Working with DataTable Events

A DataTable represents a table of a DataSet. A DataTable provides many events that can be tracked down by an application (see Table 5-5).

Table 5-5: The DataTable Events

EVENT	DESCRIPTION
ColumnChanged	This event occurs when a value of a column has been changed.
ColumnChanging	This event occurs when a new value is being added to a column.
RowChanged	This event occurs when value of a row in the table is changed.
RowChanging	This event occurs when a row in a table is changing.
RowDeleted	This event occurs when a row in a table is deleted.
RowDeleting	This event occurs when a row is being deleted.

The ColumnChangedEventHandler handles the ColumnChanged event and is defined as follows:

 Public Delegate Sub DataColumnChangeEventHandler(_    ByVal sender As Object, _    ByVal e As DataColumnChangeEventArgs _ ) 

In this example, sender is the source of the event and e is DataColumnChangedEventArgs that contains the event data.

ColumnChangingEventHandler handles the ColumnChanging event and is defined as follows:

 Public Delegate Sub DataColumnChangeEventHandler(_    ByVal sender As Object, _    ByVal e As DataColumnChangeEventArgs _ ) 

In this example, sender is the source of the event and e is DataColumnChangingEventArgs that contains the event data.

Similar to these two event handlers, RowChangedEventHandler, RowChangingEventHandler, RowDeletedEventHandler, and RowDeletedEventHandler handle the RowChanged, RowChanging, RowDeleted, and RowDeleting events, respectively. Definitions of these event handlers are similar to DataColumnChangingEventHandler and DataColumnChangedEventHandler—except the EventArgs class is different.

To test these events you'll create a DataTable, add DataRows to it, and then update and delete rows from the table.

Listing 5-9 creates a DataTable, adds three columns—id, name, and address—adds data to the table and changes the column of the table. It also calls the ColumnChanged and ColumnChanging event handlers. You write the code for the ColumnChanged and ColumnChanging event handlers in the Column_changed and Column_Changing methods. You can write this code on a button click event handler.

Listing 5-9: Writing the ColumnChanging and ColumnChanged Event Handlers

 Private Sub ColumnChange_Click(ByVal sender As System.Object, _   ByVal e As System.EventArgs) Handles ColumnChange.Click     Dim custTable As DataTable = New DataTable("Customers")     ' add columns     custTable.Columns.Add("id", Type.GetType("System.Int32"))     custTable.Columns.Add("name", Type.GetType("System.String"))     custTable.Columns.Add("address", Type.GetType("System.String"))     ' add a ColumnChanged & ColumnChanging event handler for the table.     AddHandler custTable.ColumnChanged, New DataColumnChangeEventHandler _     (AddressOf Column_Changed)     AddHandler custTable.ColumnChanging, New DataColumnChangeEventHandler _     (AddressOf Column_Changing)      ' add Two rows     custTable.Rows.Add(New Object() {1, "name1", "address1"})     custTable.Rows.Add(New Object() {2, "name2", "address2"})     ' Save changes     custTable.AcceptChanges()     ' change Address column in all the rows      Dim row As DataRow      For Each row In custTable.Rows     row("address") = "New address"     Next row     ' Removing event handlers     RemoveHandler custTable.ColumnChanged, AddressOf Column_Changed     RemoveHandler custTable.ColumnChanging, AddressOf Column_Changing   End Sub   ' Column changed event handler   Private Shared Sub Column_Changed(ByVal sender As Object, _   ByVal e As DataColumnChangeEventArgs)    MessageBox.Show("Column_Changed Event: " + " ," & _   e.Row("name") + " ," + e.Column.ColumnName + " ," & _    e.Row("name", DataRowVersion.Original))   End Sub   ' Column Changing event handler   Private Shared Sub Column_Changing(ByVal sender As Object, _   ByVal e As DataColumnChangeEventArgs)     MessageBox.Show("Column_Changing Event: " + " ," & _     e.Row("name") + " ," + e.Column.ColumnName + " ," & _     e.Row("name", DataRowVersion.Original))   End Sub 

Listing 5-10 creates a DataTable, adds three columns—id, name, and address—adds data to the table, and changes the column of the table. It also calls the RowChanging and RowChanged event handlers.

Listing 5-10: Writing the RowChanging and RowChanged Event Handlers

   ' Row Changed and Row Changing Events Caller   Private Sub UpdateRow_Click(ByVal sender As System.Object, _   ByVal e As System.EventArgs) Handles UpdateRow.Click     Dim custTable As DataTable = New DataTable("Customers")     ' add columns     custTable.Columns.Add("id", Type.GetType("System.Int32"))     custTable.Columns.Add("name", Type.GetType("System.String"))     custTable.Columns.Add("address", Type.GetType("System.String"))   ' add a ColumnChanged & ColumnChanging event handler for the table.     AddHandler custTable.RowChanged, New DataRowChangeEventHandler _     (AddressOf Row_Changed)     AddHandler custTable.RowChanged, New DataRowChangeEventHandler _     (AddressOf Row_Changing)     ' add Two rows     custTable.Rows.Add(New Object() {1, "name1", "address1"})     custTable.Rows.Add(New Object() {2, "name2", "address2"})     ' Save changes     custTable.AcceptChanges()     ' change Address column in all the rows     Dim row As DataRow     For Each row In custTable.Rows     row("name") = "New name"     Next row     ' Removing event handlers     RemoveHandler custTable.RowChanged, AddressOf Row_Changed     RemoveHandler custTable.RowChanging, AddressOf Row_Changing   End Sub   ' Row Changed event handler   Private Shared Sub Row_Changed(ByVal sender As Object, _   ByVal e As DataRowChangeEventArgs)     MessageBox.Show("Row_Changed Event:" & _     e.Row("name", DataRowVersion.Original).ToString() & _     e.Action.ToString())   End Sub   ' Row Changing event handler   Private Shared Sub Row_Changing(ByVal sender As Object, _   ByVal e As DataRowChangeEventArgs)     MessageBox.Show("Row_Changing Event:" & _     e.Row("name", DataRowVersion.Original).ToString() & _     e.Action.ToString())   End Sub 

Listing 5-11 creates a DataTable, adds three columns—id, name, and address—adds data to the table, and changes a column of the table. It also calls the RowDeleting and RowDeleted event handlers.

Listing 5-11: Writing the RowDeleting and RowDeleted Event Handlers

  ' Row Deleted and Row Deleting Events Caller  Private Sub DeleteRow_Click(ByVal sender As System.Object, _  ByVal e As System.EventArgs) Handles DeleteRow.Click    Dim custTable As DataTable = New DataTable("Customers")  ' add columns    custTable.Columns.Add("id", Type.GetType("System.Int32"))    custTable.Columns.Add("name", Type.GetType("System.String"))    custTable.Columns.Add("address", Type.GetType("System.String"))    ' add a ColumnChanged & ColumnChanging event handler for the table.    AddHandler custTable.RowDeleted, New DataRowChangeEventHandler _    (AddressOf Row_Deleted)    AddHandler custTable.RowDeleting, New DataRowChangeEventHandler _    (AddressOf Row_Deleting)     ' add Two rows    custTable.Rows.Add(New Object() {1, "name1", "address1"})    custTable.Rows.Add(New Object() {2, "name2", "address2"})    ' Save changes    custTable.AcceptChanges()    ' change Address column in all the rows     Dim row As DataRow     ' Go through all rows     For Each row In custTable.Rows    ' Delete the row    row.Delete()    Next row    ' Removing event handlers    RemoveHandler custTable.RowDeleted, AddressOf Row_Deleted    RemoveHandler custTable.RowDeleting, AddressOf Row_Deleting  End Sub  ' Row Deleted event handler  Private Shared Sub Row_Deleted(ByVal sender As Object, _  ByVal e As DataRowChangeEventArgs)    MessageBox.Show("Row_Deleted Event:" & _   e.Row("name", DataRowVersion.Original).ToString() & _   e.Action.ToString())  End Sub  'Row Deleting event handler  Private Shared Sub Row_Deleting(ByVal sender As Object, _  ByVal e As DataRowChangeEventArgs)    MessageBox.Show("Row_Deleting Event:" & _    e.Row("name", DataRowVersion.Original).ToString() & _    e.Action.ToString())  End Sub