WCF Exception when returning list of interface objects

Question

I am getting the following exception when I call the WCF Service from ASP.NET website. How can we overcome it?

Note: By applying break point in service project, I have verified that the service is returning two valid objects.

Note: In service, I am returing List of IBankAccount. [OperationContract] List<IBankAccount> GetDataUsingDataContract(int userId); The IBankAccount is an interface.

The exception says "The underlying connection was closed: The connection was closed unexpectedly". Detailed stack trace is available in the following picture.

//Website

using System;
using ServiceReference1;
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{

    Service1Client client = new Service1Client();

    string result = client.GetData(1);
    Response.Write(result);


    client.GetDataUsingDataContract(1);
    int d = 0;

}
}

//Service Interface

using System.Collections.Generic;
using System.ServiceModel;
using DTOProject;
namespace MyServiceApp
{
[ServiceContract]
public interface IService1
{
    [OperationContract]
    string GetData(int value);

    [OperationContract]
    List<IBankAccount> GetDataUsingDataContract(int userId);

}

}

//DTO

using System.Runtime.Serialization;
namespace DTOProject
{
public interface IBankAccount
{
     int Duration { get; set; }
     int AmountDeposited { get; set; }
}
}

using System.Runtime.Serialization;
namespace DTOProject
{
[DataContract]
public class FixedAccount : IBankAccount
{
    [DataMember]
    public int Duration { get; set; }

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

using System.Runtime.Serialization;
namespace DTOProject
{
[DataContract]
public class SavingsAccount : IBankAccount
{
    [DataMember]
    public int Duration { get; set; }

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

//Service Implementation

using System.Collections.Generic;
using DTOProject;
using BusinessLayer;

namespace MyServiceApp
{
public class Service1 : IService1
{
    public string GetData(int value)
    {
        return string.Format("You entered: {0}", value);
    }
    public List<IBankAccount> GetDataUsingDataContract(int userId)
    {
        BusinessLayer.AccountManager accManager = new AccountManager();
        List<IBankAccount> accounts = accManager.GetAllAccountsForUser(userId);
        return accounts;
    }

}
}

//Business Layer

 using System.Collections.Generic;
 using DTOProject;
 using DataAccessLayer;
 namespace BusinessLayer
{
public class AccountManager
{
    public List<IBankAccount> GetAllAccountsForUser(int userID)
    {
        DataAccessLayer.AccounutManagerDAL accountManager = new AccounutManagerDAL();
        List<IBankAccount> accountList = accountManager.GetAllAccountsForUser(userID);
        return accountList;
    }

}
}

//Data Access Layer

using System;
using System.Collections.Generic;
using DTOProject;
namespace DataAccessLayer
{

 public class DatabaseRecordSimulation
 {
    public string AccountType { get; set; }
    public int Duration { get; set; }
    public int DepositedAmount { get; set; }
 }

public class AccounutManagerDAL
{

    List<DatabaseRecordSimulation> dbRecords = new List<DatabaseRecordSimulation>()
    {
        new DatabaseRecordSimulation{AccountType="Savings",Duration=6,DepositedAmount=50000},
        new DatabaseRecordSimulation{AccountType="Fixed",Duration=6,DepositedAmount=50000}
    };

    public List<IBankAccount> GetAllAccountsForUser(int userID)
    {

        List<IBankAccount> accountList = new List<IBankAccount>();
        foreach (DatabaseRecordSimulation dbRecrod in dbRecords)
        {
            IBankAccount acc = AccountFactory.GetAccount(dbRecrod);
            accountList.Add(acc);
        }
        return accountList;
    }

}

public static class AccountFactory
{
    public static IBankAccount GetAccount(DatabaseRecordSimulation dbRecord)
    {
        IBankAccount theAccount = null;
        if ( String.Equals(dbRecord.AccountType, "Fixed"))
        {
            theAccount = new FixedAccount();
        }
        if (String.Equals(dbRecord.AccountType, "Savings"))
        {
            theAccount = new SavingsAccount();
        }
        return theAccount;

    }
}
}

READING: 1. WCF Object Design - OOP vs SOA

Answer 1

As per this post: http://social.msdn.microsoft.com/Forums/eu/wcf/thread/31102bd8-0a1a-44f8-b183-62926390b3c3 You cannot return an interface - you must return a class. However, you can create an abstract base type - BankAccount and apply the KnownType attribute to it. For example:

[DataContract]
[KnowType(typeof(FixedAccount))]
[KnowType(typeof(SavingsAccount))]
abstract class BankAccount { ... }

[DataContract]
class FixedAccount : BankAccount { ... }

[DataContract]
class SavingsAccount : BankAccount { ... }

The KnownType attribute let's WCF know that some other type, not explicitly referenced in the service contract will be a part of the contract.