Delphi Basics : Try command

DelphiBasics

Try
Keyword

Starts code that has error trapping

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

Description

The Try keyword is used to mark the start of a block of statements that have error trapping. If an error occurs, the program is not terminated. Instead, control is passed to either a Finally or Except section.

Try is used in a number of ways.

Version 1

In a Try-Finally construct, the Finally statement is guaranteed to be executed absolutely regardless of what happens in the Try clause. However, the Finally clause does not actually handle any exceptions - the program will terminate if no Except clause is found (see notes below).

Try-Finally is normally used by a routine to allow cleanup processing to take place, such as freeing resources, with the exception being correctly passed to the caller to handle.

Version 2

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

Version 3

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 3 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.

The following list of exceptions covers the basic types - there are hundreds of exception classes:

Exception	Base class
EAbort	Abort without dialog box
EAbstractError	Abstract method error
AssertionFailed	Assert call failed
EBitsError	Boolean array error
ECommonCalendarError	Calendar calc error
EDateTimeError	DateTime calc error
EMonthCalError	Month calc error
EConversionError	Raised by Convert
EConvertError	Object convert error
EDatabaseError	Database error
EExternal	Hardware/Windows error
EAccessViolation	Access violation
EControlC	User abort occured
EExternalException	Other Internal error
EIntError	Integer calc error
EDivByZero	Divide by zero
EIntOverflow	Integer overflow
ERangeError	Out of value range
EMathError	Floating point error
EInvalidArgument	Bad argument value
EInvalidOp	Inappropriate operation
EOverflow	Value too large
EUnderflow	Value too small
EZeroDivide	Divide by zero
EStackOverflow	Severe Delphi problem
EHeapException	Dynamic memory problem
EInvalidPointer	Bad memory pointer
EOutOfMemory	Cannot allocate memory
EInOutError	IO error
EInvalidCast	Object casting error
EInvalidOperation	Bad component op
EMenuError	Menu item error
EOSError	Operating system error
EParserError	Parsing error
EPrinter	Printer error
EPropertyError	Class property error
EPropReadOnly	Invalid property access
EPropWriteOnly	Invalid property access
EThread	Thread error
EVariantError	Variant problem

Notes

There are times when you want a construct like this :

Try
  ...
Except
  ...
Finally
  ...
End;

where exceptions are trapped and acted upon, but in all cases, a set of clean up statements are executed. This can be achieved with nested Try statements :

Try
  Try
    ...
  Except
    ...
  End;
Finally
  ...
End;

Related commands

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

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Example code : Version 1 : Zero divide with a finally clause

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));
  finally
    if number = -1 then
    begin
      ShowMessage('Number was not assigned a value - using default');
      number := 0;
    end;
  end;
end;

Show full unit code

  Number was not assigned a value - using default

  Then, the program terminates with an EDivByZero error message - the finally clause did not trap the error.

Example code : Version 2 : 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 : Version 3 : 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