System.Data Namespace

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Indicates the action that occurs when a ForeignKeyConstraint is enforced.

Member Name	Description
Cascade	Delete or update related rows. This is the default.
None	No action taken on related rows.
SetDefault	Set values in related rows to the value contained in the DefaultValue property.
SetNull	Set values in related rows to DBNull.

The Rule values are set to the UpdateRule and the DeleteRule properties of a ForeignKeyConstraint object found in a DataTable object' s ConstraintCollection.

The Rule values determine the action that occurs when a value in a column is either deleted or updated. Of the two, deleting a value is the more critical and demanding of attention when setting a rule.

In the case where a value is deleted, Cascade specifies that all rows containing that value are also deleted. SetNull specifies that values in all child columns are set to null values. SetDefault specifies that all child columns be set to the default value for the column. None specifies that no action will occur, but exceptions are generated.

In the case where a value is updated, Cascade specifies that all child columns are likewise updated with the new value. SetNull specifies that all child columns be set to null values. SetDefault specifies that all child column values be set to the default value. None specifies that no action be taken, but exceptions are generated.

Constraints on a DataSet are not enforced unless the EnforceConstraints property is true.

When the AcceptChanges method is called, the AcceptRejectRule further determines what action occurs.

   ' the next line goes into the Declarations section of the module:

   ' suppliersProducts is a class derived from DataSet.
Private myDataSet As SuppliersProducts
Private Sub SetConstraint ( )

   ' declare parent column and child column variables.
   Dim pCol As DataColumn
   Dim cCol As DataColumn
   Dim myFKC As ForeignKeyConstraint

   ' set parent and child column variables.
   pCol = myDataSet.Tables ( "Suppliers" ).Columns ( "SupplierID" )
   cCol = myDataSet.Tables ( "Products" ).Columns ( "SupplieriD" )
   myFKC = New ForeignKeyConstraint ( "SupplierFkConst", pCol, cCol )

   ' set null values when a value is deleted.
   myFKC.DeleteRule = Rule.SetNull
   myFKC.UpdateRule = Rule.Cascade
   myFKC.AcceptRejectRule = AcceptRejectRule.Cascade

   ' add the constraint, and set EnforceConstraints to true.
   myDataSet.Tables ( "Suppliers" ).Constraints.Add ( myFKC )
   myDataSet.EnforceConstraints = True
End Sub

Constraints   ForeignKeyConstraint

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