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

When To Use Exceptions
Exceptions provide an elegant way to trap runtime errors without halting the program and without awkward
conditional statements. The requirements imposed by exception handling semantics impose a code/data size and
runtime performance penalty. While it is possible to raise exceptions for almost any reason, and to protect almost
any block of code by wrapping it in a 
try...except
 or 
try...finally
 statement, in practice these tools are
best reserved for special situations.
Exception handling is appropriate for errors whose chances of occurring are low or difficult to assess, but whose
consequences are likely to be catastrophic (such as crashing the application); for error conditions that are
complicated or difficult to test for in 
if...then
 statements; and when you need to respond to exceptions raised by
the operating system or by routines whose source code you don't control. Exceptions are commonly used for
hardware, memory, I/O, and operating-system errors.
Conditional statements are often the best way to test for errors. For example, suppose you want to make sure that
a file exists before trying to open it. You could do it this way:
try 
    AssignFile(F, FileName); 
    Reset(F);     // raises an EInOutError exception if file is not found 
except 
    on Exception do ... 
end;
But you could also avoid the overhead of exception handling by using
if FileExists(FileName) then    // returns False if file is not found; raises no exception
begin 
    AssignFile(F, FileName); 
160

---

    Reset(F); 
end;
Assertions provide another way of testing a Boolean condition anywhere in your source code. When an 
Assert
statement fails, the program either halts with a runtime error or (if it uses the 
SysUtils
 unit) raises an
EAssertionFailed exception. Assertions should be used only to test for conditions that you do not expect to occur.