Take Care of Error Handling with Bound Controls

This How-To shows examples of what exceptions could occur and how to handle them using the Try, Catch, and Finally statements.. Listing 1.15 Standard Code for Handling Exceptions Privat

1.6 Take Care of Error Handling with Bound Controls

When dealing with database tasks, you are going to get runtime errors In NET, we call the various types of errors you can get exceptions This How-To shows examples of what exceptions could occur and how to handle them using the Try, Catch, and Finally

statements

Adding and deleting records is fine, but what happens if an error occurs? You tried to delete an existing company, and got the screen that appears in Figure 1.10

Figure 1.10 This is an unhandled error, called an exception in NET

How do you make it so errors are handled gracefully within the application using bound controls?

Technique

Error handling is one of the most important aspects of working with data, between when

a user is entering data and updating the server If you don't have proper error handling, your system will, in the best situation, give an ugly error message, and in the worst, blow

up and put bad data in your system In Visual Studio NET, errors are classes called exceptions Many different (inherited) types of exceptions exist

Run-time exceptions can occur in just about any part of your application They can be handled by using Try Catch Finally or they can be unhandled, where you will see the error in Figure 1.11

Figure 1.11 This error was thrown up from one subroutine to the one that called it

Take a look at a common way to trap exceptions shown in Listing 1.15

Listing 1.15 Standard Code for Handling Exceptions

Private Sub MySub()

Try

'-Code to be handled here

Catch dataException as Exception

MessageBox.Show(dataException.Message)

Finally

'-Code that will occur regardless of if an error occurs

End Try

End Sub

Following are some basic points to help you when working with exceptions and

Try Catch Finally blocks:

• The name of the exception object can be whatever you want it to be

dataException was just used as an example

• Specific types of exceptions inherit from the base class of System.Exception OleDbException is one of those classes, and you will see an example of using this class in the following steps

• You can use the Throw statement within a Catch statement to throw the exception

back up to a calling subroutine

• You can use the When clause on the Catch statement to trap for specific

exceptions

• When multiple Catch statements are used, each one is executed unless an End Try

or End Sub is placed within a Catch block, as in this How-To

• You can use multiple Catch statements to handle various possible exceptions that

can occur

You will see an example of these bullets in the following steps

Steps

Taking the form with which you have been working, you are going to modify it to trap exceptions when the record is being saved (either for additions or editing) and when the record is deleted You will also add some code in the event when closing the form

1 Modify the code in the Click event of the command button called btnSave You will surround the SaveRecord routine call with a Try Catch End Try block If an exception occurs, the Show method of the MessageBox class is called, passing the Message property of the Exception object called saveException Next, the

subroutine is exited If no errors occur, then life goes on, and so does the

subroutine

Listing 1.16 frmHowTo1_6.vb: Code Added to btnSave Click Event to Trap Save Exceptions

Private Sub btnSave_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles btnSave.Click

Try

'- Save the information

SaveRecord()

Catch saveException As Exception

MessageBox.Show("The following error has occurred: " & _

saveException.Message, "Error Saving Record")

Exit Sub

End Try

'- Disable the text boxes

ActivateEditing(False)

If mbAddNew Then

LoadList()

RefreshIndividual()

mbAddNew = False

End If

End Sub

2 Modify the SaveRecord routine to "throw" the exception up to the btnSave_Click subroutine, or whatever happens to have called the SaveRecord routine Following

is the SaveRecord subroutine with the new Try Catch End Try block added, along with the Throw statement

Listing 1.17 frmHowTo1_6.vb: Pass the Exception Back Up to the Calling Routine

Private Sub SaveRecord()

Try

'- Use the BindingContext class to end the current editing so

' that we can update the server

Me.BindingContext(Me.dsCustomerIndividual, _

"Customers").EndCurrentEdit()

'- Perform the requested task at the dataset

' level using the data adapter

odaCustomerIndividual.Update(dsCustomerIndividual, "Customers")

'- By accepting the changes, the data gets sent back to the server

dsCustomerIndividual.AcceptChanges()

Catch saveException As Exception

Throw saveException

End Try

End Sub

3 Now it's time to deal with adding exception handling to the btnDelete Click event,

as seen next The Try statement is used to wrap the code that performs deletion of the data The code then uses a Catch statement to check whether the exception that occurs is a specific OleDbException, 3621, which is an error having to do with trying to delete a record when related records exist in another table Note that you could-and in fact should-assign this to a constant value Because delException has

been caught as an OleDbException, you can look at the NativeError property of the first error in the Errors collection to get the actual OleDb error number If the error is not 3621, then the exception is trapped with the next Catch statement, and

a messagebox is displayed If errors occur, then the subroutine is exited before the final lines of code are executed

Listing 1.18 frmHowTo1_6.vb: Catching Exceptions When You're Deleting a Record

Private Sub btnDelete_Click(ByVal sender As System.Object,

ByVal e As System.EventArgs) Handles btnDelete.Click

Try

'- Mark the row for deletion using the RemoveAt

' method of the BindingContext

Me.BindingContext(Me.dsCustomerIndividual,

"Customers").RemoveAt(Me.BindingContext(Me.dsCustomerIndividual,

"Customers").Position)

'- Perform the requested task at the dataset

' level using the data adapter

odaCustomerIndividual.Update(dsCustomerIndividual, "Customers")

'- By accepting the changes, the data gets sent back to the server

dsCustomerIndividual.AcceptChanges()

Catch delException As System.Data.OleDb.OleDbException _

When delException.Errors(0).NativeError = 3621

MessageBox.Show("An error occurred because of related order records.",

"Error Deleting Customer", _

MessageBoxButtons.OK, MessageBoxIcon.Exclamation)

Exit Sub

Catch delException As Exception

MessageBox.Show(delException.Message, "Error Deleting Customer", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)

Exit Sub

End Try

'- Reload the list

LoadList()

'- Display the first record

RefreshIndividual()

'- Disable the text boxes

ActivateEditing(False)

End Sub

How It Works

When an exception occurs within the Try Catch End Try block, the Catch statements are compared when a When clause is used or when the code is simply executed If a Throw statement is used, then an execution is thrown back up a level to the

Try Catch End Try block containing the call to the routine where the Throw statement occurred You can see an example of this in Figure 1.11

Comments

Microsoft has gone to great length to let you control how you handle exceptions in NET with the various languages You can be as creative as you need to by using the

Try Catch End Try block with all the clauses available

Exceptions bring back plenty of information so that you can create pretty slick error handling You can use exceptions to create a centralized routine that logs errors, or even one that e-mails exception information to you from your applications