“系统.NullReferenceException未处理;在WCF对象中

我的wcf应用程序的服务器部分还在挣扎。

问题是，在服务器类中创建一个"回调"对象会导致一个"系统"。NullReferenceException was unhandled" error.

如果我理解正确的话，它发生在我创建这个服务器对象时- ServerClass myServer = new ServerClass();

我想我应该为服务器对象创建一个列表并创建&每当客户端建立连接时自动添加这些对象。请建议，最好的方法是什么?

下面是目前为止的代码:

namespace server2
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
            myServer.eventHappened += new EventHandler(eventFunction);
        }
        ServerClass myServer = new ServerClass();
        // here I instance a new server object.
        // yet I believe that I need to make a list, that would store this objects, so that multiple clients would be able to connect,
        // and I would be able to pick, to whom I want to send a callback.
        void eventFunction(object sender, EventArgs e)
        {
            label1.Text = myServer.clientName;
        }
        private ServiceHost duplex;
        private void Form2_Load(object sender, EventArgs e)     /// once the form loads, create and open a new ServiceEndpoint.
        {
            duplex = new ServiceHost(typeof(ServerClass));
            duplex.AddServiceEndpoint(typeof(IfaceClient2Server), new NetTcpBinding(), "net.tcp://localhost:9080/service");
            duplex.Open();
            this.Text = "SERVER *on-line*";
        }
    }


    class ServerClass : IfaceClient2Server
    {
        public event EventHandler eventHappened;
        IfaceServer2Client callback = OperationContext.Current.GetCallbackChannel<IfaceServer2Client>();
        // ERROR: System.NullReferenceException was unhandled
        public string clientName = "noname";
        public void StartConnection(string name)
        {
            clientName = name;
            MessageBox.Show(clientName + " has connected!");
            eventHappened(this, new EventArgs());
            // ERROR: System.NullReferenceException was unhandled :(
            callback.Message_Server2Client("Welcome, " + clientName);
        }
        public void Message_Cleint2Server(string msg)
        {
        }
        public void Message2Client(string msg)
        {
        }
    }


    [ServiceContract(Namespace = "server", CallbackContract = typeof(IfaceServer2Client), SessionMode = SessionMode.Required)]

    public interface IfaceClient2Server           ///// what comes from the client to the server.
    {
        [OperationContract(IsOneWay = true)]
        void StartConnection(string clientName);
        [OperationContract(IsOneWay = true)]
        void Message_Cleint2Server(string msg);
    }

    public interface IfaceServer2Client          ///// what goes from the sertver, to the client.
    {
        [OperationContract(IsOneWay = true)]
        void AcceptConnection();
        [OperationContract(IsOneWay = true)]
        void RejectConnection();
        [OperationContract(IsOneWay = true)]
        void Message_Server2Client(string msg);
    }
}

谢谢!

class ServerClass : IfaceClient2Server
{
   public event EventHandler eventHappened;
   IfaceServer2Client callback;
   public ServerClass ()
   {
      callback = OperationContext.Current.GetCallbackChannel<IfaceServer2Client>();
   }
   ...
}