Delphi Language Guide Delphi for Microsoft Win32 Delphi for the Microsoft. Net framework

Try...except Statements
Exceptions are handled within 
try...except
 statements. For example,
try 
   X := Y/Z; 
   except 
     on EZeroDivide do HandleZeroDivide; 
end;
This statement attempts to divide 
Y
 by 
Z
, but calls a routine named 
HandleZeroDivide
 if an EZeroDivide exception
is raised.
The syntax of a 
try...except
 statement is
try statementsexceptexceptionBlockend
where statements is a sequence of statements (delimited by semicolons) and exceptionBlock is either
another sequence of statements or
a sequence of exception handlers, optionally followed by
elsestatements
An exception handler has the form
onidentifier: typedostatement
where identifier: is optional (if included, identifier can be any valid identifier), type is a type used to represent
exceptions, and statement is any statement.
A 
try...except
 statement executes the statements in the initial statements list. If no exceptions are raised, the
exception block (exceptionBlock) is ignored and control passes to the next part of the program.
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If an exception is raised during execution of the initial statements list, either by a raise statement in the statements
list or by a procedure or function called from the statements list, an attempt is made to 'handle' the exception:
If any of the handlers in the exception block matches the exception, control passes to the first such handler. An
exception handler 'matches' an exception just in case the type in the handler is the class of the exception or an
ancestor of that class.
If no such handler is found, control passes to the statement in the else clause, if there is one.
If the exception block is just a sequence of statements without any exception handlers, control passes to the
first statement in the list.
If none of the conditions above is satisfied, the search continues in the exception block of the next-most-recently
entered 
try...except
 statement that has not yet exited. If no appropriate handler, else clause, or statement list
is found there, the search propagates to the next-most-recently entered 
try...except
 statement, and so forth. If
the outermost 
try...except
 statement is reached and the exception is still not handled, the program terminates.
When an exception is handled, the stack is traced back to the procedure or function containing the
try...except
 statement where the handling occurs, and control is transferred to the executed exception handler,
else clause, or statement list. This process discards all procedure and function calls that occurred after entering the
try...except
 statement where the exception is handled. The exception object is then automatically destroyed
through a call to its 
Destroy
 destructor and control is passed to the statement following the 
try...except
statement. (If a call to the 
Exit
, 
Break
, or 
Continue
 standard procedure causes control to leave the exception
handler, the exception object is still automatically destroyed.)
In the example below, the first exception handler handles division-by-zero exceptions, the second one handles
overflow exceptions, and the final one handles all other math exceptions. EMathError appears last in the exception
block because it is the ancestor of the other two exception classes; if it appeared first, the other two handlers would
never be invoked.
try 
  ... 
except 
  on EZeroDivide do HandleZeroDivide; 
  on EOverflow do HandleOverflow; 
  on EMathError do HandleMathError; 
end;
An exception handler can specify an identifier before the name of the exception class. This declares the identifier to
represent the exception object during execution of the statement that follows 
on...do
. The scope of the identifier
is limited to that statement. For example,
try 
  ... 
except 
  on E: Exception do ErrorDialog(E.Message, E.HelpContext); 
end;
If the exception block specifies an else clause, the else clause handles any exceptions that aren't handled by the
block's exception handlers. For example,
try 
  ... 
except 
  on EZeroDivide do HandleZeroDivide; 
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  on EOverflow do HandleOverflow; 
  on EMathError do HandleMathError; 
else 
  HandleAllOthers; 
end;
Here, the else clause handles any exception that isn't an EMathError.
An exception block that contains no exception handlers, but instead consists only of a list of statements, handles all
exceptions. For example,
try 
   ... 
except 
   HandleException; 
end;
Here, the 
HandleException
 routine handles any exception that occurs as a result of executing the statements
between try and except.