I've recently started working with WCF on a project at work, I've read about it and played about with it but never got into any depth with it before now. So far it's going well except for a few cryptic errors I was getting when running my unit tests for a patictular service. The error in was:

Error    1    TestCase 'CPD.Services.Test.CourseTest.Test10'
failed: System.ServiceModel.CommunicationException : An error occurred while receiving the HTTP response to https://lap001.office.aurion.co.uk/cpdengage/Services/CourseService.svc. This could be due to the service endpoint binding not using the HTTP protocol. This could also be due to an HTTP request context being aborted by the server (possibly due to the service shutting down). See server logs for more details.
  ----> System.Net.WebException : The underlying connection was closed: An unexpected error occurred on a receive.
  ----> System.IO.IOException : Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.
  ----> System.Net.Sockets.SocketException : An existing connection was forcibly closed by the remote host

Server stack trace:
   at System.ServiceModel.Channels.HttpChannelUtilities.ProcessGetResponseWebException(WebException webException, HttpWebRequest request, HttpAbortReason abortReason)
   at System.ServiceModel.Channels.HttpChannelFactory.HttpRequestChannel.HttpChannelRequest.WaitForReply(TimeSpan timeout)
   at System.ServiceModel.Channels.RequestChannel.Request(Message message, TimeSpan timeout)
   at System.ServiceModel.Channels.SecurityChannelFactory`1.SecurityRequestChannel.Request(Message message, TimeSpan timeout)
   at System.ServiceModel.Dispatcher.RequestChannelBinder.Request(Message message, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)
   at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Exception rethrown at [0]:
   at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
   at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
   at CPD.Services.Test.CourseServiceReference.CourseService.GetCourseHeader()
   at CPD.Services.Test.CourseServiceReference.CourseServiceClient.GetCourseHeader() in c:\users\damien.mcgivern\documents\dev\aurion\cpdengage\src\cpd.services.test\service references\courseservicereference\reference.cs:line 1027
   at CPD.Services.Test.CourseTest.Test10() in C:\Users\damien.mcgivern\Documents\dev\aurion\cpdengage\src\CPD.Services.Test\CourseTest.cs:line 157
--WebException
   at System.Net.HttpWebRequest.GetResponse()
   at System.ServiceModel.Channels.HttpChannelFactory.HttpRequestChannel.HttpChannelRequest.WaitForReply(TimeSpan timeout)
--IOException
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   at System.Net.FixedSizeReader.ReadPacket(Byte[] buffer, Int32 offset, Int32 count)
   at System.Net.Security._SslStream.StartFrameHeader(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security._SslStream.StartReading(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security._SslStream.ProcessRead(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.TlsStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   at System.Net.PooledStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   at System.Net.Connection.SyncRead(HttpWebRequest request, Boolean userRetrievedStream, Boolean probeRead)
--SocketException
   at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)    c:\users\damien.mcgivern\documents\dev\aurion\cpdengage\src\cpd.services.test\service references\courseservicereference\reference.cs    1027

It was the first method that used a generic collection and at first I thought this may be to fault but testing other generic collections proved that I was using them correctly (see http://msdn2.microsoft.com/en-us/library/system.runtime.serialization.collectiondatacontractattribute.aspx)

But just to make sure I wrote a test method that only returned the type that was being used for the generic collection items, and indeed it still failed with the same error.

This got me thinking that I must have defined the DataContract or DataMember attributes for the object incorrectly, going through each of them I noticed the following code:

[DataMember(IsRequired = true, EmitDefaultValue = false, Order = 5)]
public bool IsEnabled

The value for IsEnabled was always false in the test data, when I set this to true the error never happened and then changing the attributes IsRequired to false stopped the error altogether. Checking the code I found a few other value types that had IsRequired set to true these hadn't and so far no more errors. So setting a value type DataMember to true will cause thie error if the value is the value types default value, bool = false, int = 0 etc.

What bugs me is that this seems to be a serialisation issue, in that the object doesn't match the requirements defined in the DataContract, however the error message doesn't seem to indicate this at all.

Updated 24 June 2008: also see http://msdn.microsoft.com/en-us/library/aa347792.aspx#Mtps_DropDownFilterText - states that SerializationException is thrown. Although the exception details don't make it down to the client.