The .NET Framework Support for Exception Handling

A Review of .NET Exception Classes

Introduction

The .NET Framework provides various classes to handle almost any type of exception you can think of. There are so many of these classes that we can only mention a few.

There are two main ways you can use one of the classes of the .NET Framework. If you know for sure that a particular exception will be produced, pass its name to a Catch clause. Then, in the Catch section, display a custom message. The second option you have consists of using the Throw keyword. We will study it later.

In most cases, we will try to always indicate the type of exception that could be thrown if something goes wrong in a program.

Everything the user types into an application using the keyboard is primarily a string and you must convert it to the appropriate type before using it. When you request a specific .NET type of value from the user, after the user has typed it and you decide to convert it to the appropriate type, if your conversion fails, the program produces (we will use he word "throw") an error. The error is of from the FormatException class.

Here is a program that deals with a FormatException exception:

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 = "Exceptional 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 = Double.Parse(txtNumber.Text)
                Result = Number * 12.48
                txtResult.Text = CStr(Result)
            Catch ex As FormatException
                MsgBox("Inavlid Value!")
            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

1. Open the GeorgetownCleaningServices1 application from the previous lesson
2. 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 ex As FormatException 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 ex As FormatException 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 ex As FormatException 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 ex As FormatException 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 ex As FormatException 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 ex As FormatException 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 ex As FormatException 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 ex As FormatException 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

3. Execute the application and return to your programming environment

A computer application receives, processes, and produces values on a regular basis as the program is running. To better manage these values, as we saw when studying variables and data types, the compiler uses appropriate amounts of space to store its values. It is not unusual that either you the programmer or a user of your application provides a value that is beyond the allowed range based on the data type. For example, a byte uses 8 bits to store a value and a combination of 8 bits can store a number no more than 255. If you provide a value higher than 255 to be stored in a byte, you get an error. Consider the following program:

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

            Try
                Number = Byte.Parse(txtNumber.Text)
                Result = Number * 12
                txtResult.Text = CStr(Result)
            Catch ex As FormatException
                MsgBox("Inavlid Value!")
            End Try
End Sub

When a value beyond the allowable range is asked to be stored in memory, the compiler produces (the verb is "throws" as we will learn soon) an error of the OverflowException class. Here is an example of running the program with a bad number:

As with the other errors, when this exception is thrown, you should take appropriate action.

Once again, in a .NET Framework application, a value is passed to the Parse() method of its data type for analysis. For a primitive data type, the Parse() method scans the string and if the converted value is beyond a determined range, the compiler throws an ArgumentOutOfRangeException exception.

1. Under the CustomerName variable, declare a variable named OrderTime of type DateTime
    

   Public Class Form1 ' Order Information Dim CustomerName As String Dim OrderDate As DateTime Dim mm As String Dim dd As String Dim yyyy As String

2. Change the Click event of the Process button as follows:
    

   Private Sub btnProcess_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnProcess.Click . . . No Change ' Request order information from the user CustomerName = txtCustomerName1.Text Try Dim mm As Integer, dd As Integer, yyyy As Integer mm = Integer.Parse(txtMM.Text) dd = Integer.Parse(txtDD.Text) yyyy = Integer.Parse(txtYYYY.Text) OrderDate = New DateTime(yyyy, mm, dd) Catch ex As ArgumentOutOfRangeException MsgBox("The date you entered is not valid" & _ "- Please try again!") End Try . . . No Change ' Display the receipt txtCustomerName2.Text = CustomerName txtOrderDate.Text = FormatDateTime(OrderDate, DateFormat.LongDate) txtTotalOrder.Text = CStr(TotalOrder) End Sub

3. Execute the application.
4. To test it enter valid and invalid values for the controls. Here is an example:
    
5. Close the form and return to your programming environment

Division by zero is an operation to always avoid. It is so important that it is one of the most fundamental exceptions of the computer. It is addressed at the core level even by the processors. It is also addressed by the operating systems at their level. It is also addressed by most, if not all, compilers. It is also addressed by most, if not, all libraries. This means that this exception is never welcomed anywhere. The .NET Framework also provides it own class to face this operation.

If an attempt to divide a value by 0, the compiler throws a DivideByZeroException exception.

As mentioned above, the Exception class is equipped with a Message property that carries a message for the error that occurred. We also mentioned that the message of this property may not be particularly useful to a user. Fortunately, you can create your own message and pass it to the Exception object. To be able to receive custom messages, the Exception class provides the following constructor:

Public Sub New(message As String)

To use it, in the section where you are anticipating the error, type the Throw keyword followed by a New instance of the Exception class using the constructor that takes a string. Here is an example:

Private Sub Button1_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles Button1.Click
        Dim Operand1 As Double
        Dim Operand2 As Double
        Dim Result As Double = 0.0
        Dim Oper As String = "."

        Try
            Operand1 = Double.Parse(TextBox1.Text)
            Oper = TextBox2.Text
            Operand2 = Double.Parse(TextBox3.Text)

            If (Oper <> "+") And _
               (Oper <> "-") And _
               (Oper <> "*") And _
               (Oper <> "/") Then
                Throw New Exception(Oper)
            End If

            Select Case Oper

                Case "+"
                    Result = Operand1 + Operand2

                Case "-"
                    Result = Operand1 - Operand2

                Case "*"
                    Result = Operand1 * Operand2

                Case "/"
                    Result = Operand1 / Operand2

                Case Else
                    MsgBox("Bad Operation")

            End Select

            TextBox4.Text = CStr(Result)
            
        Catch Ex As Exception
            MsgBox("Operation Error: " & Ex.Message & _
                   vbCrLf & Oper & " is not a valid operator")
        End Try
End Sub

In the above examples, when we anticipated some type of problem, we instructed the compiler to use our default catch section. We left it up to the compiler to find out when there was a problem and we provided a catch section to deal with it. A method with numerous or complex operations and requests can also produce different types of errors. With such a type of program, you should be able to face different problems and deal with them individually, each by its own kind. To do this, you can create different catch sections, each made for a particular error. The formula used would be:

Try 
	' Code to Try
Catch Arg1

	' One Exception
Catch Arg2

	' Another Exception
End Try

The compiler would proceed in a top-down:

1. Following the normal flow of the program, the compiler enters the try block
2. If no exception occurs in the Try block, the rest of the Try block is executed
   If an exception occurs in the Try block, the compiler registers the type of error that occurred. If there is a Throw line, the compiler registers it also:
   1. The compiler gets out of the Try section
   2. The compiler examines the first Catch. If the first Catch matches the thrown error, that catch executes and the exception handling routine may seize. If the first Catch does not match the thrown error, the compiler proceeds with the next Catch
   3. The compiler checks the next match, if any, and proceeds as in the first match. This continues until the compiler finds a Catch clause that matches the thrown error
   4. If one of the catches matches the thrown error, its body executes. If no Catch matches the thrown error, the compiler calls the Exception class and uses the default message

Multiple catches are written if or when a try block is expected to throw different types of errors. For example, in our calculator, we want to consider only the addition, the subtraction, the multiplication, and the division. It is also likely that the user may type one or two invalid numbers. This leads us to know that our program can produce at least two types of errors at this time. Based on this, we can address them using two catch clauses as follows:

Private Sub Button1_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles Button1.Click
        Dim Operand1 As Double
        Dim Operand2 As Double
        Dim Result As Double = 0.0
        Dim Oper As String = "."

        Try
            Operand1 = Double.Parse(TextBox1.Text)
            Oper = TextBox2.Text
            Operand2 = Double.Parse(TextBox3.Text)

            If (Oper <> "+") And _
               (Oper <> "-") And _
               (Oper <> "*") And _
               (Oper <> "/") Then
                Throw New Exception(Oper)
            End If

            Select Case Oper

                Case "+"
                    Result = Operand1 + Operand2

                Case "-"
                    Result = Operand1 - Operand2

                Case "*"
                    Result = Operand1 * Operand2

                Case "/"
                    Result = Operand1 / Operand2

                Case Else
                    MsgBox("Bad Operation")

            End Select

            TextBox4.Text = CStr(Result)

        Catch ex As FormatException
            MsgBox("You type an invalid number. Please correct it")
        Catch Ex As Exception
            MsgBox("Operation Error: " & Ex.Message & _
                   vbCrLf & Oper & " is not a valid operator")
        End Try
End Sub

This program works fine as long as the user types two valid numbers and a valid arithmetic operator. Anything else, such an invalid number or an unexpected operator would cause an error to be thrown:

Obviously various bad things could happen when this program is running. Imagine that the user wants to perform a division. You need to tell the compiler what to do if the user enters the denominator as 0 (or 0.00). If this happens, one of the options you should consider is to display a message and get out. Fortunately, the .NET Framework provides the DivideByZeroException class to deal with an exception caused by division by zero. As done with the message passed to the Exception class, you can compose your own message and pass it to the DivideByZeroException(string message) constructor.

Exception is the parent of all exception classes. It corresponds to the type of Catch that takes no argument. Therefore, if you write various catch blocks, the one that either takes nor argument or is of the Exception type must be the last.

Here is an example that catches two types of exceptions:

Private Sub Button1_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles Button1.Click
        Dim Operand1 As Double
        Dim Operand2 As Double
        Dim Result As Double = 0.0
        Dim Oper As String = "."

        Try
            Operand1 = Double.Parse(TextBox1.Text)
            Oper = TextBox2.Text
            Operand2 = Double.Parse(TextBox3.Text)

            If (Oper <> "+") And _
               (Oper <> "-") And _
               (Oper <> "*") And _
               (Oper <> "/") Then
                Throw New Exception(Oper)
            End If

            Select Case Oper

                Case "+"
                    Result = Operand1 + Operand2

                Case "-"
                    Result = Operand1 - Operand2

                Case "*"
                    Result = Operand1 * Operand2

                Case "/"
                    If Operand2 = 0 Then
                        Throw New DivideByZeroException("Division by zero is not allowed")
                    End If

                    Result = Operand1 / Operand2

                Case Else
                    MsgBox("Bad Operation")

            End Select

            TextBox4.Text = CStr(Result)

        Catch ex As FormatException
            MsgBox("You type an invalid number. Please correct it")
        Catch ex As DivideByZeroException
            MsgBox(ex.Message)
        Catch
            MsgBox("Invalid Operation: " & vbCrLf & Oper & " is not a valid operator")
        End Try
End Sub

1. To catch various exceptions, change the code as follows:
    

   Public Class Form1 ' Order Information Dim CustomerName As String Dim OrderDate As DateTime 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 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 Try Dim mm As Integer, dd As Integer, yyyy As Integer mm = Integer.Parse(txtMM.Text) dd = Integer.Parse(txtDD.Text) yyyy = Integer.Parse(txtYYYY.Text) OrderDate = New DateTime(yyyy, mm, dd) ' This exception is thrown if the user types a value that cannot ' be converted into a valid number Catch ex As FormatException MsgBox("Error: " & ex.Message & _ vbCrLf & "The value you entered " & _ "is not a valid number") ' This exception is thrown if the values that user had typed cannot ' produce a valid date value Catch ex As ArgumentOutOfRangeException MsgBox("The date you entered is not valid" & _ "- Please try again!") End Try ' Request the quantity of each category of items ' Number of Shirts Try NumberOfShirts = CInt(txtQtyShirts.Text) Catch ex As FormatException 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") ' This exception is thrown if the user types a negative value Catch Ex As OverflowException MsgBox("The number you typed is negative but " & _ "we cannot accept a negative number of shirts") End Try ' Number of Pants Try NumberOfPants = CInt(txtQtyPants.Text) Catch ex As FormatException 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") Catch Ex As OverflowException MsgBox("The number you typed is negative but " & _ "we cannot accept a negative number of shirts") End Try ' Number of other items Try NumberOfOther = CInt(txtQtyOther.Text) Catch ex As FormatException 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") Catch Ex As OverflowException MsgBox("The number you typed is negative but " & _ "we cannot accept a negative number of shirts") End Try ' Unit Prices of items Try PriceOneShirt = CDbl(txtUnitPriceShirts.Text) If PriceOneShirt < 0 Then Throw New Exception("Negative numbers are not allowed " & _ "for the price of a shirt") End If Catch ex As FormatException 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.") Catch Ex As Exception MsgBox("Something bad happened") End Try Try PriceAPairOfPants = CDbl(txtUnitPricePants.Text) If PriceAPairOfPants < 0 Then Throw New Exception("Negative numbers are not allowed " & _ "for the price of a pair of pants") End If Catch ex As FormatException 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.") Catch Ex As Exception MsgBox("Something bad happened") End Try Try PriceOther = CDbl(txtUnitPriceOther.Text) If PriceOther < 0 Then Throw New Exception("Negative numbers are " & _ "not allowed for the price") End If Catch ex As FormatException 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.") Catch Ex As Exception MsgBox("Something bad happened") End Try ' 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 = FormatDateTime(OrderDate, DateFormat.LongDate) txtTotalOrder.Text = CStr(TotalOrder) 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 If TaxRate < 0 Then Throw New Exception("Negative numbers are not " & _ "allowed for a tax rate") End If Catch ex As FormatException 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.") Catch Ex As Exception MsgBox("Something bad happened") 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) If AmountTended < 0 Then Throw New Exception("Negative numbers are not " & _ "allowed forthe amount tended") End If Catch ex As FormatException MsgBox("The value you entered for the amount " & _ "tended is not valid. Only natural or " & _ "decimal numbers are allowed." & _ "Please try again.") Catch Ex As Exception MsgBox("Something bad happened") 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 and return to your programming environment

The calculator simulator we have studied so far performs a division as one of its assignments. We learned that, in order to perform any operation, the compiler must first make sure that the user has entered a valid operator. Provided the operator is one of those we are expecting, we also must make sure that the user typed valid numbers. Even if these two criteria are met, it is still possible that the user would enter 0 for the denominator. The block that is used to check for a non-zero denominator depends on the exception that validates the operators. The exception that could result from a zero denominator depends on the user first entering a valid number for the denominator.

You can create an exception inside of another. This is referred to as nesting an exception. This is done by applying the same techniques used to nest conditional statements. This means that you can write an exception that depends on, and is subject to, another exception. To nest an exception, create a Try clause in the body of the parent exception. The nested Try clause must be followed by its own Catch clause(s). To effectively handle the exception, make sure you include an appropriate Throw in the Try block. Here is an example:

Private Sub Button1_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles Button1.Click
        Dim Operand1 As Double
        Dim Operand2 As Double
        Dim Result As Double = 0.0
        Dim Oper As String = "."

        Try
            Operand1 = Double.Parse(TextBox1.Text)
            Oper = TextBox2.Text
            Operand2 = Double.Parse(TextBox3.Text)

            If (Oper <> "+") And _
               (Oper <> "-") And _
               (Oper <> "*") And _
               (Oper <> "/") Then
                Throw New Exception(Oper)
            End If

            Select Case Oper

                Case "+"
                    Result = Operand1 + Operand2
                    TextBox4.Text = CStr(Result)

                Case "-"
                    Result = Operand1 - Operand2
                    TextBox4.Text = CStr(Result)

                Case "*"
                    Result = Operand1 * Operand2
                    TextBox4.Text = CStr(Result)

                Case "/"
                    Try
                        If Operand2 = 0 Then
                            Throw New DivideByZeroException( _
					"Division by zero is not allowed")
                        End If
                        Result = Operand1 / Operand2

                    Catch ex As DivideByZeroException
                        MsgBox(ex.Message)
                    End Try

                Case Else
                    MsgBox("Bad Operation")

            End Select


        Catch ex As FormatException
            MsgBox("You type an invalid number. Please correct it")
        Catch
            MsgBox("Invalid Operation: " & vbCrLf & _
			Oper & " is not a valid operator")
        End Try
End Sub

One of the most effective techniques used to deal with code is to isolate assignments in different functions. For example, the Select Case statement that was performing the operations in the “normal” version of our program could be written as follows:

Private Function Calculate(ByVal Value1 As Double, _
                               ByVal Value2 As Double, _
                               ByVal symbol As Char) As Double

        Dim Result As Double = 0.0

        Select Case symbol

            Case "+"
                Result = Value1 + Value2

            Case "-"
                Result = Value1 - Value2

            Case "*"
                Result = Value1 * Value2

            Case "/"
                Result = Value1 / Value2
        End Select

        Calculate = Result
End Function

Private Sub Button1_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles Button1.Click
        Dim Operand1 As Double
        Dim Operand2 As Double
        Dim Result As Double = 0.0
        Dim Oper As String = "."

        Try
            Operand1 = Double.Parse(TextBox1.Text)
            Oper = TextBox2.Text
            Operand2 = Double.Parse(TextBox3.Text)

            If (Oper <> "+") And _
               (Oper <> "-") And _
               (Oper <> "*") And _
               (Oper <> "/") Then
                Throw New Exception(Oper)
            End If

            If Oper = "/" Then
                If Operand2 = 0 Then
                    Throw New DivideByZeroException("Division by zero is not allowed")
                End If
            End If

            Result = Calculate(Operand1, Operand2, Oper)
            TextBox4.Text = CStr(Result)

        Catch ex As FormatException
            MsgBox("You type an invalid number. Please correct it")
        Catch ex As DivideByZeroException
            MsgBox(ex.Message)
        Catch
            MsgBox("Invalid Operation: " & vbCrLf & Oper & " is not a valid operator")
        End Try
End Sub

This is an example of running the program:

You can still use regular functions that handle exceptions and each function can handle its own exception(s). Here is an example:

Private Function Addition(ByVal Value1 As Double, _
                               ByVal Value2 As Double) As Double
        Addition = Value1 + Value2
End Function

Private Function Subtraction(ByVal Value1 As Double, _
                                 ByVal Value2 As Double) As Double
        Subtraction = Value1 - Value2
End Function

Private Function Multiplication(ByVal Value1 As Double, _
                                    ByVal Value2 As Double) As Double
        Multiplication = Value1 * Value2
End Function

Private Function Division(ByVal Value1 As Double, _
                              ByVal Value2 As Double) As Double

        Dim Result As Double = 0.0

        Try

            If Value2 = 0 Then
                Throw New DivideByZeroException("Division by zero is not allowed")
            End If

            Result = Value1 + Value2

        Catch ex As DivideByZeroException
            MsgBox(ex.Message)
        End Try

        Division = Result
End Function

Private Sub Button1_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles Button1.Click
        Dim Operand1 As Double
        Dim Operand2 As Double
        Dim Result As Double = 0.0
        Dim Oper As String = "."

        Try
            Operand1 = Double.Parse(TextBox1.Text)
            Oper = TextBox2.Text
            Operand2 = Double.Parse(TextBox3.Text)

            If (Oper <> "+") And _
               (Oper <> "-") And _
               (Oper <> "*") And _
               (Oper <> "/") Then
                Throw New Exception(Oper)
            End If

            Select Case Oper

                Case "+"
                    Result = Addition(Operand1, Operand2)

                Case "-"
                    Result = Subtraction(Operand1, Operand2)

                Case "*"
                    Result = Multiplication(Operand1, Operand2)

                Case "/"
                    Result = Division(Operand1, Operand2)
            End Select

            TextBox4.Text = CStr(Result)

        Catch ex As FormatException
            MsgBox("You type an invalid number. Please correct it")
        Catch ex As DivideByZeroException
            MsgBox(ex.Message)
        Catch
            MsgBox("Invalid Operation: " & vbCrLf & Oper & " is not a valid operator")
        End Try
End Sub

1. Create a Windows Application named GeometricCalculations
2. Add a class named Sphere to the project. The class must be equipped with properties that can produce the diameter, the circumference, the area, and the volume
3. Design the form so it can take a radius value and can display the diameter, the circumference, the area, and the volume
4. Write code that can display the results of the diameter, the circumference, the area, and the volume calculated from the Sphere class
5. Add the tooltips to all controls on the form
6. Create a help file that  will assist the user with the application

1. Open the GeorgetownCleaningServices1 application from the previous lesson
2. Add the tooltips to all controls on the form
3. Create a help file that  will assist the user with the application

1. Create a Windows Application named ClarskvilleIceCream1a
2. Design the form as follows:
    
   
3. Add tool tips and help to the application