Lesson 24: Mastering Errors Handling in VB2022

Private Sub BtnCalculate_Click(sender As Object, e As EventArgs) Handles BtnCalculate.Click
    LblErrorMessage.Visible = False
    Dim num1, num2, result As Double

    ' Activate error handling
    On Error GoTo ErrorHandler

    num1 = CDbl(TxtNumber1.Text)
    num2 = CDbl(TxtNumber2.Text)
    
    ' Explicit check for division by zero
    If num2 = 0 Then
        Err.Raise(11) ' Division by zero error
    End If

    result = num1 / num2
    LblResult.Text = result.ToString("N2")

    ' Exit before error handler
    Exit Sub

ErrorHandler:
    ' Handle specific error types
    Select Case Err.Number
        Case 6 ' Overflow
            LblErrorMessage.Text = "Number too large or too small"
        Case 11 ' Division by zero
            LblErrorMessage.Text = "Cannot divide by zero"
        Case 13 ' Type mismatch
            LblErrorMessage.Text = "Please enter valid numbers"
        Case Else
            LblErrorMessage.Text = $"Unexpected error: {Err.Description}"
    End Select

    LblErrorMessage.Visible = True
    LblResult.Text = "Error"
    Resume Next
End Sub

Private Sub BtnReadFile_Click(sender As Object, e As EventArgs) Handles BtnReadFile.Click
    Dim fileReader As System.IO.StreamReader = Nothing
    
    Try
        ' Attempt to open and read file
        fileReader = New System.IO.StreamReader("C:\data\important.txt")
        TxtContent.Text = fileReader.ReadToEnd()
        LblStatus.Text = "File read successfully"
        
    Catch ex As System.IO.FileNotFoundException
        LblStatus.Text = "File not found"
        MessageBox.Show("The file was not found: " & ex.Message, "File Error", 
                        MessageBoxButtons.OK, MessageBoxIcon.Error)
        
    Catch ex As System.IO.IOException
        LblStatus.Text = "I/O Error"
        MessageBox.Show("An I/O error occurred: " & ex.Message, "I/O Error", 
                        MessageBoxButtons.OK, MessageBoxIcon.Error)
        
    Catch ex As Exception
        LblStatus.Text = "General Error"
        MessageBox.Show("An unexpected error occurred: " & ex.Message, "Error", 
                        MessageBoxButtons.OK, MessageBoxIcon.Error)
        
    Finally
        ' Ensure file is closed even if error occurred
        If fileReader IsNot Nothing Then
            fileReader.Close()
        End If
    End Try
End Sub

Private Sub BtnConvert_Click(sender As Object, e As EventArgs) Handles BtnConvert.Click
    LblErrorMessage.Visible = False
    
    Try
        Dim fahrenheit = CDbl(TxtFahrenheit.Text)
        
        ' Validate temperature range
        If fahrenheit < -459.67 Then
            Throw New ArgumentOutOfRangeException("Temperature below absolute zero")
        End If
        
        Dim celsius = (fahrenheit - 32) * 5 / 9
        LblCelsius.Text = celsius.ToString("N1") & " °C"
        
    Catch ex As FormatException
        LblErrorMessage.Text = "Please enter a valid number"
        LblErrorMessage.Visible = True
        
    Catch ex As OverflowException
        LblErrorMessage.Text = "Number is too large or too small"
        LblErrorMessage.Visible = True
        
    Catch ex As ArgumentOutOfRangeException
        LblErrorMessage.Text = "Temperature cannot be below absolute zero (-459.67°F)"
        LblErrorMessage.Visible = True
        
    Catch ex As Exception
        LblErrorMessage.Text = $"Unexpected error: {ex.Message}"
        LblErrorMessage.Visible = True
        
    End Try
End Sub

Private Sub LoadCustomerData()
    Dim conn As New SqlConnection("connection_string")
    Dim cmd As New SqlCommand("SELECT * FROM Customers", conn)
    Dim reader As SqlDataReader = Nothing
    
    Try
        conn.Open()
        reader = cmd.ExecuteReader()
        
        ' Process data
        While reader.Read()
            ' Add data to list
        End While
        
    Catch ex As SqlException
        MessageBox.Show($"Database error: {ex.Message}", "Database Error", 
                      MessageBoxButtons.OK, MessageBoxIcon.Error)
        
    Catch ex As Exception
        MessageBox.Show($"General error: {ex.Message}", "Error", 
                      MessageBoxButtons.OK, MessageBoxIcon.Error)
        
    Finally
        ' Cleanup resources in reverse order of creation
        If reader IsNot Nothing AndAlso Not reader.IsClosed Then
            reader.Close()
        End If
        
        If conn.State = ConnectionState.Open Then
            conn.Close()
        End If
        
        ' Dispose of objects
        reader?.Dispose()
        cmd.Dispose()
        conn.Dispose()
        
        LblStatus.Text = "Operation completed"
    End Try
End Sub