Delphi Basics : On command

DelphiBasics

On
Keyword

Defines exception handling in a Try Except clause

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

Description

The On keyword defines an exception handling statement in the Except part of a Try statement.

If the specified Exception is raised in the Try statements, then the Do statement is executed.

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

If the exception raised finds no matching On clause, 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).

For all exceptions, an Exception object is created, and it is this that is referenced in the On clause. These objects are normally derived from the Exception class.

You can determine the type of exception that occured by looking at the ClassName property, which gives the exception type, such as 'EDivByZero', as shown in the example code.

Related commands

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

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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

Example code : Divide by zero with multiple Except On clauses

var
  number, zero : Integer;
begin
  // Try to divide an integer by zero - to raise an exception
  number := -1;
  Try
    zero   := 0;
    number := 1 div zero;
    ShowMessage('number / zero = '+IntToStr(number));
  except
    // Our first exception will not be nmatched
    On E : EInOutError do
      ShowMessage('IO error : '+E.Message);
    // This exception will be matched
    On E : EDivByZero do
      ShowMessage('Div by zero error : '+E.Message);
    // Catch other errors
    Else
      ShowMessage('Unknown error');
  end;
end;

Show full unit code

Div by zero error : Division by zero