Introduction

• In unSEH, you have to remember to include On Error GoTo sometimes in a random area inside the function. Besides On Error GoTo, you have to remember to create a label section. The name of the label has to be faithfully included in the On Error GoTo line
• When using the On Error GoTo technique, you have to make sure you get out of the function at the right time, which is done using an Exit Sub line. If you create this Exit Sub line in the wrong area in your function, either the whole code would not be considered or an desired section of the function would still execute, possibly producing an unpredictable result. In the same way, a bizarre way of creating a Resume or a Resume Next line would prevent the compiler from reaching or considering some sections of a function
• As mentioned previously, the On Error GoTo system provides a global variable named Err. As flexible as good intentioned as it may appear, to effectively use Err, you have to proceed by trial and error because Err identifies errors by a number but its documentation does not possibly provide a list of error numbers. This is because Err not only depends on the application (Visual Basic, VBA, etc) but also it depends on the circumstance in which it is invoked. The only possible solution is to cause an error in your code in order to cause Err to show the numeric error that occurred. Then you have to use that error number and create a custom error message

Because of these uncertainties, you should abandon the On Error GoTo traditional error handling and use SEH in all of your new code. Because SEH and unSEH techniques are inherently different, you cannot use both in the same function.

1. Start Microsoft Visual Basic if necessary.
   Create a new Windows Application named GeorgetownCleaningServices1
2. Design the form as follows:
    

   Control Name Text Additional Properties Form     Size: 378, 408 Label   Customer Name:   TextBox txtCustomerName1     Label   mm   Label   dd   Label   yyyy   Label   Order Date:   TextBox txtMM 1   TextBox txtDD 1   TextBox txtYYYY 2000   Label   Item Types   Label   Qty   Label   Unit Price   Label   Sub-Total   Label   Shirts   TextBox txtQtyShirts 0   TextBox txtUnitPriceShirts 1.15   TextBox txtSubTotalShirts 0.00   Label   Pants   TextBox txtQtyPants 0   TextBox txtUnitPricePants 1.95   TextBox txtSubTotalPants 0.00   Label   Other   TextBox txtQtyOther 0   TextBox txtUnitPriceOther 3.50   TextBox txtSubTotalOther 0.00   Button btnProcess Process   Label   Customer Name:   TextBox txtCustomerName2     Label   Order date:   TextBox txtOrderDate     Label   Tax Rate:   TextBox txtTaxRate 5.75   Label   %   Button btnTax Tax   Label   Total Order:   TextBox txtTotalOrder 0.00   Label   Tax Amount:   TextBox txtTaxAmount 0.00   Label   Net Price:   TextBox txtNetPrice 0.00   Label   Amount Tended:   TextBox txtAmountTended 0.00   Button btnDifference Diff   Label   Difference:   TextBox txtDifference 0.00

3. To arrange the tab sequence, on the main menu, click View -> Tab Order
4. On the form, click only the following controls whose squares have a white background, in the indicated order:
    
   
5. Press Esc
6. Right-click the form and click View Code
7. Declare a few variables as follows:
    

   Public Class Form1 ' Order Information Dim CustomerName As String Dim mm As String Dim dd As String Dim yyyy As String ' Quantities of items Dim NumberOfShirts As Integer Dim NumberOfPants As Integer Dim NumberOfOther As Integer ' Price of items Dim PriceOneShirt As Double Dim PriceAPairOfPants As Double Dim PriceOther As Double ' Each of these sub totals will be used for cleaning items Dim SubTotalShirts As Double Dim SubTotalPants As Double Dim SubTotalOther As Double ' Values used to process an order Dim TaxRate As Double Dim TotalOrder As Double Dim TaxAmount As Double Dim SalesTotal As Double End Class

8. In the Class Name combo box, select btnProcess
9. In the Method Name, select Click and implement its event as follows:
    

   Private Sub btnProcess_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnProcess.Click If btnProcess.Text = "Process" Then Height = 408 btnProcess.Text = "Reset" Else Height = 232 txtCustomerName1.Text = "" txtMM.Text = "1" txtDD.Text = "1" txtYYYY.Text = "2000" txtQtyShirts.Text = "0" txtQtyPants.Text = "0" txtQtyOther.Text = "0" txtSubTotalShirts.Text = "0.00" txtSubTotalPants.Text = "0.00" txtSubTotalOther.Text = "0.00" btnProcess.Text = "Process" End If ' Request order information from the user CustomerName = txtCustomerName1.Text mm = txtMM.Text dd = txtDD.Text yyyy = txtYYYY.Text ' Request the quantity of each category of items ' Number of Shirts NumberOfShirts = CInt(txtQtyShirts.Text) ' Number of Pants NumberOfPants = CInt(txtQtyPants.Text) ' Number of Dresses NumberOfOther = CInt(txtQtyOther.Text) ' Unit Prices of items PriceOneShirt = CDbl(txtUnitPriceShirts.Text) PriceAPairOfPants = CDbl(txtUnitPricePants.Text) PriceOther = CDbl(txtUnitPriceOther.Text) ' Perform the necessary calculations SubTotalShirts = NumberOfShirts * PriceOneShirt SubTotalPants = NumberOfPants * PriceAPairOfPants SubTotalOther = NumberOfOther * PriceOther txtSubTotalShirts.Text = CStr(SubTotalShirts) txtSubTotalPants.Text = CStr(SubTotalPants) txtSubTotalOther.Text = CStr(SubTotalOther) ' Calculate the "temporary" total of the order TotalOrder = SubTotalShirts + SubTotalPants + SubTotalOther ' Display the receipt txtCustomerName2.Text = CustomerName txtOrderDate.Text = mm + "/" & dd + "/" & yyyy txtTotalOrder.Text = CStr(TotalOrder) End Sub

10. In the Class Name combo box, select btnTax
11. In the Method Name, select Click and implement its event as follows:
     

    Private Sub btnTax_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnTax.Click ' Get the tax rate TaxRate = CDbl(txtTaxRate.Text) / 100 ' Calculate the tax amount using a constant rate TaxAmount = TotalOrder * TaxRate ' Add the tax amount to the total order SalesTotal = TotalOrder + TaxAmount txtTaxAmount.Text = TaxAmount.ToString() txtNetPrice.Text = CStr(SalesTotal) End Sub

12. In the Class Name combo box, select btnDifference
13. In the Method Name, select Click and implement its event as follows:
     

    Private Sub btnDifference_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnDifference.Click Dim AmountTended As Double = 0.0 Dim Difference As Double = 0.0 ' Request money for the order AmountTended = CDbl(txtAmountTended.Text) ' Calculate the difference owed to the customer ' or that the customer still owes to the store Difference = AmountTended - SalesTotal txtDifference.Text = CStr(Difference) End Sub

14. Return to the form and resize it form to appear as follows:
     
    
15. To execute the application, on the Standard toolbar, click the Start Without Debugging button
     
16. Close the form and return to your programming environment
17. Execute the application again. This time, type a letter such as d for the quantity of shirts and click Process
     
    
18. Click Quit to close the form and return to your programming environment

As mentioned already, errors are likely going to occur in your program. The more you anticipate them and take action, the better your application can be. We have already seen that syntax errors are usually human mistakes such as misspelling, bad formulation of expressions, etc. The compiler will usually help you fix the problem by pointing it out.

SEH is based on two main keywords: Try and Catch. An exception handling section starts with the Try keyword and stops with the End Try statement. Between Try and End Try, there must by at least one Catch section. Therefore, exception handling uses the following formula:

Try
    ' Code to execute in case everything is alright
Catch
    ' If something bad happened, deal with it here
End Try

Exception handling always starts with the Try keyword. Under the Try line, Write the normal code that the compiler must execute. Here is an example:

Private Sub CalculateClicked(ByVal sender As Object, _
                                              ByVal e As EventArgs) _
                                              Handles btnCalculate.Click
            Dim Number As Double
            Dim Result As Double

            Try
                Number = CDbl(InputBox("Enter a number:"))
            
            End Try
End Sub

As the compiler is treating code in the Try section, if it encounters a problem, it "gets out" of the Try section and starts looking for a Catch section. Therefore, you MUST always have a Catch section. If you do not, the program will not compile. A Catch section must be written before the End Try line:

Private Sub CalculateClicked(ByVal sender As Object, _
                                              ByVal e As EventArgs) _
                                              Handles btnCalculate.Click
            Dim Number As Double
            Dim Result As Double

            Try
                Number = CDbl(InputBox("Enter a number:"))
            Catch
                
            End Try
End Sub

When the Catch keyword is simply written as above, it would be asked to treat any error that occurs. For example, if you execute the above code with a number such as 35$.75 instead of 35$.75, nothing would appear to happen. This would indicate that the error was found and vaguely dealt with. One problem in this case is that the compiler would not bother to let the user know why there is no result displayed. Because there can be various types of errors in a program, you also should make your program more intuitive and friendlier so that, when an error occurs, the user would know the type of problem. This is also useful if somebody calls you and says that your program is not functioning right. If there is a way the user can tell you what exact type of error is displaying, maybe you would find the solution faster.

1. To introduce exceptions, access the form's code and change the events of the buttons as follows:
    

   Public Class Form1 . . . No Change Private Sub btnProcess_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnProcess.Click . . . No Change ' Request the quantity of each category of items ' Number of Shirts Try NumberOfShirts = CInt(txtQtyShirts.Text) Catch End Try ' Number of Pants Try NumberOfPants = CInt(txtQtyPants.Text) Catch End Try ' Number of Dresses Try NumberOfOther = CInt(txtQtyOther.Text) Catch End Try ' Unit Prices of items Try PriceOneShirt = CDbl(txtUnitPriceShirts.Text) Catch End Try Try PriceAPairOfPants = CDbl(txtUnitPricePants.Text) Catch End Try Try PriceOther = CDbl(txtUnitPriceOther.Text) Catch End Try . . . No Change End Sub Private Sub btnTax_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnTax.Click ' Get the tax rate Try TaxRate = CDbl(txtTaxRate.Text) / 100 Catch End Try ' Calculate the tax amount using a constant rate TaxAmount = TotalOrder * TaxRate ' Add the tax amount to the total order SalesTotal = TotalOrder + TaxAmount txtTaxAmount.Text = TaxAmount.ToString() txtNetPrice.Text = SalesTotal.ToString() End Sub Private Sub btnDifference_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnDifference.Click Dim AmountTended As Double = 0.0 Dim Difference As Double = 0.0 ' Request money for the order Try AmountTended = CDbl(txtAmountTended.Text) Catch End Try ' Calculate the difference owed to the customer ' or that the customer still owes to the store Difference = AmountTended - SalesTotal txtDifference.Text = CStr(Difference) End Sub End Class

2. Execute the application. This time, type invalid values in the quantity text boxes and other text boxes where the user is supposed to enter some values
3. Click Process
    
   
4. Return to your programming environment

As mentioned already, if an error occurs when processing the program in the Try section, the compiler transfers the processing to the next Catch section. You can then use the catch section to deal with the error. At a minimum, you can display a message to inform the user. To do this, you can create a message box in the Catch section. Here is an example:

Imports System.Drawing
Imports System.Windows.Forms

Module Exercise

    Public Class Starter
        Inherits Form

        Private lblNumber As Label
        Private txtNumber As TextBox
        Friend WithEvents btnCalculate As Button
        Private lblResult As Label
        Private txtResult As TextBox

        Dim components As System.ComponentModel.Container

        Public Sub New()
            InitializeComponent()
        End Sub

        Public Sub InitializeComponent()
            Text = "Exception Behavior"

            lblNumber = New Label
            lblNumber.Location = New Point(17, 23)
            lblNumber.Text = "Number:"
            lblNumber.AutoSize = True

            txtNumber = New TextBox
            txtNumber.Location = New Point(78, 20)
            txtNumber.Size = New Size(83, 20)

            btnCalculate = New Button
            btnCalculate.Location = New Point(78, 45)
            btnCalculate.Text = "Calculate"
            btnCalculate.Size = New Size(83, 23)

            lblResult = New Label
            lblResult.Location = New Point(17, 75)
            lblResult.Text = "Result:"
            lblResult.AutoSize = True

            txtResult = New TextBox
            txtResult.Location = New Point(76, 72)
            txtResult.Size = New Size(83, 20)

            Controls.Add(lblNumber)
            Controls.Add(txtNumber)
            Controls.Add(btnCalculate)
            Controls.Add(lblResult)
            Controls.Add(txtResult)

        End Sub

        Private Sub CalculateClicked(ByVal sender As Object, _
                                     	      ByVal e As EventArgs) _
                                     	      Handles btnCalculate.Click
            Dim Number As Double
            Dim Result As Double

            Try
                Number = CDbl(txtNumber.Text)
                Result = Number * 12.48
                txtResult.Text = CStr(Result)
            Catch
                MsgBox("Something bad happened")
            End Try
        End Sub
    End Class

    Function Main() As Integer

        Dim frmStart As Starter = New Starter

        Application.Run(frmStart)

        Return 0
    End Function

End Module

Of course, your message may not be particularly clear but this time, the program will not crash.

1. To display custom messages to the user, change the code as follows:
    

   Public Class Form1 . . . No Change Private Sub btnProcess_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnProcess.Click . . . No Change ' Request the quantity of each category of items ' Number of Shirts Try NumberOfShirts = CInt(txtQtyShirts.Text) Catch MsgBox("The value you typed for the number of " & _ "shirts is not a valid number." & _ vbCrLf & "Please enter a natural number such " & _ "as 2 or 24 or even 248") End Try ' Number of Pants Try NumberOfPants = CInt(txtQtyPants.Text) Catch MsgBox("The value you typed for the number of " & _ "pair or pants is not a valid number." & _ vbCrLf & "Please enter a natural number such " & _ "as 2 or 24 or even 248") End Try ' Number of other items Try NumberOfOther = CInt(txtQtyOther.Text) Catch MsgBox("The value you typed for the number of " & _ "other items is not a valid number." & _ vbCrLf & "Please enter a natural number such " & _ "as 2 or 24 or even 248") End Try ' Unit Prices of items Try PriceOneShirt = CDbl(txtUnitPriceShirts.Text) Catch MsgBox("The value you entered for the unit price " & _ "of a shirt is not a recognizable currency " & _ "amount." & vbCrLf & _ "Only natural or decimal numbers " & _ "are allowed. Please consult the management " & _ "to know the valid prices.") End Try Try PriceAPairOfPants = CDbl(txtUnitPricePants.Text) Catch MsgBox("The value you entered for the unit price of " & _ "a pair of pants is not a recognizable " & _ "currency amount." & vbCrLf & _ "Only natural or decimal " & _ "numbers are allowed. You can consult the " & _ "management to find out about " & _ "the allowable prices.") End Try Try PriceOther = CDbl(txtUnitPriceOther.Text) Catch MsgBox("The value you entered for the unit " & _ "price of other items is not a valid amount." & _ vbCrLf & "You must enter only a natural or a " & _ "decimal number. For more information, " & _ "please consult the management to get " & _ "the right prices.") End Try . . . No Change End Sub Private Sub btnTax_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnTax.Click ' Get the tax rate Try TaxRate = CDbl(txtTaxRate.Text) / 100 Catch MsgBox("The value you entered is not " & _ "recognized as a valid tax rate." & _ vbCrLf & "A valid tax rate is a value " & _ "between 0 and 100.00" & _ vbCrLf & "Please try again.") End Try ' Calculate the tax amount using a constant rate TaxAmount = TotalOrder * TaxRate ' Add the tax amount to the total order SalesTotal = TotalOrder + TaxAmount txtTaxAmount.Text = TaxAmount.ToString() txtNetPrice.Text = SalesTotal.ToString() End Sub Private Sub btnDifference_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnDifference.Click Dim AmountTended As Double = 0.0 Dim Difference As Double = 0.0 ' Request money for the order Try AmountTended = CDbl(txtAmountTended.Text) Catch MsgBox("The value you entered for the amount " & _ "tended is not valid. Only natural or " & _ "decimal numbers are allowed." & _ "Please try again.") End Try ' Calculate the difference owed to the customer ' or that the customer still owes to the store Difference = AmountTended - SalesTotal txtDifference.Text = CStr(Difference) End Sub End Class

2. Test the application with valid and invalid values. Here is an example:
    
3. Return to Notepad