Events and exceptions in 3-layer architecture - vb.net

Question

I have another question considering events and exceptions
A small introduction to my project: I'm working in a 3 layer architecture, so I have a GUI(form), BLL(Business logic layer) and a DAL(data acces layer).
So my GUI is linked to a BLL which is connected to several DAL classes. For example my BLL has a class clsCSV (which reads and writes CSV files)
I now want to raise events and/or catch exceptions from this class to inform the user and log exceptions. There are 3 events: "ErrorLoad", "ErrorWrite", "Ready"
Public Class clsCSV
Public Sub New()
End Sub
Public Function Load(sFilePath As String)
Dim oFileHelper As New DelimitedFileEngine(Of clsItem)
Dim oList As New List(Of clsItem)
' Load a CSV file
Try
oList = oFileHelper.ReadFileAsList(sFilePath)
Catch ex As Exception
'RaiseEvent ErrorLoad("")
clsErrorLog.Log(ex)
Finally
End Try
' If the list is empty return nothing
If Not IsNothing(oList) Then
Return oList
Else
Return Nothing
End If
End Function
Public Sub Write(sFilePath As String, oList As List(Of clsTranslatedItem))
Dim oFileHelper As New DelimitedFileEngine(Of clsTranslatedItem)
oFileHelper.WriteFile(sFilePath, oList)
RaiseEvent Ready()
End Sub
Public Event ErrorLoad(ByVal sender As Object, ByVal e As EventArgs)
Public Event ErrorWrite(sMessage As String)
Public Event Ready()
End Class
There is a try-catch in my public sub load which catches any exception and logs it to a text file by calling clsErrorLog.log(ex) which then calls this class and logs the error:
Public Shared Sub Log(ex As Exception)
Try
Dim oBestand As New System.IO.StreamWriter(My.Computer.FileSystem.SpecialDirectories.Desktop & "\ErrorLog.txt", True)
If Not IsNothing(oBestand) Then
oBestand.WriteLine("")
oBestand.WriteLine(My.Computer.Clock.LocalTime.ToString)
oBestand.WriteLine(ex.Message)
oBestand.WriteLine(ex.StackTrace)
oBestand.WriteLine("_________________________________________")
oBestand.Close()
End If
Catch exc As Exception
End Try
End Sub
This works fine, but now I want to inform the user that an error occured, so I was thinking to raise an event called "ErrorLoad" (in clsCSV) and catch this in my BLL. But what can I do then? Add a sub that handles this event, which then raises an event in the BLL, caught by the GUI? or is this a long way to this?
I also want to do this for the other events offcourse. But the whole event system in the layered architecture is a bit confusing.
Anyone who can enlighten me?
Thanks in advance!

Answer 1

Without knowing the implementation details of your UI, I can only give you some general advice.
You need some kind of two directional communication between your GUI and your BLL. For example in WPF this is usually done using INotifyPropertyChanged interface and the event it defines.
So the BLL raises a event and the GUI is listening. The basically only has the task of informing the GUI that it has to check a specific property or function or anything like this from the BLL. The rest is up to the GUI to do so and inform the user.
I wouldn't do that using exceptions. These are there to handle the error path of specific functions. This path is done once your BLL noticed the error and prepared it for the UI handling.