The DataSet Classes

Unlike the data provider classes, only one set of DataSet classes exists. The DataSet classes are all contained in the System.Data namespace, and they represent data in an abstract form that's not tied to any particular database implementation. This section introduces you to the DataSet class and the other objects it contains, including

DataTable
DataRelation
DataRow
DataColumn
DataView

The DataSet class itself is a self-contained, memory-resident representation of relational data. A DataSet object contains other objects, such as DataTable and DataRelation objects, that hold the actual data and information about the design of the data. Table 7.5 shows the most important members of the DataSet class.

Table 7.5. Important Members of the DataSet Class

Member	Type	Description
AcceptChanges()	Method	Marks all changes in the dataset as having been accepted
Clear()	Method	Removes all data from the dataset
GetChanges()	Method	Gets a DataSet object that contains only the changed data in the dataset
GetXml()	Method	Gets an XML representation of the dataset
GetXmlSchema()	Method	Gets an XML Schema Definition (XSD) representation of the dataset schema
Merge()	Method	Merges two datasets
ReadXml()	Method	Loads the dataset from an XML file
ReadXmlSchema()	Method	Loads the dataset schema from an XSD file
Relations	Property	Specifies a collection of DataRelation objects
Tables	Property	Specifies a collection of DataTable objects
WriteXml()	Method	Writes the dataset to an XML file
WriteXmlSchema()	Method	Writes the dataset schema to an XSD file

The DataTable Class

The DataTable class represents a single table within the DataSet object. A single DataSet object can contain many DataTable objects. Table 7.6 shows the most important members of the DataTable class.

Table 7.6. Important Members of the DataTable Class

Member	Type	Description
ChildRelations	Property	Specifies a collection of DataRelation objects that refer to children of the DataTable class
Clear()	Method	Removes all data from the DataTable class
ColumnChanged	Event	Occurs when the data in any row of a specified column has been changed
ColumnChanging	Event	Occurs when the data in any row of a specified column is about to be changed
Columns	Property	Specifies a collection of DataColumn objects
Constraints	Property	Specifies a collection of Constraint objects
DataSet	Property	Retrieves the DataSet object to which the DataTable object belongs
NewRow()	Method	Creates a new, blank row in the data table
ParentRelations	Property	Specifies a collection of DataRelation objects that refer to parents of the DataTable
PrimaryKey	Property	Specifies an array of DataColumn objects that provide the primary key for the DataTable
RowChanged	Event	Occurs when any data in a DataRow has been changed
RowChanging	Event	Occurs when any data in a DataRow is about to be changed
RowDeleted	Event	Occurs when a row has been deleted
RowDeleting	Event	Occurs when a row is about to be deleted
Rows	Property	Specifies a collection of DataRow objects
Select()	Method	Selects an array of DataRow objects that meet specified criteria
TableName	Property	Specifies the name of the DataTable

As you can see, you can manipulate a DataTable as either a collection of DataColumn objects or a collection of DataRow objects. The DataTable class also provides events that you can use to monitor data changes. For example, you might bind a DataTable to a DataGrid control and use these events to track the user 's operations on the data in the DataGrid control.

The DataRelation Class

The DataRelation object stores information on the relationships between DataTable objects in a DataSet object. The most important members of the DataRelation class are listed in Table 7.7.

Table 7.7. Important Members of the DataRelation Class

Member	Type	Description
ChildColumns	Property	Specifies a collection of DataColumn objects that defines the foreign key side of the relationship
ChildKeyConstraint	Property	Returns a ForeignKeyConstraint object for the relationship
ChildTable	Property	Specifies a DataTable object from the foreign key side of the relationship
DataSet	Property	Retrieves the DataSet object to which the DataRelation object belongs
ParentColumns	Property	Specifies a collection of DataColumn objects that defines the primary key side of the relationship
ParentKeyConstraint	Property	Returns a PrimaryKeyConstraint object for the relationship
ParentTable	Property	Specifies a DataTable from the primary key side of the relationship
RelationName	Property	Specifies the name of the DataRelation

The DataRow Class

A DataRow object represents a single row of data. When you're selecting, inserting, updating, or deleting data in a DataSet object, you're normally working with DataRow objects. Table 7.8 shows the most important members of the DataRow class.

Table 7.8. Important Members of the DataRow Class

Member	Type	Description
BeginEdit()	Method	Starts editing the DataRow object
CancelEdit()	Method	Discards an edit in progress
Delete()	Method	Deletes the DataRow object from its parent DataTable object
EndEdit()	Method	Ends an edit in progress, saving the changes
IsNull()	Method	Returns true if a specified column contains a null value
ItemArray	Property	Allows the manipulation of a DataRow object through an array of values
RowState	Property	Returns information on the current state of a DataRow object (for example, whether it has been changed since it was last saved to the database)
Table	Property	Retrieves the DataTable object to which the DataRow object belongs

The DataColumn Class

A DataColumn object represents a single column in a DataTable object. By manipulating the DataColumn objects, you can determine and even change, the structure of the DataTable object. Table 7.9 lists the most important members of the DataColumn class.

Table 7.9. Important Members of the DataColumn Class

Member	Type	Description
AllowDbNull	Property	Indicates whether the DataColumn object can contain null values
AutoIncrement	Property	Indicates whether the DataColumn object is an identity column
ColumnName	Property	Specifies the name of the DataColumn object
DataType	Property	Specifies the data type of the DataColumn object
DefaultValue	Property	Specifies the default value of this DataColumn object for new rows of data
Expression	Property	Specifies an expression that can be used to filter rows, calculate the values in a column, or create an aggregate column
MaxLength	Property	Specifies the maximum length of a text DataColumn object
Unique	Property	Indicates whether values in the DataColumn object must be unique across all rows in the DataTable object

The DataView Class

The DataView object represents a view of the data contained in a DataTable object. A DataView object might contain every DataRow object from the DataTable object, or it might be filtered to contain only specific rows. That filtering can be done by SQL expressions (for example, returning only rows for customers in France) or by row state (for example, returning only rows that have been modified). The most important members of the DataView class are detailed in Table 7.10.

Table 7.10. Important Members of the DataView Class

Member	Type	Description
AddNew()	Method	Adds a new row to the DataView object
AllowDelete	Property	Indicates whether deletions can be performed through this DataView object
AllowEdit	Property	Indicates whether updates can be performed through this DataView object
AllowNew	Property	Indicates whether insertions can be performed through this DataView object
Count	Property	Returns the number of rows in this DataView object
Delete()	Method	Deletes a row from this DataView object
Find()	Method	Searches for a row in the DataView object
FindRows()	Method	Returns an array of rows matching a filter expression
RowFilter	Property	A string expression that specifies how the rows are filtered
RowStateFilter	Property	One of the values from the DataViewRowState enumeration that specifies how the rows are filtered
Sort()	Method	Sorts the data in a DataView object