Delphi Data Validation Options

• Non-database data validation
• Database data validation:
  • Table-level validation
  • Field-level validation
  • Record-level validation
  • Form-level validation
• see also:
  • Delphi Programming Main Index
  • Delphi database programming 1
  • Database systems

Non-Database Data Validation:

• these techniques can be used for databound controls too
• use a control with set value options:
  • checkbox, radiobutton, listbox, combobox, lookupcombobox, calendar controls, etc.
• validating a TEdit component:
  • set TEdit properties:
    • MaxLength
    • CharCase = [ecUpperCase, ecLowerCase]
  • via TEdit's OnExit event to call a validation function which raises an exception which prevents user from exiting the control
  • NB. if use the OnChange or OnKeyDown/Up events, these will require much more coding as are called each time user enters a character

Database Data Validation:

• Table-level validation:
  • validation rules embedded within database structure & exceptions are raised when a record is attempted to be posted
  • advantages:
    • validation is stored with the data
    • all applications that access the table will be validated irrespective of application's validations
    • only needs to be created once, thus no need to provide the validation code in applications
  • disadvantages:
    • may delay user being notified of invalid data until record or transaction is being posted which may create confusion and frustration
    • limited flexibility depending of file format
    • some features can be over-ridden or ignored
  • examples:
    • field data type (eg. integer/date/float/char)
    • primary keys to avoid duplicate unique record ID's thereby producing key violation exceptions
    • validity checks (eg. Paradox tables) but these are not utilised by Delphi ???
    • mandatory fields (ie. cannot be left blank) - not used by Delphi
    • field format masks (eg. Paradox "pictures") only used in Delphi if use Infopower components
    • triggers in SQL tables
    • foreign keys (lookup tables) ensures data must already exist in a lookup table
• Field-level validation:
  • set input mask properties for data entry fields (NOT the DisplayFormat property!!):
    • Delphi's EditMask:
      • has 3 parts, each separated by semicolons:
        • mask (eg. ! = leading blanks will not be saved, 0 = digit required)
        • 0/1 to indicate whether literal characters in mask will be saved with data (eg. hyphens)
        • character used in mask to refer to blank spaces (eg. an underscore)
    • Infopower's Paradox-style picture masks:
      • these are tested via InfoPower's OnInvalidValue event
      • can set user alerts via changing controls color, etc via InfoPower's OnCheckValue event
      • cannot define them in Delphi if use Paradox table with picture mask already defined, as it uses them automatically if UsePictureMask property set to true
      • examples:
        • # = any digit; ? = any letter; & = any letter, converted to uppercase; @ = any character;
        • ! = any character converted to uppercase; ; = next character to be used literally;
        • [abc] = optional sequence of characters abc
        • { } = grouping operator
          • eg. {Red, Green, Blue}
          • eg. {Red,Gr[ay,een],B{l{ack,ue},rown},White,Yellow}
        • * = repeat the following character any number of times:
          • eg. *5{#} = specify 5 numbers in a row
          • eg. *! = any number or any character with all letters converted to uppercase
          • eg. &*? = letters with 1st letter capitalised
  • set TField "required" property to true to prevent field being left blank
    • NB. if using EditMask with literals saved then this may not trigger an exception if edited then blanked!!
  • via TField onValidation event:
    • eg. if field.value < 1 then raise Exception.Create('Positive number required');
    • an exception occurring in this event prevents the data from being posted and displays the message
  • see also non-database data validation
• Record-level validation:
  • via TDataset's "BeforePost" event:
    • can check for conditional validations that depend on values in more than one field
    • raising an exception or calling abort prevents the record from being posted
• Form-level validation:
  • via Form's "OnCloseQuery" event:
    • NB. closing a form does not destroy it, you need to call either Release or Free methods to do that!
    • this event is aborted by setting CanClose := False;

References:

• Delphi Informant Sept 1995 p50-56
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