Constraints
ConstraintCollection cc = DataTable.Constraints;
Accesses the ConstraintCollection for the
DataTable, providing access to the
Constraint objects belonging to the
DataTable. The Constraints
property can be used to add, remove, and examine the
UniqueConstraint and
ForeignKeyConstraint objects in a
DataTable.
There are two methods that can add a constraint to a table. The
Add( ) method takes an argument specifying a
reference to an existing constraint or takes arguments specifying
whether a unique constraint or foreign key constraint is being added.
The following example demonstrates adding a constraint by specifying
a reference to an existing constraint:
// add a unique constraint by reference
UniqueConstraint uc = new UniqueConstraint(dt.Columns["MyColumn"]);
dt.Constraints.Add(uc);
// add a foreign key constraint by reference
ForeignKeyConstraint fc = new ForeignKeyConstraint(
dtParent.Columns["ParentColumn"],
dtChild.Columns["ChildColumn"]);
dt.Constraints.Add(fc);Two overloads of the Add( ) method create and add
a UniqueConstraint in one statement. The methods
take a constraint name, either a reference to a
DataColumn or a DataColumn
array, and an argument indicating whether the column or columns are a
primary key.
// add a unique constraint that is also a primary key
dt.Constraints.Add("MyUniqueConstraint", dt.Columns["MyColumn"], true);The other two overloads of the Add( ) method
create and add a ForeignKeyConstraint in one
statement. The methods take a constraint name, and either two
DataColumn references or two
DataColumn arrays.
// add a foreign key constraint based on two columns
dt.Constraints.Add("MyForeignKeyConstraint",
dtParent.Columns["ParentCol1"],
dtChild.Columns["ChildCol2"]);The AddRange( ) method adds an array of
Constraint objects to the end of the constraint
collection:
Constraint c1, c2;
// ... code to define constraints c1 and c2
// add the constraints c1 and c2 to the ConstraintCollection for the table
dt.Constraints.AddRange(new Constraint[] {c1, c2});The ConstraintCollection class derives its
standard functionality from the
InternalDataCollectionBase class from which it
inherits.
The commonly used public properties of the
ConstraintCollection are listed and described in
Table 23-9.
The commonly used public methods of the
ConstraintCollection are listed and described in
Table 23-10.