public OperationResult CreateEmployee(Model.Employee model)
 {
     using (IUnitOfWork unitofwork = new EntityFrameworkContext())
     {
         IEmployeeRepository employeeRepository = new EmployeeRepository(unitofwork);
         if (!employeeRepository.ExistsUserName(model.UserName))
         {
             employeeRepository.Insert(model);
             unitofwork.Commit();
             return new OperationResult(OperationResultType.Success, "用户注册成功.");
         }
         return new OperationResult(OperationResultType.NoChanged, "用户名已存在.");
     }
 }