Represents a constraint that can be enforced on one or more DataColumn objects.
Assembly: System.Data (in System.Data.dll)
Thetype exposes the following members.
|CheckStateForProperty||Infrastructure. Gets the DataSet to which this constraint belongs.|
|Equals(Object)||Determines whether the specified object is equal to the current object. (Inherited from Object.)|
|Finalize||Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)|
|GetHashCode||Serves as the default hash function. (Inherited from Object.)|
|GetType||Gets the Type of the current instance. (Inherited from Object.)|
|MemberwiseClone||Creates a shallow copy of the current Object. (Inherited from Object.)|
|SetDataSet||Sets the constraint's DataSet.|
|ToString||Gets the ConstraintName, if there is one, as a string. (Overrides Object.ToString.)|
A constraint is a rule used to maintain the integrity of the data in the DataTable. For example, when you delete a value that is used in one or more related tables, a 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 UniqueConstraint, on the other hand, just makes sure that all values within a particular table are unique. For more information, see DataTable Constraints.
A base constructor is not used. Primary or unique key constraints are created by using the UniqueConstraint constructor, and foreign key constraints are created by using the ForeignKeyConstraint constructor.
The following example examines the collection of constraints for a DataTable and determines whether each constraint is a UniqueConstraint or a ForeignKeyConstraint. The properties of the constraint are then displayed.
Private Sub GetConstraints(dataTable As DataTable) Console.WriteLine() ' Print the table's name. Console.WriteLine("TableName: " & dataTable.TableName) ' Iterate through the collection and print ' each name and type value. Dim constraint As Constraint For Each constraint In dataTable.Constraints Console.WriteLine("Constraint Name: " _ & constraint.ConstraintName) Console.WriteLine("Type: " _ & constraint.GetType().ToString()) ' If the constraint is a UniqueConstraint, ' print its properties using a function below. If TypeOf constraint Is UniqueConstraint Then PrintUniqueConstraintProperties(constraint) End If ' If the constraint is a ForeignKeyConstraint, ' print its properties using a function below. If TypeOf constraint Is ForeignKeyConstraint Then PrintForeigKeyConstraintProperties(constraint) End If Next constraint End Sub Private Sub PrintUniqueConstraintProperties( _ constraint As Constraint) Dim uniqueConstraint As UniqueConstraint uniqueConstraint = CType(constraint, UniqueConstraint) ' Get the Columns as an array. Dim columnArray() As DataColumn columnArray = uniqueConstraint.Columns ' Print each column's name. Dim i As Integer For i = 0 To columnArray.Length - 1 Console.WriteLine("Column Name: " _ & columnArray(i).ColumnName) Next i End Sub Private Sub PrintForeigKeyConstraintProperties( _ constraint As Constraint) Dim fkConstraint As ForeignKeyConstraint fkConstraint = CType(constraint, ForeignKeyConstraint) ' Get the Columns as an array. Dim columnArray() As DataColumn columnArray = fkConstraint.Columns ' Print each column's name. Dim i As Integer For i = 0 To columnArray.Length - 1 Console.WriteLine("Column Name: " _ & columnArray(i).ColumnName) Next i Console.WriteLine() ' Get the related columns and print each columns name. columnArray = fkConstraint.RelatedColumns For i = 0 To columnArray.Length - 1 Console.WriteLine("Related Column Name: " _ & columnArray(i).ColumnName) Next i Console.WriteLine() End Sub