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1. If the Status property is equal to Continue and the value of the RecordsAffected property is 1, the update operation was successful. In most cases, you have little else to do, and you can exit the event handler. 2. If the Status property is equal to ErrorsOccurred, you can check the Errors prop erty to understand what went wrong. Frequent causes of errors are violations of database constraints or referential integrity rules, such as a duplicated value in a primary key or a unique column or a foreign key that doesn t point to any row in the parent table. 3. If you get a System.Data.DBConcurrencyException exception, it means that the WHERE clause in the SQL command failed to locate the row in the data source. What you do next depends on your application s business logic. Typically, you test the StatementType property to determine whether it was an insert, delete, or update operation; in the last two cases, the conflict is likely caused by another user who deleted or modified the record you re trying to delete or update. 4. You can issue a SELECT query against the database to determine what columns caused the conflict, in an attempt to resynchronize the DataSet with the data source and reconcile the conflicting row. For example, if you have a conflict in an update operation (the most frequent case), you can issue a SELECT command to again read the values now in the database table. If you have a conflict in a delete operation, you can issue a SELECT command to check whether the DELETE com mand failed because the record was deleted or because another user changed one of the fields listed in your WHERE clause. 5. In all cases, you must decide whether the update operation should continue. You can set the Status property to Continue or SkipCurrentRow to ignore the conflict for now, or SkipAllRemainingRows to end the update operation without raising an error in the application. You can also leave the value set to ErrorsOccurred, in which case the Update method is terminated right away and an exception is thrown to the main application. You don t have to perform all the preceding operations from inside a RowUpdate event handler, however, and you don t even need to write this event handler in some cases. For example, you can postpone the resychronization step until after the Update method has completed, and you might even decide not to resynchronize at all. The sections that follow illustrate three possible resynchronization strategies that you can adopt when dealing with update conflicts:
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You don t reconcile at all and just display a warning to the user, mentioning which rows failed to be updated correctly. In this case, you just set the ContinueUpdateOnError property to True and don t have to intercept the RowUpdated event. You reconcile with the data source after the Update method, using a SELECT com mand that reads again all the rows that failed to update. In this case, you set the
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ContinueUpdateOnError property to True to avoid an exception when a conflict ing row is found and place the resync code in the main application after the Update method.
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You reconcile with the data source on a row-by-row basis for each row that failed to update correctly. In this case, you place the resync code right inside the RowUpdated event handler and set the Status property of its argument to Con tinue. (Otherwise, the Update method will fail when the first conflict is found.)
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In the first strategy for managing conflicts, you don t even try to reconcile them and limit your actions to just displaying the records that failed the update operation. This strategy can be implemented quite simply, as this code demonstrates:
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A class-level DataAdapter that has been correctly initialized Dim da As OleDbDataAdapter Sub UpdateRecords() Ensure that conflicting rows don t throw an exception. da.ContinueUpdateOnError = True Send changes to the database, exit if no update conflicts. da.Update(ds, Publishers ) If Not ds.HasChanges Then Exit Sub If we get here, there s at least one conflicting row.
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Dim dt As DataTable = ds.Tables( Publishers )
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