Sometimes you need to change an exceptions type (usually to a custom exception type) but still want to maintain stacktrace information (read about this issue in part1). Well it is really easy just look at this example

1

2 Public Sub Layer2()

3 Throw New NotImplementedException

4 End Sub

5

6 Public Sub LogError(ByVal ex As Exception)

7 'TODO: log the error

8 End Sub

9

10 Private Sub Layer1()

11 Try

12 Layer2()

13 Catch ex As Exception

14 Throw New CustomException("Layer1 error", ex)

15 End Try

16 End Sub

17

18 Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

19 Try

20 Layer1()

21 Catch ex As Exception

22 LogError(ex)

23 Throw

24 End Try

25 End Sub


By throwing a new exception and adding the caught exception as the inner exception we maintain the stacktrace information.

FYI my stack output looks like this:

System.Exception: Layer1 error ---> System.NotImplementedException: The method or operation is not implemented.
at WindowsApplication1.Form1.Layer2() in C:\...\Form1.vb:line 3
at WindowsApplication1.Form1.Layer1() in C:\...\Form1.vb:line 12
--- End of inner exception stack trace ---
at WindowsApplication1.Form1.Layer1() in C:\..\Form1.vb:line 14
at WindowsApplication1.Form1.Button1_Click(Object sender, EventArgs e) in C:\..\Form1.vb:line 20

you can see that the inner exception has all the trace information down to where the original exception occured but the outer exception shows only that at Layer1() did an exception occur.