Constraint Class
Definition
Represents a constraint that can be enforced on one or more DataColumn objects.
[System.ComponentModel.TypeConverter(typeof(System.Data.ConstraintConverter))]
public abstract class Constraint
- Inheritance
-
Constraint
- Derived
- Attributes
Inherited Members
System.Object
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)
{
PrintForeigKeyConstraintProperties(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 PrintForeigKeyConstraintProperties(
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.
Constructors
| Constraint() |
Initializes a new instance of the Constraint class. |
Properties
| _DataSet |
Gets the DataSet to which this constraint belongs. |
| ConstraintName |
The name of a constraint in the ConstraintCollection. |
| ExtendedProperties |
Gets the collection of user-defined constraint properties. |
| Table |
Gets the DataTable to which the constraint applies. |
Methods
| CheckStateForProperty() |
Gets the DataSet to which this constraint belongs. |
| SetDataSet(DataSet) |
Sets the constraint's DataSet. |
| ToString() |
Gets the ConstraintName, if there is one, as a string. |
Thread Safety
This type is safe for multithreaded read operations. You must synchronize any write operations.