Constraint Class

Definition

public ref class Constraint abstract

[System.ComponentModel.TypeConverter(typeof(System.Data.ConstraintConverter))]
public abstract class Constraint

public abstract class Constraint

[System.ComponentModel.TypeConverter(typeof(System.Data.ConstraintConverter))]
[System.Serializable]
public abstract class Constraint

[<System.ComponentModel.TypeConverter(typeof(System.Data.ConstraintConverter))>]
type Constraint = class

type Constraint = class

[<System.ComponentModel.TypeConverter(typeof(System.Data.ConstraintConverter))>]
[<System.Serializable>]
type Constraint = class

Public MustInherit Class Constraint

Inheritance

Object

Constraint

Derived

System.Data.ForeignKeyConstraint

System.Data.UniqueConstraint

Attributes

TypeConverterAttribute SerializableAttribute

Examples

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 void GetConstraints(DataTable dataTable)
{
 Console.WriteLine();

 // Print the table's name.
 Console.WriteLine("TableName: " + dataTable.TableName);

 // Iterate through the collection and
 // print each name and type value.
 foreach(Constraint 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(constraint is UniqueConstraint)
 {
 PrintUniqueConstraintProperties(constraint);
 }
 // If the constraint is a ForeignKeyConstraint,
 // print its properties using a function below.
 if(constraint is ForeignKeyConstraint)
 {
 PrintForeignKeyConstraintProperties(constraint);
 }
 }
}

private void PrintUniqueConstraintProperties(
 Constraint constraint)
{
 UniqueConstraint uniqueConstraint;
 uniqueConstraint = (UniqueConstraint) constraint;

 // Get the Columns as an array.
 DataColumn[] columnArray;
 columnArray = uniqueConstraint.Columns;

 // Print each column's name.
 for(int i = 0;i<columnArray.Length ;i++)
 {
 Console.WriteLine("Column Name: "
 + columnArray[i].ColumnName);
 }
}

private void PrintForeignKeyConstraintProperties(
 Constraint constraint)
{
 ForeignKeyConstraint fkConstraint;
 fkConstraint = (ForeignKeyConstraint) constraint;

 // Get the Columns as an array.
 DataColumn[] columnArray;
 columnArray = fkConstraint.Columns;

 // Print each column's name.
 for(int i = 0;i<columnArray.Length ;i++)
 {
 Console.WriteLine("Column Name: "
 + columnArray[i].ColumnName);
 }
 Console.WriteLine();

 // Get the related columns and print each columns name.
 columnArray = fkConstraint.RelatedColumns ;
 for(int i = 0;i<columnArray.Length ;i++)
 {
 Console.WriteLine("Related Column Name: "
 + columnArray[i].ColumnName);
 }
 Console.WriteLine();
}

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

Remarks

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 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.

Thread Safety

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

See also

• ConstraintCollection
• Constraints
• ForeignKeyConstraint
• UniqueConstraint