Unfortunately, building applications includes coding. Regardless of how carefully you write/debug your program, it will be impossible to imagine every situation that can go wrong. Inexperienced user might, for example, try to open a nonexisting file or input a bad value into a data field.
Users make mistakes and we should be prepared to handle/prevent these errors wherever and whenever possible.
Errors, Exceptions?
An exception is generally an error condition or other event that interrupts normal flow of execution in an application. Whenever an error results from processing a line of code, Delphi creates (raises) an object descendent from TObject called the exception object.Guarded Blocks
An application responds to an exception either by executing some termination code, handling the exception, or both. The way to enable error/exception trapping within a given code, the exception must occur within a guarded block of statements. The general code looks like: try {guarded block of code} except on <ESomeException> do begin {exception block-handles SomeException} end; end; Example:
... Zero:=0; try dummy:= 10 / Zero; except on EZeroDivide do MessageDlg('Can not divide by zero!', mtError, [mbOK], 0) ; end; ... Protecting Resources
When a section of code acquires a resource, it is often necessary to ensure that the resource is released again (or you might get a memory leak), regardless of whether the code completes normally or is interrupted by an exception. In this case, the syntax uses finally keyword and looks like: {some code to allocate resources} try {guarded block of code} finally {termination blok - code to free resources} end; ... AboutBox:=TAboutBox.Create(nil) ; try AboutBox.ShowModal; finally AboutBox.Release; end; ... 
