You only need the Action / ReplyAction properties if you want to customize those values in the messages (and they’re reflected in the WSDL). If you don’t have them, the default is <serviceContractNamespace> + <serviceContractName> + <operationName> for the Action, and <serviceContractNamespace> + <serviceContractName> + <operationName> + "Response" for ReplyAction.

The code below prints out the Action/ReplyAction properties of all operations in the service.

public class StackOverflow_6470463
{
    [ServiceContract(Namespace = "http://schemas.mycompany.com/", Name = "MyService")]
    public interface IMyService
    {
        [OperationContract(Name = "MyOperation")]
        string MyOperation(string request);
    }
    public class Service : IMyService
    {
        public string MyOperation(string request) { return request; }
    }
    public static void Test()
    {
        string baseAddress = "http://" + Environment.MachineName + ":8000/Service";
        ServiceHost host = new ServiceHost(typeof(Service), new Uri(baseAddress));
        host.AddServiceEndpoint(typeof(IMyService), new BasicHttpBinding(), "");
        host.Open();
        Console.WriteLine("Host opened");

        foreach (ServiceEndpoint endpoint in host.Description.Endpoints)
        {
            Console.WriteLine("Endpoint: {0}", endpoint.Name);
            foreach (var operation in endpoint.Contract.Operations)
            {
                Console.WriteLine("  Operation: {0}", operation.Name);
                Console.WriteLine("    Action: {0}", operation.Messages[0].Action);
                if (operation.Messages.Count > 1)
                {
                    Console.WriteLine("    ReplyAction: {0}", operation.Messages[1].Action);
                }
            }
        }

        Console.Write("Press ENTER to close the host");
        Console.ReadLine();
        host.Close();
    }
}