Catching exceptions thrown from an event...

jacob · December 5th, 2005, 12:19 PM

There is probably a simple answer to this, but I cannot seem to find out where to catch exceptions thrown from an event, say from within a RowDeleting event of DataTable. I have done a small application with a DataView and when I delete a row I would like a confirmation dialog.

I have read at MSDN that one has the throw an exception in order to cancel the deletion of the row. Fine, but where do I catch this exception? It is the main thread so I am clueless. This is the method implementing the event actions...

Code:

        private void Table_RowDeleting(object sender, DataRowChangeEventArgs ea)
        {
            XmlDocument xml;
            DialogResult result;
            try
            {
                result = MessageBox.Show("Delete element for " + (string)ea.Row[1] + "?", 
                    "Deleting...", MessageBoxButtons.OKCancel);

                if(result == DialogResult.OK)
                {
                    if(File.Exists(tbFilename.Text))
                    {
                        xml = new XmlDocument();
                        xml.Load(tbFilename.Text);
                        xml.DocumentElement.RemoveChild(
                            xml.DocumentElement.SelectSingleNode("item[@id='" 
                                + (int)ea.Row[0] + "']"));
                        xml.Save(tbFilename.Text);
                    }
                    else
                        throw new FileNotFoundException(
                            "The specified file was not found.");
                }
                else
                    throw new Exception("canceled");
            }
            catch(Exception e)
            {
                MessageBox.Show(e.Message);
                throw;
            }            
        }

If the exception throwing solution is not the way to cancel row deletion in a DataTable, please let me know as well.

Thanks,
Jacob.

Ankur_Verma · December 5th, 2005, 06:06 PM

If you r deleting the rows using DataRow's Delete function, then u put this function call in try-catch. Like this

            try
            {
                // Delete all the rows
                foreach( DataRow row in employeeTable.Rows )
                    row.Delete();
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

Any exception thrown from within the row_deleting event handler will be caught here.
And yes- throwing the exception, indeed halts the deletion process.

Regards
Ankur Verma

jacob · December 6th, 2005, 06:50 AM

Thanks Ankur. I am not using the Delete method since the DataTable is shown as a DataView in the application and the RowDeleting event is therefore triggered when the Delete button is clicked and the row is highlightend.

There must be a way to catch these exceptions as well, right?

Thanks, Jacob.

jacob · December 6th, 2005, 10:39 AM

Finally, found the solution. The following code is the interesting parts of the solution...

Code:

        ...
        [STAThread]
        static void Main() 
        {
            Application.ThreadException += 
                new System.Threading.ThreadExceptionEventHandler(Application_ThreadException);
            Application.Run(new MainForm());
        }
        ...
        private static void Application_ThreadException(object sender, 
            System.Threading.ThreadExceptionEventArgs ea)
        {
            MessageBox.Show("error");
        }
        ...
        private void Table_RowDeleting(object sender, DataRowChangeEventArgs ea)
        {
            XmlDocument xml;
            DialogResult result;
            try
            {
                result = MessageBox.Show("Delete element for " + (string)ea.Row[1] + "?", 
                    "Deleting...", MessageBoxButtons.OKCancel);

                if(result == DialogResult.OK)
                {
                    ...
                }
                else
                    throw new Exception("canceled");
            }
            catch(Exception e)
            {
                MessageBox.Show(e.Message);
                throw;
            }            
        }
        ...

I found the solution at this page.

Thanks anyway, Jacob.

Ankur_Verma · December 6th, 2005, 12:38 PM

Okay, so you are basically creating an exception handler for all the exceptions occuring at the main thread level. I was thinking on the line of putting the whole database operation in a new thread.
Other solution could be to trap the key press and do the deletion yourself calling the DataRow's Delete function based on the validation, when delete key is pressed.
But that seemed like a lot of work.
Nice solution, this one. Thanks for updating the forum.

Regards
Ankur Verma

jacob · December 6th, 2005, 12:48 PM

Yes, that would be possible too, thanks, regards, Jacob.