Title: Using TRY EXCEPT and TRY FINALLY statements
Question: When writing applications that need to be fail-safe the TRY EXCEPT and TRY FINALLY statements are a very good way of avoiding and handling errors.
Answer:
bIntroduction/b
When writing applications that are designated to run on their own
in server environments or applications that need to be fail-safe
the TRY EXCEPT END and TRY FINALLY END statements can be used to handle or avoid fatal errors.
bException Handling Basics/b
If an exception is caused somewhere in a code block the current
executing of the block immediately ends, and jumps to the next
EXCEPT or FINALLY statement immediately. If no exception handler
is written then Delphi applications display an EXCEPTION dialog
by default that usually says very little to the person using the
program.
The EXCEPT block is mainly used for handling the error situation.
The FINALLY block is mainly used for cleanup code, since it is
executed regardless of error or not.
By combining these two blocks effective exception handlers can
be written.
The Delphi compiler also generates WARNINGS if certain code
statements are placed in the wrong place outside/inside the
blocks.
uExample:/u
Function CauseException(): Boolean;
Var
X : Integer;
L : TStringList;
Begin
// Default result must be here or the compiler generates a warning
Result := False;
// We encapsulate everything in a TRY EXCEPT to make sure we
// catch ALL exceptions
{ TRY EXCEPT BLOCK }
bTry/b
// Initialize variables (Zero them so we know they are NIL)
L := NIL;
{ TRY FINALLY BLOCK }
bTry/b
// Create objects etc.
L := TStringList.Create;
// Do what we need to do
X := 0;
Inc(X);
X := X DIV (X-1); // Causes DIVISION BY ZERO exception
bFinally/b
If Assigned(L) then FreeAndNil(L);
bEnd;/b
// Code after the FINALLY statement, that is executed
// if NO exception occurred in the TRY FINALLY block
// Note: Do not place any error prone code here.
Result := True;
bExcept/b
On E:Exception do // This handles ALL exceptions
Begin
// Place Exception code here
Result := False;
End;
bEnd;/b
End;
In the example above the code generates an EXCEPTION at the DIV
statement, and will jump Directly to the FINALLY part of the TRY
FINALLY block. After that the code again jumps to EXCEPTION part
of the TRY EXCEPT block !
Please note the code flow, It skips the statements after the try
finally block!!!
If the code would not generate the error the TRY FINALLY block
would still be executed (so the list is freed) but the code would
NOT jump to the EXCEPT block.
bAnother example with unhandeled and handeled execptions/b
Below is another example with two procedures. One of them has
exception handling, the other has not. As you can see when you
run the code the GenError1 exception is logged in the timer and
thus makes it hard to find. The GenError0 procedure exception is
handeled inside the procedure so it is very easy to find.
To test the example create a new application, add one Timer from
the system tab and paste the code below. Also attach the timers
OnTimer event to the pasted code.
Procedure Log(S:String);
Begin
// Here you can log the exception in a file or database
Beep;
End;
Procedure GenError0;
Var
L : TStringList;
Begin
Try
L.Clear;
Except
On E:Exception do Log('GenError0: '+E.Message);
End;
End;
Procedure GenError1;
Var
L : TStringList;
Begin
L.Clear;
End;
procedure TForm1.Timer1Timer(Sender: TObject);
Var
X : Integer;
begin
Try
Timer1.Enabled := False;
Try
GenError0; // Handles it's own exception
GenError1; // Causes exception, catched below
Finally
// This code is always executed
Timer1.Enabled := True;
End;
ShowMessage('This will NEVER occur !!!');
Except
On E:Exception do Log('TForm1.Timer1Timer: '+E.Message);
End;
end;
bWhere to next ?/b
If you want more information on exceptions check the Delphi Help
File (search for "exception"), and also search the forums on the
net for more information.
As a pointer for further reading you can check the RAISE statement,
that Re-Raises an exception so that it can be forwarded to another exception handler ...
Enjoy !