WCF DataContractSerializer

Question

I am getting the following error when trying to return a Questionnaire object to the client. I added the KnowType[typeof(...)] in the Data Contract as suggested, but it still doesn't work. Not knowing which type is unknown to the Serializer, I just threw in all classes that are in the EF model. Can someone help? Thanks.

Here is the Service Contract

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.ServiceModel.Web;
using System.Text;
using QuestionnaireWcfServiceApp.Models;

namespace QuestionnaireWcfService
{
    [ServiceContract]
    public interface IQuestionnaireService
    {
        [OperationContract]
        QuestionnaireContract GetQuestionnaire(string questionnaireName);

        [OperationContract]
        QuestionChain LoadQuestion(int questionnaireID, int? questionID, int? userResponse);
    }

    [DataContract]
    [KnownType(typeof(Questionnaire))]
    [KnownType(typeof(Question))]
    [KnownType(typeof(Choice))]
    [KnownType(typeof(Decision))]
    [KnownType(typeof(QuestionFlow))]
    public class QuestionChain
    {
        [DataMember]
        public Question Question { get; set; }

        [DataMember]
        public int? Decision {get;set;}
    }

    [DataContract]
    [KnownType(typeof(Questionnaire))]
    [KnownType(typeof(Question))]
    [KnownType(typeof(Choice))]
    [KnownType(typeof(Decision))]
    [KnownType(typeof(QuestionFlow))]
    public class QuestionnaireContract
    {
        [DataMember]
        public Questionnaire Questionnaire { get; set; }
    }
}

Here is the Service.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.ServiceModel.Web;
using System.Text;
using QuestionnaireWcfServiceApp.Models;

namespace QuestionnaireWcfService
{
    public class QuestionnaireService : IQuestionnaireService
    {
        QuestionnaireWcfServiceApp.Models.QuestionnaireEntities db = new QuestionnaireEntities();

        public QuestionnaireContract GetQuestionnaire(string questionnaireName)
        {
            QuestionnaireContract questionnaireContract = new QuestionnaireContract();
            if (!string.IsNullOrEmpty(questionnaireName))
            {
                Questionnaire thisQuestionnaire = (from q in db.Questionnaires where q.Name.Equals(questionnaireName) select q).FirstOrDefault();
                if (thisQuestionnaire == null)
                    throw new ArgumentNullException("Questionnaire ID is not found.");
                else
                {
                    questionnaireContract.Questionnaire = thisQuestionnaire;
                    return questionnaireContract;
                }
            }
            else
                throw new ArgumentException("Questionnaire name is not specified.");
        }

        public QuestionChain LoadQuestion(int questionnaireID, int? currentQuestionID, int? userResponse)
        {
            QuestionChain qc = new QuestionChain();
            QuestionFlow thisFlow = null;
            Question nextQuestion = null;           
            Questionnaire thisQuestionnaire = (from q in db.Questionnaires where q.QuestionnaireId == questionnaireID select q).FirstOrDefault();

            if (thisQuestionnaire == null)
                throw new ArgumentNullException("Questionnaire ID is not found");   //InvalidOperationException;

            if (currentQuestionID.HasValue)
            {
                //QuestionID should never be changed after setup.  Change the QuestionText around the QuestionID
                Question thisQuestion = thisQuestionnaire.Questions.Where(q => q.PKey.Equals(currentQuestionID)).FirstOrDefault();
                if (thisQuestion == null)
                    throw new ArgumentNullException("Question ID is not found");
                else
                {
                    if (userResponse.HasValue)
                    {
                        thisFlow = thisQuestion.QuestionFlows.First(f => f.QuestionId.Equals(currentQuestionID) && f.ChoiceId.Equals(userResponse));
                        if (thisFlow.Question1 != null)
                        {
                            nextQuestion = thisFlow.Question1;
                            qc.Question = nextQuestion;
                        }
                        else
                        {
                            qc.Question = null;
                            qc.Decision = thisFlow.Decision.Value;
                        }
                    }
                    else
                    {
                        //can't happen. when reaches here, a userResponse must not be null
                    }
                }
            }           
            else
            {
                //default to question 1
                nextQuestion = thisQuestionnaire.Questions.First(q => q.QuestionId.Equals(1));
                if (nextQuestion == null)
                    throw new ArgumentNullException("Question ID");
                else
                    qc.Question = nextQuestion;
            }
            return qc;
        }
    }
}

This is the exception in the Windows Application Log.

Exception: System.ServiceModel.CommunicationException: There was an error while trying to serialize parameter http://tempuri.org/:GetQuestionnaireResult. The InnerException message was 'Type System.Data.Entity.DynamicProxies.Questionnaire_EF00247BEFB9F733C947A4C3E57FD12709E91510AC0DA534D137ED75FCCAC342' 
with data contract name 'Questionnaire_EF00247BEFB9F733C947A4C3E57FD12709E91510AC0DA534D137ED75FCCAC342:
http://schemas.datacontract.org/2004/07/System.Data.Entity.DynamicProxies' 
is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.'.  Please see InnerException for more details. ---> System.Runtime.Serialization.SerializationException: Type 'System.Data.Entity.DynamicProxies.Questionnaire_EF00247BEFB9F733C947A4C3E57FD12709E91510AC0DA534D137ED75FCCAC342' with data contract name 'Questionnaire_EF00247BEFB9F733C947A4C3E57FD12709E91510AC0DA534D137ED75FCCAC342:http://schemas.datacontract.org/2004/07/System.Data.Entity.DynamicProxies' is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.
           at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeAndVerifyType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, Boolean verifyKnownType, RuntimeTypeHandle declaredTypeHandle, Type declaredType)
           at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiTypeAtTopLevel(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle originalDeclaredTypeHandle, Type graphType)
           at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
           at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
           at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
           at System.Runtime.Serialization.XmlObjectSerializer.WriteObject(XmlDictionaryWriter writer, Object graph)
           at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameterPart(XmlDictionaryWriter writer, PartInfo part, Object graph)
           --- End of inner exception stack trace ---
           at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameterPart(XmlDictionaryWriter writer, PartInfo part, Object graph)
           at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameter(XmlDictionaryWriter writer, PartInfo part, Object graph)
           at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeBody(XmlDictionaryWriter writer, MessageVersion version, String action, MessageDescription messageDescription, Object returnValue, Object[] parameters, Boolean isRequest)
           at System.ServiceModel.Dispatcher.OperationFormatter.SerializeBodyContents(XmlDictionaryWriter writer, MessageVersion version, Object[] parameters, Object returnValue, Boolean isRequest)
           at System.ServiceModel.Dispatcher.OperationFormatter.OperationFormatterMessage.OperationFormatterBodyWriter.OnWriteBodyContents(XmlDictionaryWriter writer)
           at System.ServiceModel.Channels.BodyWriterMessage.OnBodyToString(XmlDictionaryWriter writer)
           at System.ServiceModel.Channels.Message.ToString(XmlDictionaryWriter writer)
           at System.ServiceModel.Diagnostics.MessageLogTraceRecord.WriteTo(XmlWriter writer)
           at System.ServiceModel.Diagnostics.MessageLogger.LogInternal(MessageLogTraceRecord record)
           at System.ServiceModel.Diagnostics.MessageLogger.LogMessageImpl(Message& message, XmlReader reader, MessageLoggingSource source)
           at System.ServiceModel.Diagnostics.MessageLogger.LogMessage(Message& message, XmlReader reader, MessageLoggingSource source)
         Process Name: WebDev.WebServer40
         Process ID: 11620

        Event Xml:
        <Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
          <System>
            <Provider Name="System.ServiceModel 4.0.0.0" />
            <EventID Qualifiers="49154">5</EventID>
            <Level>2</Level>
            <Task>7</Task>
            <Keywords>0x80000000000000</Keywords>
            <TimeCreated SystemTime="2012-10-18T07:32:11.000000000Z" />
            <EventRecordID>36499</EventRecordID>
            <Channel>Application</Channel>
            <Computer>Jon-PC</Computer>
            <Security UserID="S-1-5-21-334737869-2079735299-2176000493-1000" />
          </System>
          <EventData>
            <Data>System.ServiceModel.CommunicationException: There was an error while trying to serialize parameter http://tempuri.org/:GetQuestionnaireResult. The InnerException message was 'Type 'System.Data.Entity.DynamicProxies.Questionnaire_EF00247BEFB9F733C947A4C3E57FD12709E91510AC0DA534D137ED75FCCAC342' with data contract name 'Questionnaire_EF00247BEFB9F733C947A4C3E57FD12709E91510AC0DA534D137ED75FCCAC342:http://schemas.datacontract.org/2004/07/System.Data.Entity.DynamicProxies' is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.'.  Please see InnerException for more details. ---&gt; System.Runtime.Serialization.SerializationException: Type 'System.Data.Entity.DynamicProxies.Questionnaire_EF00247BEFB9F733C947A4C3E57FD12709E91510AC0DA534D137ED75FCCAC342' with data contract name 'Questionnaire_EF00247BEFB9F733C947A4C3E57FD12709E91510AC0DA534D137ED75FCCAC342:http://schemas.datacontract.org/2004/07/System.Data.Entity.DynamicProxies' is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.
           at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeAndVerifyType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, Boolean verifyKnownType, RuntimeTypeHandle declaredTypeHandle, Type declaredType)
           at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiTypeAtTopLevel(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle originalDeclaredTypeHandle, Type graphType)
           at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
           at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
           at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
           at System.Runtime.Serialization.XmlObjectSerializer.WriteObject(XmlDictionaryWriter writer, Object graph)
           at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameterPart(XmlDictionaryWriter writer, PartInfo part, Object graph)
           --- End of inner exception stack trace ---
           at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameterPart(XmlDictionaryWriter writer, PartInfo part, Object graph)
           at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameter(XmlDictionaryWriter writer, PartInfo part, Object graph)
           at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeBody(XmlDictionaryWriter writer, MessageVersion version, String action, MessageDescription messageDescription, Object returnValue, Object[] parameters, Boolean isRequest)
           at System.ServiceModel.Dispatcher.OperationFormatter.SerializeBodyContents(XmlDictionaryWriter writer, MessageVersion version, Object[] parameters, Object returnValue, Boolean isRequest)
           at System.ServiceModel.Dispatcher.OperationFormatter.OperationFormatterMessage.OperationFormatterBodyWriter.OnWriteBodyContents(XmlDictionaryWriter writer)
           at System.ServiceModel.Channels.BodyWriterMessage.OnBodyToString(XmlDictionaryWriter writer)
           at System.ServiceModel.Channels.Message.ToString(XmlDictionaryWriter writer)
           at System.ServiceModel.Diagnostics.MessageLogTraceRecord.WriteTo(XmlWriter writer)
           at System.ServiceModel.Diagnostics.MessageLogger.LogInternal(MessageLogTraceRecord record)
           at System.ServiceModel.Diagnostics.MessageLogger.LogMessageImpl(Message&amp; message, XmlReader reader, MessageLoggingSource source)
           at System.ServiceModel.Diagnostics.MessageLogger.LogMessage(Message&amp; message, XmlReader reader, MessageLoggingSource source)</Data>
            <Data>WebDev.WebServer40</Data>
            <Data>11620</Data>
          </EventData>
        </Event>

Answer 1

All types that are used with WCF must be either a data contract or marked as [Serializable]. This includes any objects that are in a WCF data contract already. If you have the ability, you must go and either add the [Serializable] tag to the classes, or add the WCF specific tags to them. This no kidding means that even though you add a Type to the KnownTypes, it doesn't mean that WCF will know that it is serializable.

If neither of these options are available, I would suggest creating "proxy" objects that can contain the values you want to pass and have those convert to and from your target objects. Sounds crazy but that is the way it goes...

ADDED EXAMPLE:

WCF utilizes a serializer containined in the System.Runtime.Serialization namespace to serialize and deserialize data into different formats (XML, SOAP, JSON, Binary). In order for this to work, the objects that are going to be serialized must be some type of serializable type (marked with an attribute). The built in WCF attributes for data objects looks like this:

//This is marked with the DataContract attribute, which is WCF specific
[DataContract]
public class Foo
{
    //The DataMember attribute is also WCF specific and specifies what data
    // is included in the serialization.  Any properties (or variables) must be
    // accessable, as in not read only.
    [DataMember]
    public string Property1{get;set;}

    //This variable will be serialized as well, even though it is private.  This
    // works great if you have a readonly property but still need to pass the data
    [DataMember]
    private int Id = 0;


    //This does not have a DataMember attribute and will not be serialized
    private string var1;
}

WCF can also utilize classes (such as the DataTable and DataSet) that are marked as Serializable.

//This is marked with the Serializable attribute.  All public and private
// fields are automatically serialized (unless there is a containing object
// that is not serializable then you get a SerializationException
[Serializable]
public class Bar
{
    //gets serialized
    public string Property1{get;set;}

    //gets serialized
    private string var1;
}

The idea of creating "proxy" objects is the same that is what is generated by Visual Studio when you add a service reference, only reversed. Say you have class "Foo2" that is not serializable, and has certain properties.

public class Foo2
{
    public string Property1{get;set;}

    public string Property2{get;set;}

    public int Property3{get;set;}
}

You can make a proxy class (in nearly any way you want) that will allow you to pass the proerties back and forth to and from your service.

[DataContract]
public class Foo2Proxy
{
    [DataMember]
    public string Property1{get;set;}

    [DataMember]
    public string Property2{get;set;}

    [DataMember]
    public int Property3{get;set;}

    public Foo2Proxy()
    {
    }

    public Foo2Proxy(Foo2 foo)
    {
        this.Property1 = foo.Property1;
        this.Property2 = foo.Property2;
        this.Property3 = foo.Property3;
    }

    public static Foo2 Create(Foo2Proxy fProxy)
    {
        var foo = new Foo2();
        foo.Property1 = fProxy.Property1;
        foo.Property2 = fProxy.Property2;
        foo.Property3 = fProxy.Property3;
        return foo;
    }
}

I'm sure that there are probably 100,000 different ways and opinions on how to do this, but this is just an example of a possibility that worked for me at the time. If you take a look at this article on CodeProject you can see what I was shooting for here.

In your particular case, you may need to create "proxy" objects (or wrapper objects whatever you want to call it) for your Question and Decision types that are created by EF, since it would seem that they are not inherently serializable, unless you can get into the code that is generated for those objects and add the Attributes as described above. One additional consideration, is if they are derived from another class (or abstract class) that base class MUST ALSO be marked as either serializable or as a DataContract!

Answer 2

The problem here is the objects you get when you query the database using QuestionnaireEntities are of a proxy type and not of Questionnaire (or other types defined by you) type.

You can test that calling .GetType() of any of the returned objects.

Proxies are created to support Lazy Loading. They load data only when you try to access a property.

In case you are returning them from a WCF service, you have to tell Entity Framework to stop creating proxies.

You can do that by writing folloing code:

db.Configuration.ProxyCreationEnabled = false;

Note: As far as I know, turning off proxies should effectively turn off Lazy Loading as well. From my experience with Entity Framework, you have two options, Lazy Loading or No Loading At All.

So, with Lazy Loading disabled, all the navigation properties will be null. To get EF to load values for these properties as well, you will need to use Include() method in your query.