Creating a Custom Constraint

A custom constraint is not imported from the server like other constraints. It is a constraint that you declare, implement, and enforce in your local application. As such, custom constraints can be useful for offering a prevalidation enforcement of data entry, but a custom constraint cannot be applied against data received from or sent to a server application.

To create a custom constraint, set the CustomConstraint property to specify a constraint condition, and set ConstraintErrorMessage to the message to display when a user violates the constraint at runtime.

CustomConstraint is an SQL string that specifies any application-specific constraints imposed on the field's value. Set CustomConstraint to limit the values that the user can enter into a field. CustomConstraint can be any valid SQL search expression such as

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x > 0 and x < 100

The name used to refer to the value of the field can be any string that is not a reserved SQL keyword, as long as it is used consistently throughout the constraint expression.

Note: Custom constraints are only available in BDE-enabled and client datasets.

Custom constraints are imposed in addition to any constraints to the field's value that come from the server. To see the constraints imposed by the server, read the ImportedConstraint property.