Chapter 15 - Hand-coding data access pages

Antonius Block · August 10th, 2015, 07:04 PM

Hello Imar,

in chapter 15 you create a hand-coded version of AddEditReview page - based on the Entity Framework and not on a SqlDataSourceControl.
However there is no handling of errors that may occur in the DB access.
For instance, if I try to update a review which, in the meantime, was deleted by another user of the applicaton I get a System.InvalidOperationException.
I've chosen to manage the exception in a try/catch and Response.Redirect the user to an error page, but I would like to know if there is a more elegant way to manage the DB exception within Entity Framework
like you did on page 562 where you caught a SqlException 547 from a SqlDataSource control.

Below my code.
Thanks
Antonius

Code:

    protected void SaveButton_Click(object sender, EventArgs e)
    {
      try
      {
        using (PlanetWroxEntities istanzaDb = new PlanetWroxEntities())
        {
          Review myReview = null;
          if (_id == -1) // Insert new item
          {
            myReview = new Review();
            myReview.CreateDateTime = DateTime.Now;
            myReview.UpdateDateTime = myReview.CreateDateTime;
            istanzaDb.Reviews.Add(myReview);
          }
          else // update existing item
          {
            var counter = (from record in istanzaDb.Reviews
                              where record.Id == _id
                              select record).Count();
            if (counter != 1)
            {
              ErrorMessage.Text = "The review was not found in the DB any more";
              return;             
             }
            else
            {
              myReview = (from record in istanzaDb.Reviews
                          where record.Id == _id
                          select record).Single();
              myReview.UpdateDateTime = DateTime.Now;
            } // end else 
          }  // end else 
          myReview.Title = TitleText.Text;
          myReview.Summary = SummaryText.Text;
          myReview.Body = BodyText.Text;
          myReview.GenreId = Convert.ToInt32(GenreList.SelectedValue);
          myReview.Authorized = Authorized.Checked;
          istanzaDb.SaveChanges();
          Response.Redirect("Reviews.aspx");
        } // end using
      } // end try
      catch (System.InvalidOperationException)
      {
        Response.Redirect("~/Errors/ErrorDbGeneric.aspx");
      }
    }

Imar · August 19th, 2015, 03:08 PM

Hi there,

In general your approach is good: you use try/catch to catch any exceptions.

You can check if an item exists in the database without raising an exception though. On page 498 you can see that Single throws an exception when the item doesn't exist, but SingleOrDefault does not. By replacing your call to Single with SingleOrDefault, you get back null when the item doesn't exist. You can then check for null instead.

You can use the same try/catch principle as on page 562 to handle other (database) exceptions here.

Hope this helps,

Imar