Lesson 24 Errors Handling

24.1 Introduction to Error Handling

Therefore a good programmer should be more alert to the parts of the program that could trigger errors and should write errors handling code to help the user in managing the errors. Writing errors handling code is a good practice for Visual Basic 2017 programmers, so do not try to finish a program fast by omitting the error handling code. However, there should not be too many errors handling code in the program as it might create problems for the programmer to maintain and troubleshoot the program later.

Visual Basic 2019 has improved a lot in its built-in errors handling capabilities compared to Visual Basic 6. For example, when the user attempts to divide a number by zero, Visual Basic 2019 will not return an error message but gives the ‘infinity’ as the answer (although this is mathematically incorrect because it should be undefined)

24.2 Using On Error GoTo Syntax

Visual Basic 2019 still supports the vb6 errors handling syntax, that is the On Error GoTo program_label structure. Although it has a more advanced error handling method, we shall deal with that later. We shall now learn how to write errors handling code in Visual Basic 2019. The syntax for errors handling is

On Error GoTo program_label

where program_label is the section of code that is designed by the programmer to handle the error committed by the user. Once an error is detected, the program will jump to the program_label section for error handling.

Example 24.1: The Division Error

In this example, we will deal with the error of entering non-numeric data into the text boxes that suppose to hold numeric values. The program_label here is error_handler. When the user enters non-numeric values into the text boxes, the error message will display the phrase "One or both of the entries is/are non-numeric!". If no error occurs, it will display the correct answer. Try it out yourself.

The Code

Private Sub BtnCal_Click(sender As Object, e As EventArgs) Handles BtnCal.Click
Lbl_ErrMsg.Visible = False
Dim firstNum, secondNum As Double

On Error GoTo error_handler
firstNum = TxtNum1.Text
secondNum = TxtNum2.Text
Lbl_Answer.Text = firstNum / secondNum

Exit Sub  'To prevent error handling even the inputs are valid

error_handler:

Lbl_Answer.Text = "Error"
Lbl_ErrMsg.Visible = True
Lbl_ErrMsg.Text = " One or both of the entries is/are non-numeric! Try again!"

End Sub

The runtime interface

*Please Note that division by zero in Visual Basic 2015 no longer gives an error message, but it displays the answer as Infinity.

24.3 Errors Handling using Try…..Catch….End Try Structure

Visual Basic 2017 has adopted a new approach in handling errors or rather exceptions handling. It is supposed to be more efficient than the old On Error Goto method, where it can handle various types of errors within the Try…Catch…End Try structure.

The structure looks like this

Try

statements

Catch exception_variable as Exception

statements to deal with exceptions

End Try

The Code

Private Sub BtnCal_Click(sender As Object, e As EventArgs) Handles BtnCal.Click
Lbl_ErrMsg.Visible = False
Dim firstNum, secondNum, answer As Double

Try

firstNum = TxtNum1.Text
secondNum = TxtNum2.Text
answer = firstNum / secondNum
Lbl_Answer.Text = answer

Catch ex As Exception

Lbl_Answer.Text = "Error"
Lbl_ErrMsg.Visible = True
Lbl_ErrMsg.Text = " One of the entries is not a number! Try again!"

End Try

End Sub

The runtime interface