Delphi Basics : Except command

DelphiBasics

Except
Keyword

Starts the error trapping clause of a Try statement

1	keyword Except(Try Statement {Statement...} Except Statement {Statement...} End;
2	Try Statement {Statement...} Except On {Name :} Exception type Do Statement {Else Statement} End;

Description

The Except keyword is used to mark the start of a block of statements that handle an exception in a Try clause. If the Except block can handle the exception, then the program is not terminated.

Except has two different syntaxes:

Version 1

In this version, if the Try clause generates an exception the Except clause is executed. This is used to take alternative action when something unexpected goes wrong. The except clause cannot determine the error type however.

Version 2

This is similar to version 2, but specifies different actions for different exception types, such as EInOutError. An Else clause can be used as a catch all for unexpected exception types. The general exception type Exception can be used to catch all exception types.

By assigning a Name to the exception, the message text of the exception (Name.Message) can be obtained for display or other uses.

When an exception is raised in a version 2 setup, if the exception is not acted upon by On or Else statements, then a check is made to see if we are in a nested Try block. If so, the Except clause of this parent Try is processed. If no On or Else clause is found, the program terminates.

The Else clause is not really necessary - it is better to use On E:Exception Do, the generic exception handling, since it still provides the error message (E.Message).

Important : you can determine the type of error that occured by using the generic exception handling - On E:Exception Do. E is a pointer to the exception object that is created by the exception condition. E.ClassName gives the exception type, such as 'EDivByZero', as shown in the final example code.

Related commands

Finally		Starts the unconditional code section of a Try statement
On		Defines exception handling in a Try Except clause
Raise		Raise an exception
Try		Starts code that has error trapping

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Example code : Zero divide with a plain Except block

var
  number, zero : Integer;
begin
  // Try to divide an integer by zero - to raise an exception
  Try
    zero   := 0;
    number := 1 div zero;
    ShowMessage('number / zero = '+IntToStr(number));
  Except
    ShowMessage('Unknown error encountered');
  end;
end;

Show full unit code

Unknown error encountered

Example code : Divide by zero with an Except On clause

var
  number, zero : Integer;
begin
  // Try to divide an integer by zero - to raise an exception
  Try
    zero   := 0;
    number := 1 div zero;
    ShowMessage('number / zero = '+IntToStr(number));
  Except
    on E : Exception do
      ShowMessage(E.ClassName+' error raised, with message : '+E.Message);
  end;
end;

Show full unit code

EDivByZero error raised with message : Division by zero