private async void InsertUser(tbl_user u)
 {
     try
     {
         var   call = userTable.InsertAsync(u);
         await call;
         if (call.IsFaulted)
         {
             MessageBox.Show("Registration failed");
         }
         if (call.IsCompleted)
         {
             MessageBox.Show("Registration success");
         }
         if (call.IsCanceled)
         {
             MessageBox.Show("Registration cancelled");
         }
     }
     catch (MobileServicePreconditionFailedException)
     {
         MessageBox.Show("Registration failed");
     }
     //items.Add(u);
 }
        private void Button_Click(object sender, RoutedEventArgs e)
        {
            tbl_user u = new tbl_user();

            u.firstname = tbfirstname.Text;
            u.lastname  = tblastname.Text;
            u.username  = tbuname.Text;
            u.password  = tbpwd.Text;
            u.email     = tbemail.Text;
            InsertUser(u);
        }