A constraint is a rule used to maintain the integrity of the data in the System.Data.DataTable. For example, when you delete a value that is used in one or more related tables, a System.Data.ForeignKeyConstraint determines whether the values in the related tables are also deleted, set to null values, set to default values, or whether no action occurs. A System.Data.UniqueConstraint, on the other hand, just makes sure that all values within a particular table are unique. For more information, see Adding Constraints to a Table.

A base System.Data.Constraint constructor is not used. Primary or unique key constraints are created by using the System.Data.UniqueConstraint constructor, and foreign key constraints are created by using the System.Data.ForeignKeyConstraint constructor.