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Article #15394: Checking for NULL in OnUpdateData handler

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FAQ394D.txt   Checking for NULL in OnUpdateData handler
Category   :DCOM/MultiTier/ActiveForm
Platform    :All
Product    :Delphi 3.x  

How can I tell if a user has changed a string field to null
in the OnUpdateData event handler?   How can I differentiate 
between a field that was changed to a null string and a
field that was not changed at all.

Use the NewValue property on the TField when reading the
second record (the one which contains the edits).  If the 
returned variant is empty (or Unassigned) then that field
was not modified. Here is some code that demonstrates this:

     NewVal: Variant;
     NewVal := DataSet.FieldByName('MyStrField').NewValue;
     if VarIsEmpty(NewVal) then
        ShowMessage('Field was not edited')
     else if VarIsNull(NewVal) then
        ShowMessage('Field was blanked out')
        ShowMessage('New Field Value: ' + String(NewVal));

If you look at the source for the RecError form (in the 
repository), you will see how it uses this information to 
display the '' string when showing the reconcile 
errors.  On the server you add record level constraints
using the constraints property of your TQuery/TTable or
field level constraints using persistent TField objects
(either CustomConstraint or ImportedConstraint).  If you use
field level constraints, they are enforced when data is
posted to the field (i.e. when you tab out of a data aware
control associated with the field).

7/16/98 4:31:28 PM

Last Modified: 01-SEP-99