asked on

Consecutive exceptions (or nested exceptions) on Delphi

I'm writing a Delphi component, all brand new code. Note that it is a component, not an application.

I don't quite understand how exceptions are handled, when they are raised at several levels of the code.

I have an Object, that owns another object, which again owns and call methos from another object.
All method code is surrounded with try..except.

Now, when I get an exception on the 3rd inner object, this also causes the 2nd object to raise another exception, and this case the 1st object to cause another exception.
If I run the code through the debugger, I see this clearly, raising 3 exceptions.

But when I run my application outside the debugger I only get one exception: the last exception in the outer object.
I see this is the most practical way, but I'm losing the error message created in the inner objects, and I only see the more generic message I have in the code.

Clearly there's something wrong either in my design or my ideas on how to implement nested exceptions.
Could you please, provide some guide on how to do this? a sample, and article to read, any thing is appreciated.

Thank you!

Ephraim Wangoya

The original message only disappears if you are displaying your own message, otherwise just pass through the original exception. In your except block, you can do this

try
//your code
except
on E: Exception do
begin
//do your cleanup code
raise; //re-raise the exception so that the message is not lost
end;
end;

fischermx

ASKER

Hi, Ewangoya:

Yes, it seems I can do that for the 2nd and 3rd level. The message persisted.
But now, in the 1st level, I do want to add some other text, my own message, PLUS the original message (which actually comes from a 3rd party library).
I think I would need some system to aggregate error messages !?

ASKER CERTIFIED SOLUTION

Geert G

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samenglish

If you raise exceptions trapped in your deeper methods, then you can access the text of those exceptions and add your own to it like this (example below)

try
// your code
except
on E: Exception do
ShowMessage('Exception in my deeper method' + #10#13 + 'ClassName: '
+ E.ClassName + '; Message: ' + E.Message); // or format string as you like

end;

fromer

For Evangoya's code,

try
//your code
except
on E: Exception do
begin
//do your cleanup code
raise Exception.Create(E.Message + ' I want to add some detail'); //re-raise the exception so that the message is not lost
end;
end;

fromer

OR .Net Approach...
make you own Exception class like,

MyException = class(Exception)
    protected
      FInnerException : Exception;
    public
      property InnerException : Exception read FInnerException;
      constructor Create(_Message : String; _InnerException : Exception);
  end;


constructor MyException.Create(_Message : String; _InnerException : Exception);
begin
  inherited Create(_Message);
  FInnerException := _InnerException;
end;

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in the first level,

try
      //your code
   except
      on E: Exception do
      begin
         //do your cleanup code
         raise MyException.Create(E.Message + ' I want to add some detail', E);    //re-raise the exception so that the message is not lost
      end;
   end;

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So you event don't lose the inner Exception class...

SOLUTION

8080_Diver