<!--For debugging, it might be cool to have some more error information.
       to get this, set includeExceptionDetailInFaults to true-->
					<serviceDebug includeExceptionDetailInFaults="true"/>
				</behavior>
			</serviceBehaviors>
		</behaviors>
...
		<serviceHostingEnvironment aspNetCompatibilityEnabled="true"/>
		<services>
			<service behaviorConfiguration="SilverlightWCF.Web.MyCoolServiceBehavior" name="SilverlightWCF.Web.MyCoolService">
				<!--Set the behaviorConfiguration of the endpoint-->
				<endpoint address="" binding="customBinding" bindingConfiguration="customBinding0" contract="SilverlightWCF.Web.MyCoolService" behaviorConfiguration="myFaultBehavior"/>
				<endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange"/>
			</service>
		</services>
	</system.serviceModel>

3. On Server-side Throw you FaultException

        [OperationContract]
        public void Foo()
        {
            throw new FaultException("this is my Exception");
        }

4. On Client-side Catch the Exception like this:

        void serviceClient_FooCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
        {
            if (e.Error == null)
            {
                // In case of success
            }
            else if (e.Error is FaultException)
            {
                FaultException fault = e.Error as FaultException;
                string text =  e.Error.Message;
            }
        }

via http://www.codeproject.com/KB/silverlight/SilverlightWCFService.aspx