WCF with EntityFramework - "System.ServiceModel.CommunicationException"

Question

i'm working on a Web Service using WCF and EntityFrameWork and an object called "TestsAuto". I'm using a WebForm to communicate with my localDB.

I have no problem to make INSERT, DELETE or UPDATE (sending a TestsAuto from my ASP.NET Web Form to a service's method which interacts whit my DB. But querying my DB to show the results on the Webform raises an Exception and i don't understand at all why.. Maybe it's a serialization problem, i've also heared about DataContract and DataMember but i really need the advices of an expert... ;) I am a beginner in these technologies.

This is the exception raised (i'm french):

Une exception de type 'System.ServiceModel.CommunicationException' s'est 
produite dans mscorlib.dll mais n'a pas été gérée dans le code utilisateur

Informations supplémentaires : Une erreur s'est produite lors de la réception 
de la réponse HTTP à http://localhost:13939/ServiceTA.svc. La raison peut en 
être que la liaison de point de terminaison de service n'utilise pas le 
protocole HTTP. Cela peut également être dû au fait qu'un contexte de requête 
HTTP a été ignoré par le serveur.

Here's some samples of my code :

The class TestsAuto (generated by Entity with DataFirst) :

using System;
using System.Collections.Generic;

public partial class TestsAuto
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public TestsAuto()
    {
        this.Resultats = new HashSet<Resultat>();
    }

    public int testid { get; set; }
    public string nom { get; set; }
    public string perimetre { get; set; }
    public string si { get; set; }
    public string intorrec { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Resultat> Resultats { get; set; }

My Interface and the implementation of the specific method:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.Text;

namespace WcfService1
{
[ServiceContract]
public interface IServiceTA
{
    [OperationContract]
    void insertTA(TestsAuto ta);
    [OperationContract]
    void insertRes(Resultat res);
    [OperationContract]
    void insertCom(Commentaire com);
    [OperationContract]
    void insertParam(Param param);



    [OperationContract]
    void updateTA(TestsAuto ta);
    [OperationContract]
    int updateRes(Resultat res);
    [OperationContract]
    int updateCom(Commentaire com);
    [OperationContract]
    int updateParam(Param param);



    [OperationContract]
    void deleteTA(TestsAuto ta);
    [OperationContract]
    int deleteRes(Resultat res);
    [OperationContract]
    int deleteCom(Commentaire com);
    [OperationContract]
    int deleteParam(Param param);

    [OperationContract]
    TestsAuto findTA(int id);
    [OperationContract]
    TestsAuto findRes(int id);
    [OperationContract]
    TestsAuto findCom(int id);

    [OperationContract]
    List<TestsAuto> listTA();
    [OperationContract]
    List<Resultat> listRes();
    [OperationContract]
    List<Commentaire> listCom();
    [OperationContract]
    List<Param> listParam();


  }
}

the method :

    public TestsAuto findTA(int id)
    {
        return tee.TestsAutoes.Single(p => p.testid == id);
    }

The code of the WebForm :

    protected void btnSelect_Click(object sender, EventArgs e)
    {
        ServiceTAClient client = new ServiceTAClient();

        TestsAuto taa = client.findTA(20);


    }

Step-by-Step : -> I've created the WCF service with the code above. -> I've create a model with EF - DataFirst. -> I've created a simple webform to try CRUD operations and i added the ServiceReference. -> INSERT, UPDATE AND DELETE are working with no problems but If i want to SELECT it raises an exception.

Thanks, i think this is not a huge problem but i didn't find any answers on the web..