Lesson 35: Errors Handling in VB6

Implement robust error handling to create reliable applications

Key Takeaway

Robust error handling is essential for creating professional, reliable VB6 applications that can gracefully handle unexpected situations.

Welcome to Lesson 35 of our Visual Basic 6 Tutorial! In this lesson, you'll learn how to implement robust error handling in your VB6 applications. Proper error handling prevents crashes, improves user experience, and makes your applications more reliable.

35.1 Understanding Errors

In Visual Basic programming, effective error handling is a crucial practice to ensure the seamless operation of a program. By addressing errors, programmers can significantly enhance the program's reliability and performance. A smoothly running, error-free program minimizes the likelihood of issues like program crashes or system hang-ups, offering users a hassle-free experience.

Common errors often stem from inaccurate user inputs. For instance, attempting to instruct the computer to divide a number by zero is a classic mistake that leads to a system error. Similarly, inputting text (string) into a field designed exclusively for numeric values, such as a person's weight, can disrupt the program's ability to perform arithmetic calculations, resulting in an error. These types of errors, referred to as synchronous errors, highlight the importance of robust error-handling mechanisms in Visual Basic programming.

User Input Errors

Invalid data types, out-of-range values, or unexpected formats

Resource Access

Missing files, locked databases, or unavailable network resources

Calculation Errors

Division by zero, overflow conditions, or invalid math operations

Best Practice

A good programmer should be alert to parts of the program that could trigger errors and should write error handling code to help manage them. However, avoid excessive error handling as it can complicate maintenance.

35.2 Error Handling Syntax

The core syntax for error handling in VB6 is:

VB6 Syntax

On Error GoTo program_label

Where program_label is the section of code designed to handle errors. When an error occurs, the program jumps to this label.

Example 35.1: Division by Zero

This example demonstrates handling division by zero errors:

DivisionError.frm

Private Sub CmdCalculate_Click()
    Dim firstNum, secondNum As Double
    firstNum = Txt_FirstNumber.Text
    secondNum = Txt_SecondNumber.Text
    
    ' Enable error handling
    On Error GoTo error_handler
    
    Lbl_Answer.Caption = firstNum / secondNum
    ' Prevent executing error handler when no error
    Exit Sub
    
' Error handling section
error_handler:
    Lbl_Answer.Caption = "Error"
    Lbl_ErrorMsg.Visible = True
    Lbl_ErrorMsg.Caption = "You attempted to divide by zero! Try again!"
End Sub

Private Sub Txt_FirstNumber_GotFocus()
    Lbl_ErrorMsg.Visible = False
End Sub

Private Sub Txt_SecondNumber_GotFocus()
    Lbl_ErrorMsg.Visible = False
End Sub

Explanation

The Exit Sub statement is crucial to prevent the error handler from executing when there are no errors. The GotFocus events clear any previous error messages when the user interacts with the textboxes again.

Figure 35.1: Division by Zero Error Handling

35.3 Nested Error Handling

For more complex scenarios, you can implement nested error handling to manage multiple potential errors:

NestedErrors.frm

Private Sub CmdCalculate_Click()
    Dim firstNum, secondNum As Double
    
    ' First error handler: non-numeric inputs
    On Error GoTo error_handler1
    
    firstNum = Txt_FirstNumber.Text
    secondNum = Txt_SecondNumber.Text
    
    ' Second error handler: division by zero
    On Error GoTo error_handler2
    
    Lbl_Answer.Caption = firstNum / secondNum
    Exit Sub
    
' Division by zero handler
error_handler2:
    Lbl_Answer.Caption = "Error"
    Lbl_ErrorMsg.Visible = True
    Lbl_ErrorMsg.Caption = "Division by zero error! Try again!"
    Exit Sub  ' Prevent falling through to error_handler1
    
' Non-numeric input handler
error_handler1:
    Lbl_Answer.Caption = "Error"
    Lbl_ErrorMsg.Visible = True
    Lbl_ErrorMsg.Caption = "Non-numeric input! Try again!"
End Sub

Important

Each error handler needs its own Exit Sub to prevent executing subsequent error handlers. The order of handlers matters - put more specific handlers before more general ones.

Figure 35.2: Nested Error Handling in Action

Figure 35.3: Error Handling Flow Diagram

Lesson Summary

In this lesson, you've learned essential error handling techniques in VB6:

Error Handling Syntax

Master the On Error GoTo statement to handle runtime errors

Error Prevention

Identify common error sources like invalid user inputs

Nested Handling

Implement multiple error handlers for complex scenarios

User Experience

Provide clear error messages to guide users

Pro Tip

Combine error handling with input validation to prevent errors before they occur. Use IsNumeric() for numeric fields and proper data type checking.

Next Lesson

Continue your VB6 journey with Lesson 36: Compiling and Distributing Applications.

Related Resources

VB6 Error Codes

Complete list of VB6 runtime error codes and descriptions

View Resource

Advanced Error Handling

Learn centralized error handling techniques

View Resource

Input Validation

Prevent errors with proper input validation

View Lesson

Debugging Techniques

Essential debugging tools in VB6

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