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Constraint Class

Represents a constraint that can be enforced on one or more DataColumn objects.

Namespace:  System.Data
Assembly:  System.Data (in System.Data.dll)

'Declaration
Public MustInherit Class Constraint
'Usage
Dim instance As Constraint

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 (ADO.NET).

A base Constraint 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

This type is safe for multithreaded read operations. You must synchronize any write operations.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0
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