Answer.

In Visual Basic, the error handling constructs On Error (VB6) and Try...Catch (VB.NET) are used for error handling.

• On Error GoTo <Label> — an outdated method from VB6 that implements a jump to the specified label on error. It does not support nesting and can lead to convoluted code.
• Try...Catch...Finally — the modern method (VB.NET) that allows working with exceptions, providing the ability to handle different types of errors in different Catch blocks and ensuring final actions are always executed in Finally.

Examples:

VB6:

Sub OpenFile()
    On Error GoTo ErrorHandler
    Open "file.txt" For Input As #1
    ' Reading the file
    Close #1
    Exit Sub
ErrorHandler:
    MsgBox "Error: " & Err.Description
End Sub

VB.NET:

Sub OpenFileVBNet()
    Try
        Using reader As New StreamReader("file.txt")
            Dim line As String = reader.ReadLine()
        End Using
    Catch ex As IOException
        MsgBox("Error: " & ex.Message)
    Finally
        ' Always executed
    End Try
End Sub

Trick question.

If an error occurs in VB6 in the On Error Resume Next block, how can you find out where exactly it happened and how to handle it correctly?

Correct answer: In On Error Resume Next mode, the program continues execution at the next line. To determine if an error occurred, you need to check the Err object after the call.

On Error Resume Next
' some code
If Err.Number <> 0 Then
    MsgBox "Error: " & Err.Description
    Err.Clear
End If

Examples of real errors due to ignorance of the subtleties of the topic.

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Story

After transitioning from VB6 to VB.NET, some error handlers were left in the On Error GoTo style, leading to implicit swallowing of errors. The logic of processing was mixed up, causing data in the database to remain semi-empty, and the bug was discovered only months later.

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Story

Did not use Err.Clear, resulting in Err.Number remaining non-zero after one handled error. During further checks, the program incorrectly thought a new error had occurred and went into incorrect branches of handling.

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Story

During refactoring, forgot to remove On Error Resume Next, which caused errors to be "swallowed" and no messages to be generated. Diagnosing the problems took hours, as the application seemed to be functioning normally but was actually losing important data.