Mega Code Archive

Try - starts code that has error trapping

1 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