MetadataExchangeBindings/ServiceMetadataBehavior

Question

I have the following code, that I am testing...

 public static void Main()
 {
  try
  {
  ServiceHost host = null;
   host = new ServiceHost(typeof(StockService), new Uri("http://localhost:8080/EssentialWCF"));
  host.AddServiceEndpoint(typeof(IStockService),new BasicHttpBinding(), "");

  ServiceMetadataBehavior behavior = new ServiceMetadataBehavior();
  behavior.HttpGetEnabled = true;
  host.Description.Behaviors.Add(behavior);    

  host.AddServiceEndpoint(typeof(IMetadataExchange), MetadataExchangeBindings.CreateMexHttpBinding(), "mex");

  host.Open();
  Console.WriteLine("Host opened. Waiting...");
  Console.Read();

  host.Close();
  }
  catch(Exception e)
  {
   Console.WriteLine(e.Message);
   Console.Read();
  }
 }

I am able to open the Host and listen on the port but if I switch the lines like this...

 public static void Main()
 {
  try
  {
  ServiceHost host = null;
   host = new ServiceHost(typeof(StockService), new Uri("http://localhost:8080/EssentialWCF"));
  host.AddServiceEndpoint(typeof(IStockService),new BasicHttpBinding(), "");

  host.AddServiceEndpoint(typeof(IMetadataExchange), MetadataExchangeBindings.CreateMexHttpBinding(), "mex");

  ServiceMetadataBehavior behavior = new ServiceMetadataBehavior();
  behavior.HttpGetEnabled = true;
  host.Description.Behaviors.Add(behavior);    

  host.Open();
  Console.WriteLine("Host opened. Waiting...");
  Console.Read();

  host.Close();
  }
  catch(Exception e)
  {
   Console.WriteLine(e.Message);
   Console.Read();
  }
 }

Basically, define the Service end point for Mex first and then add the service behavior, I get the following exception with the message

The contract name 'IMetadataExchange' could not be found in the list of contract s implemented by the service EssentialWCF.StockService. Add a ServiceMetadataBe havior to the configuration file or to the ServiceHost directly to enable suppor t for this contract.

what is the dependency that I have to define them in the correct order...??

Thanks Harsha