Re: Is it possible for a try...finally...end construct to fail?

From: Craig Stuntz [TeamB] (cstuntz_at_nospam.please)
Date: 11/25/03 

Date: 25 Nov 2003 05:57:02 -0700

Mark wrote:

> Firstly using exceptions is it possible to have error messages like
> assertions where the line number is provided? This would be really
> helpful in some situations.

        Yes and no. This feature isn't built in, but there are a number of
third-party packages, such as the Jedi debugging library from the
Delphi JEDI project and MadExcept which provide this. They'll also
give you call stacks and other useful information.

        In case you want to know how this works, read this article:

http://www.undu.com/Articles/010729d.html
 
> I assume the best way to code nested try/except/finally blocks is
> like this:

        Actually, the best way to code nested try/except/finally blocks is not
to do it. :) try/finally and try/except are completely different
constructs. Use of try/finally should be *very* common -- use it
whenever action B must happen after action A when there is something in
the middle which might raise an exception.

        Use of try/except, on the other hand, should be fairly rare.
Exceptions are not a means of showing a dialog to the user (that's a
side effect of Delphi's global exception handler). Rather, they're a
means of interrupting program flow. The fact that an exception was
raised means that a method's contract was violated. There are two
possible correct responses to an exception being raised:

1. Halt execution -- kick the current instruction pointer to the top
of the call stack. Probably show an error message to the user at this
point.
2. Fix the conditions which caused the error and re-run the method
from the start. For example, if a method is trying to login to the
database and the user has entered an incorrect password, instead of
simply showing them an error you can give them a chance to retype their
password, and then try the login method again.

        What this means is that statistically speaking nested
try/except/finally constructs will be rarer than try/except, which
should already be fairly rare.

        Note that if your only reason for catching an exception is to show a
different error message than the default you can do this in an
Application.OnException event (or use the TApplicationEvents
component). This gives you the ability to change the exception message
globally -- for the entire app, not just one bit of code.

> Allocmem;
> try
> try
> dosomething;
> except
> // exception code here
> end;
> try
> dosomethingelse;
> except
> // handle the exception - suppress it <g>
> end;
> finally
> freemem;
> end;

        With the caveats above and elsewhere in this thread taken into
account, the code above is fine.

        -Craig 

-- 
Craig Stuntz [TeamB] . Vertex Systems Corp. . Columbus, OH
  Delphi/InterBase Weblog : http://delphi.weblogs.com
InterBase in Multi-tier World -- Use multiple RDMs, IB features in 
  your appserver: http://delphi.weblogs.com/stories/storyReader$195

Relevant Pages

  • RE: Biztalk 2006 Tutorial 3
    ... There are some known issues w/ the tutorials. ... Below is the error message that appears in the event ... Exception type: ServiceCreationException ... md, Objectargs, Object server, Int32 methodPtr, Boolean fExecuteInContext, ...
    (microsoft.public.biztalk.general)
  • RE: Concerns about exception string revealing internals/data about
    ... exceptions could return certain information as part of the error message. ... exception occurs in my program, is it possible that the exception error ... built in ADO objects in Visual Studio for Microsoft SQL Server. ... the build-in controls in SQL Server2000 or .Net winform/Asp.net controls? ...
    (microsoft.public.dotnet.framework.clr)
  • Re: Concerns about exception string revealing internals/data about
    ... Sorry in last post I meant an "unhandled" exception should never be shown to ... exceptions could return certain information as part of the error message. ... built in ADO objects in Visual Studio for Microsoft SQL Server. ... the build-in controls in SQL Server2000 or .Net winform/Asp.net controls? ...
    (microsoft.public.dotnet.framework.clr)
  • RE: Biztalk 2006 Tutorial 3
    ... Below is the error message that appears in the event ... Exception type: ServiceCreationException ... Microsoft.BizTalk.XLANGs.BTXEngine.BTXSession._createOrRehydrateRealService(Guid& instanceId, IBTMessage currMsg) ... md, Objectargs, Object server, Int32 methodPtr, Boolean fExecuteInContext, ...
    (microsoft.public.biztalk.general)
  • RE: Single Sign-On User Credentials Question
    ... error message means that GetCredentialEntryURL can't find an Enterprise ... different EA names in its calls to GetCredentials and GetCredentialEntryURL. ... This seems to indicate that it can't find the login URL for my application. ... I get the same error message when an unhandled exception occurrs. ...
    (microsoft.public.sharepoint.portalserver)
Loading