How to Determine Exception Subtype - vb.net

Question

I'm wondering if there's a standard way to determine the sub-type of an exception. For example, for the File.Copy() method, IOException indicates that the destination file exists OR a general I/O error occurred. There are other such cases. In my exception handler, how can I determine which it is? I'm checking the end of ex.Message for the string already exists., which works, but seems awfully kludgy and unreliable.
While it is possible to check File.Exists() on the destination file, confirm overwrite with the user if it exists and then perform File.Copy() this is not atomic, which is to say, between checking and copying, it is possible for the conditions to change, for example if some other process created or copied a file into the destination location.
EDIT:
I had already changed the code based on comments here, but I just rolled it back and will post it here, just to show what I was doing:
Try
File.Copy(SrcFile, DstFile, OverWrite)
Catch ex As DirectoryNotFoundException
MsgBox(ex.Message)
Catch ex As FileNotFoundException
MsgBox("File not found: " & ex.FileName)
Catch ex As UnauthorizedAccessException
MsgBox("You do not have write access to the destination.")
Catch ex As IOException
' IOException represents an existing destination file OR a general IO error.
If SubStr(ex.Message, -15) = "already exists." Then
OverwriteCheck = MsgBox(
"Overwrite " & IO.Path.GetFileName(SrcFile) & " in destination directory?",
MsgBoxStyle.YesNo
)
If OverwriteCheck = DialogResult.Yes Then
Try
File.Copy(SrcFile, DstFile, OverWrite)
Catch iex As Exception
MsgBox("Unable to copy " & SrcFile & ":" & vbNewLine & iex.Message)
End Try
End If
Else
Throw ex
End If
Catch ex As ArgumentException
' The user left a blank line in the text box. Just skip it.
End Try

Answer 1

Here is an option using FileStreams to get more granular information about your exception
Sub Main()
Try
copyTo("C:\t\output3.txt", "C:\t\output1.txt", True)
Catch ex As Exception
Console.WriteLine(ex.Message)
End Try
Console.ReadLine()
End Sub
Private Sub copyTo(source As String, destination As String, Optional overwrite As Boolean = False)
' raises FileNotFoundException if source doesn't exist
Using fsSource As New FileStream(source, FileMode.Open, FileAccess.Read, FileShare.None)
If Not overwrite AndAlso File.Exists(destination) Then
' Raises exception when destination file exists and not overwrite
Throw New Exception(
String.Format("Destination file '{0}' exists and overwrite is false.", destination))
Else
Using fsDestination As New FileStream(destination, FileMode.OpenOrCreate, FileAccess.Write, FileShare.None)
fsSource.CopyTo(fsDestination)
End Using
End If
End Using
End Sub
This is a rudimentary example, but you can see how you can differentiate different exception cases, while having atomicity between checking file existence and copying.

Answer 2

I believe you looking for this pattern:
Try
IO.File.Copy("source", "Dest", True)
Catch exUnAuth As System.UnauthorizedAccessException
Catch exArg As System.ArgumentException
Catch exNotFound As IO.FileNotFoundException
Catch exGeneral As System.Exception
End Try
Place the list of specific exceptions first in the sequence. The last exception tested for should be the least derived.
You should read through the documentation: How to use structured exception handling in Visual Basic .NET or in Visual Basic 2005. Yes this is an old reference, but that is an indication how long this has been part of the language.