Tuesday, October 31, 2006

Catch When What?

Joe Duffy's excellent blog has just taught me something (in guideline 16 of this article) that I didn't know about structured exception handling in VB.NET, and which is not explained in the language spec (see 10.10.1).

What do you think this code should do?


    2 Module Module1


    4     Private _value As Integer


    6     Sub Main()

    7         _value = 42

    8         Try

    9             Trouble()

   10         Catch ex As Exception When _value = 7

   11             MsgBox("Oh, that's all right then.")

   12         Catch ex As Exception

   13             MsgBox("The non-7 value is: " & _value.ToString)

   14         End Try

   15     End Sub


   17     Sub Trouble()

   18         Try

   19             Throw New Exception

   20         Finally

   21             _value = 7

   22         End Try

   23     End Sub


   25 End Module


It prints "The non-7 value is: 7" of course.

Without a great deal of research (i.e. none) it seems the two passes mentioned by Joe are:

  1. find the target of the exception (i.e. the closest matching catch block on the stack);
  2. transfer control to that catch block via each finally block between it and the throw point.

Therefore no finally-block code will have executed when the When clause is executed, so invariants could still be broken.

No big deal (since exception filters should be kept simple), but worth knowing.

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