Applied XML Programming for Microsoft .NET.pdf - Csbdu.in

If no match is found, ReadXml deduces that the row in the section wasdeleted from the table when it was at the position that the msdata:rowOrder attributeindicates. The method inserts a new row in the table at the same position andpopulates it with the values read from the section. Next the row ismarked for deletion using the Delete method of the DataRow object.The final step consists of reading the values from the section andupdating accordingly the RowError property of the corresponding DataRow object in thetable.The Row Commit ModelThe DataSet, DataTable, and DataRow objects maintain a local cache of changes.When a row is modified, deleted, or added, its state changes to one of the values of theDataRowState enumeration. (See the .NET Framework documentation for details.)Similarly, when a row is added, modified, or deleted from a table, the internal state ofthe table is altered, resulting in pending changes for the affected rows.Pending changes can be either accepted or rejected at the DataSet, DataTable, orDataRow level. Accepting a pending change means that the row (changes alwaysinvolve a row) updates are committed to the table. Rejecting a pending change rollsback the state of the table, and the table appears as though the change never occurred.A DiffGram can track pending changes—that is, in-memory changes that have not yetbeen committed. Table 10-3 lists the allowable states for a DataRow object.Table 10-3: States of a DataRow ObjectStateAddedDeletedDetachedModifiedUnchangedDescriptionThe row has been added to the table, but AcceptChangeshas not yet been called.The row is marked for deletion from the parent table.Either the row has been created but not yet added to thetable, or the row has been removed from the rows collection.Some columns within the row have been changed.No changes have been made since the last call toAcceptChanges. This is also the state of all rows when thetable is first created.The AcceptChanges method has the power to commit all the changes and accept thecurrent values as the new original values of the table, clearing pending changes.RejectChanges rolls back all the pending changes. We'll encounter the row commitmodel again in the section "A Save-And-Resume Application," on page 464, when welook at save-and-resume applications.The Original Data SectionThe DiffGram has a layered structure in which current values, original values for themodified rows, and pending errors are stored in distinct sections. The state of theDataSet object is rebuilt by combining the contents of these sections. The originalvalues are stored in the section as a change with respect to the currentdata instance.The DataRow object maintains several versions of itself that are internally stored in anarray of rows. The versions are grouped in the DataRowVersion enumeration, shown inTable 10-4.368