Tài liệu Exploring the DataAdapter and DataTable Events docx

Exploring the DataAdapter and DataTable Events You'll find all the code examples shown in this section in the program UsingEvents.cs located in the ch11 directory. The listing for this program is omitted from this book for brevity. If you compile and run that program, you'll see the order in which the events fire when you add, modify, and remove a row from a DataTable that contains rows retrieved from the Customers table. The DataAdapter Events The events exposed by a SqlDataAdapter object are shown in Table 11.11 . Table 11.11: SqlDataAdapter EVENTS EVENT EVENT HANDLER DESCRIPTION FillError FillErrorEventHandler Fires when an error occurs during a call to the Fill() method. RowUpdating RowUpdatingEventHandler Fires before a row is added, modified, or deleted in the database as a result of calling the Update() method. RowUpdated RowUpdatedEventHandler Fires after a row is added, modified, or deleted in the database as a result of calling the Update() method. The FillError Event The FillError event fires when you call the Fill() method and an error occurred. The following are a couple of scenarios that would cause an error: • An attempt is made to add a number from the database to a DataColumn that couldn't be converted to the .NET type of that DataColumn without losing precision. • An attempt is made to add a row from the database to a DataTable that violates a constraint in that DataTable. The following example event handler, named FillErrorEventHandler(), checks for the precision conversion error: public static void FillErrorEventHandler( object sender, FillErrorEventArgs myFEEA ) { if (myFEEA.Errors.GetType() == typeof(System.OverflowException)) { Console.WriteLine("A loss of precision occurred"); myFEEA.Continue = true; } } The first parameter is an object of the System.Object class, and it represents the object that raises the event. The second parameter is an object of the FillErrorEventArgs class, which like all the EventArgs classes, is derived from the System.EventArgs class. The EventArgs class is the base class for event data and represents the details of the event. Table 11.12 shows the FillErrorEventArgs properties. Table 11.12: FillErrorEventArgs PROPERTIES PROPERTY TYPE DESCRIPTION Continue bool Gets or sets a bool that indicates whether you want to continue filling your DataSet even though an error has occurred. The default is false. DataTable DataTable Gets the DataTable that was being filled when the error occurred. Errors Exception Gets the Exception containing the error that occurred. Values object[] Gets the DataColumn values of the DataRow in which the error occurred. These values are returned in an object array. You indicate that mySqlDataAdapter is to call the FillErrorEventHandler() method when the FillError event fires using the following code: mySqlDataAdapter.FillError += new FillErrorEventHandler(FillErrorEventHandler); The RowUpdating Event The RowUpdating event fires before a row is updated in the database as a result of you calling the Update() method of your DataAdapter. This event fires once for each DataRow you've added, modified, or deleted in a DataTable. The following actions are performed behind the scenes for each DataRow when you call the Update() method of your DataAdapter: 1. The values in your DataRow are copied to the parameter values of the appropriate Command in the InsertCommand, UpdateCommand, or DeleteCommand property of your DataAdapter. 2. The RowUpdating event of your DataAdapter fires. 3. The Command is run to push the change to the database. 4. Any output parameters from the Command are returned. 5. The RowUpdated event of your DataAdapter fires. 6. The AcceptChanges() method of your DataRow is called. The second parameter to any event handler method you write to handle the RowUpdating event of a SqlDataAdapter object is of the SqlRowUpdatingEventArgs class, and Table 11.13 shows the properties of this class. Table 11.13: SqlRowUpdatingEventArgs PROPERTIES PROPERTY TYPE DESCRIPTION Command SqlCommand Gets or sets the SqlCommand that is run when the Update() method is called. Errors Exception Gets the Exception for any error that occurred. Row DataRow Gets the DataRow to send to the database through the Update() method. StatementType StatementType Gets the type of the SQL statement that is to be run. StatementType is an enumeration in the System.Data namespace that contains the following members: • Delete • Insert • Select • Update Status UpdateStatus Gets or sets the UpdateStatus of the Command object. UpdateStatus is an enumeration in the System.Data namespace that contains the following members: • Continue, which indicates that the DataAdapter is to continue processing rows. • ErrorsOccurred, which indicates that the update is to be treated as an error. • SkipAllRemainingRows, which indicates that the current row and all remaining rows are to be skipped and not updated. Table 11.13: SqlRowUpdatingEventArgs PROPERTIES PROPERTY TYPE DESCRIPTION • SkipCurrentRow, which indicates that the current row is to be skipped and not updated. The default is Continue. TableMapping DataTableMapping Gets the DataTableMapping object that is sent to the Update() method. A DataTableMapping object contains a description of a mapped relationship between a source table and a DataTable (see Chapter 10, "Using DataSet Objects to Store Data"). The following example event handler, named RowUpdatingEventHandler(), prevents any new rows from being added to the database with a CustomerID of J5COM: public static void RowUpdatingEventHandler( object sender, SqlRowUpdatingEventArgs mySRUEA ) { Console.WriteLine("\nIn RowUpdatingEventHandler()"); if ((mySRUEA.StatementType == StatementType.Insert) && (mySRUEA.Row["CustomerID"] == "J5COM")) { Console.WriteLine("Skipping current row"); mySRUEA.Status = UpdateStatus.SkipCurrentRow; } } You indicate that mySqlDataAdapter is to call the RowUpdatingEventHandler() method when the RowUpdating event fires using the following code: mySqlDataAdapter.RowUpdating += new SqlRowUpdatingEventHandler(RowUpdatingEventHandler); If you then call the AddDataRow() method shown earlier to attempt to add a row to the Customers table, the RowUpdating event will fire and call the RowUpdatingEventHandler() method. This method causes the new row to be skipped and prevents it from being added to the Customers database table. The RowUpdated Event The RowUpdated event fires after a row is updated in the database as a result of you calling the Update() method of your DataAdapter. This event fires once for each DataRow you've added, modified, or deleted in a DataTable. The second parameter to any method you write to handle the RowUpdated event of a SqlDataAdapter object is of the SqlRowUpdatedEventArgs class. The properties of this class are the same as those shown earlier in Table 11.13 , plus one additional property shown in Table 11.14 . Table 11.14: ADDITIONAL SqlRowUpdatedEventArgs PROPERTY PROPERTY TYPE DESCRIPTION RecordsAffected int Gets an int containing the number of rows added, modified, or removed when the appropriate Command is run by the Update() method. The following example event handler, named RowUpdatedEventHandler(), displays the number of records affected by the following Command: public static void RowUpdatedEventHandler( object sender, SqlRowUpdatedEventArgs mySRUEA ) { Console.WriteLine("\nIn RowUpdatedEventHandler()"); Console.WriteLine("mySRUEA.RecordsAffected = " + mySRUEA.RecordsAffected); } You indicate that mySqlDataAdapter is to call the RowUpdatedEventHandler() method when the RowUpdated event fires using the following code: mySqlDataAdapter.RowUpdated += new SqlRowUpdatedEventHandler(RowUpdatedEventHandler); The DataTable Events The events exposed by a DataTable object are shown in Table 11.15 . Table 11.15: DataTable EVENTS EVENT EVENT HANDLER DESCRIPTION ColumnChanging DataColumnChangeEventHandler Fires before a changed Table 11.15: DataTable EVENTS EVENT EVENT HANDLER DESCRIPTION DataColumn value is committed in a DataRow. ColumnChanged DataColumnChangeEventHandler Fires after a changed DataColumn value is committed in a DataRow. RowChanging DataRowChangeEventHandler Fires before a changed DataRow is committed in a DataTable. RowChanged DataRowChangeEventHandler Fires after a changed DataRow is committed in a DataTable. RowDeleting DataRowChangeEventHandler Fires before a DataRow is deleted from a DataTable. RowDeleted DataRowChangeEventHandler Fires after a DataRow is deleted from a DataTable. The ColumnChanging and ColumnChanged Events The ColumnChanging event fires before a change to a DataColumn value is committed in a DataRow. Similarly, the ColumnChanged event fires after a change to a DataColumn value is committed in a DataRow. These two events are always fired before the RowChanging and RowChanged events. What is meant by "commit" in this context? If you simply set a new value for a DataColumn, then the change is automatically committed in the DataRow. However, if you start the change to the DataRow using the BeginEdit() method, then the change is committed only when you call the EndEdit() method of that DataRow. You can also reject the change to the DataRow using the CancelEdit() method. The second parameter to any event handler you write to handle the ColumnChanging or ColumnChanged events of a DataTable object is of the DataColumnChangeEventArgs class. Table 11.16 shows the properties of this class. Table 11.16: DataColumnChangeEventArgs PROPERTIES PROPERTY TYPE DESCRIPTION Column DataColumn Gets the DataColumn with the value that is changing. ProposedValue object Gets or sets the new value for the DataColumn. Row DataRow Gets the DataRow that contains the DataColumn with the value that is changing. The following example method handlers, named ColumnChangingEventHandler() and ColumnChangedEventHandler(), display the Column and ProposedValue properties: public static void ColumnChangingEventHandler( object sender, DataColumnChangeEventArgs myDCCEA ) { Console.WriteLine("\nIn ColumnChangingEventHandler()"); Console.WriteLine("myDCCEA.Column = " + myDCCEA.Column); Console.WriteLine("myDCCEA.ProposedValue = " + myDCCEA.ProposedValue); } public static void ColumnChangedEventHandler( object sender, DataColumnChangeEventArgs myDCCEA ) { Console.WriteLine("\nIn ColumnChangedEventHandler()"); Console.WriteLine("myDCCEA.Column = " + myDCCEA.Column); Console.WriteLine("myDCCEA.ProposedValue = " + myDCCEA.ProposedValue); } The next example creates a DataTable named customersDataTable and adds the previous two methods to the ColumnChanging and ColumnChanged events of customersDataTable: DataTable customersDataTable = myDataSet.Tables["Customers"]; customersDataTable.ColumnChanging += new DataColumnChangeEventHandler(ColumnChangingEventHandler); customersDataTable.ColumnChanged += new DataColumnChangeEventHandler(ColumnChangedEventHandler); The RowChanging and RowChanged Events The RowChanging event fires before a change to a DataRow is committed in a DataTable. Similarly, the RowChanged event fires after a change to a DataRow value is committed in a DataTable. The second parameter to any event handler you write to handle the RowChanging or RowChanged events of a DataTable object is of the DataRowChangeEventArgs class. Table 11.17 shows the properties of this class. Table 11.17: DataRowChangeEventArgs PROPERTIES PROPERTY TYPE DESCRIPTION Action DataRowAction Gets the DataRowAction that has occurred for the DataRow. The DataRowAction enumeration is defined in the System.Data namespace and contains the following members: • Add, which indicates the DataRow has been added to the DataTable. • Change, which indicates the DataRow has been modified. • Commit, which indicates the DataRow has been committed in the DataTable. • Delete, which indicates the DataRow has been removed from the DataTable. • Nothing, which indicates the DataRow has not changed. • Rollback, which indicates the change to the DataRow has been rolled back. Row DataRow Gets the DataRow that contains the DataColumn with the value that is changing. The following example event handlers, named RowChangingEventHandler() and RowChangedEventHandler(), display the Action property: public static void RowChangingEventHandler( object sender, DataRowChangeEventArgs myDRCEA ) { Console.WriteLine("\nIn RowChangingEventHandler()"); Console.WriteLine("myDRCEA.Action = " + myDRCEA.Action); } public static void RowChangedEventHandler( object sender, DataRowChangeEventArgs myDRCEA ) { Console.WriteLine("\nIn RowChangedEventHandler()"); Console.WriteLine("myDRCEA.Action = " + myDRCEA.Action); } The next example adds the previous two methods to the RowChanging and RowChanged events of customersDataTable: customersDataTable.RowChanging += new DataRowChangeEventHandler(RowChangingEventHandler); customersDataTable.RowChanged += new DataRowChangeEventHandler(RowChangedEventHandler); The RowDeleting and RowDeleted Events The RowDeleting event fires before a DataRow is deleted from a DataTable. Similarly, the RowDeleted event fires after a DataRow is deleted from a DataTable. The second parameter to any event handler you write to handle the RowDeleting or RowDeleted events of a DataTable is of the DataRowChangeEventArgs class, and Table 11.17 shown earlier shows the properties of this class. The following example method handlers, named RowDeletingEventHandler() and RowDeleted-EventHandler(), display the Action property: public static void RowDeletingEventHandler( object sender, DataRowChangeEventArgs myDRCEA ) { Console.WriteLine("\nIn RowDeletingEventHandler()"); Console.WriteLine("myDRCEA.Action = " + myDRCEA.Action); } public static void RowDeletedEventHandler( object sender, DataRowChangeEventArgs myDRCEA ) { Console.WriteLine("\nIn RowDeletedEventHandler()"); Console.WriteLine("myDRCEA.Action = " + myDRCEA.Action); } The next example adds the previous two methods to the RowDeleting and RowDeleted events of customersDataTable: customersDataTable.RowDeleting += new DataRowChangeEventHandler(RowDeletingEventHandler); customersDataTable.RowDeleted += new DataRowChangeEventHandler(RowDeletedEventHandler); Note You'll find all the code examples shown in this section in the program UsingEvents.cs located in the ch11 directory. This program listing is omitted from this book for brevity. . Exploring the DataAdapter and DataTable Events You'll find all the code examples shown in this section in the program UsingEvents.cs located in the. SqlRowUpdatedEventHandler(RowUpdatedEventHandler); The DataTable Events The events exposed by a DataTable object are shown in Table 11.15 . Table 11.15: DataTable EVENTS EVENT EVENT HANDLER