Comments on HelloData
The HelloData application steps through the basic operations of a typical ADO application: getting, examining, editing, and updating data. When you start the application, click the first button, Get Data. This will run the GetData subroutine.
GetData places a valid connection string into a module-level variable, m_sConnStr. For more information about connection strings, see Creating the Connection String.
Assign an error handler using a Visual Basic OnError statement. For more information about error handling in ADO, see Error Handling. A new Connection object is created, and the CursorLocation property is set to adUseClient because the HelloData example creates a disconnected Recordset. This means that as soon as the data has been fetched from the data source, the physical connection with the data source is broken, but you can still work with the data that is cached locally in your Recordset object.
After the connection has been opened, assign an SQL string to a variable (sSQL). Then create an instance of a new Recordset object, m_oRecordset1. In the next line of code, open the Recordset over the existing Connection, passing in sSQL as the source of the Recordset. You help ADO in making the determination that the SQL string you have passed as the source for the Recordset is a textual definition of a command by passing adCmdText in the final argument to the Recordset Open method. This line also sets the LockType and CursorType associated with the Recordset.
The next line of code sets the MarshalOptions property equal to adMarshalModifiedOnly. MarshalOptions indicates which records should be marshaled to the middle tier (or Web server). For more information about marshaling, see the COM documentation. When you use adMarshalModifiedOnly with a client-side cursor (CursorLocation = adUseClient), only records that have been modified on the client are written back to the middle tier. Setting MarshalOptions to adMarshalModifiedOnly can improve performance because fewer rows are marshaled.
Next, disconnect the Recordset by setting its ActiveConnection property equal to Nothing. For more information, see the section "Disconnecting and Reconnecting the Recordset" in Updating and Persisting Data.
Close the connection to the data source and destroy the existing Connection object. This releases the resources that it consumed.
The final step is to set the Recordset as the DataSource for the Microsoft DataGrid Control on the form so that you can easily display the data from the Recordset on the form.
Click the second button, Examine Data. This runs the ExamineData subroutine.
ExamineData uses various methods and properties of the Recordset object to display information about the data in the Recordset. It reports the number of records by using the RecordCount property. It loops through the Recordset and prints the value of the AbsolutePosition property in the display text box on the form. Also while in the loop, the value of the Bookmark property for the third record is placed into a variant variable, vBookmark, for later use.
The routine navigates directly back to the third record using the bookmark variable that it stored earlier. The routine calls the WalkFields subroutine, which loops through the Fields collection of the Recordset and displays details about each Field in the collection.
Finally, ExamineData uses the Filter property of the Recordset to screen for only those records with a CategoryId equal to 2. The result of applying this filter is immediately visible in the display grid on the form.
For more information about the functionality shown in the ExamineData subroutine, see Examining Data.
Next, click the third button, Edit Data. This will run the EditData subroutine.
When the code enters the EditData subroutine, the Recordset is still filtered on CategoryId equal to 2, so that only those items that meet the filter criteria are visible. It first loops through the Recordset and increases the price of each visible item in the Recordset by 10 percent. The value of the Price field is changed by setting the Value property for that field equal to a new, valid amount.
Remember that the Recordset is disconnected from the data source. The changes that were made in EditData are made only to the locally cached copy of the data. For more information, see Editing Data.
The changes will not be made on the data source until you click the fourth button, Update Data. This will run the UpdateData subroutine.
UpdateData first removes the filter that has been applied to the Recordset. The code removes and resets m_oRecordset1 as the DataSource for the Microsoft Bound DataGrid on the form so that the unfiltered Recordset appears in the grid.
The code then checks to see whether you can move backward in the Recordset by using the Supports method with the adMovePrevious argument.
The routine moves to the first record using the MoveFirst method and displays the field's original and current values, by using the OriginalValue and Value properties of the Field object. These properties, together with the UnderlyingValue property (not used here), are discussed in Updating and Persisting Data.
Next, a new Connection object is created and used to reestablish a connection to the data source. You reconnect the Recordset to the data source by setting the new Connection as the ActiveConnection for the Recordset. To send the updates to the server, the code calls UpdateBatch on the Recordset.
If the batch update succeeds, a module-level flag variable, m_flgPriceUpdated, is set to True. This will remind you later to clean up all the changes that were made to the database.
Finally, the code moves back to the first record in the Recordset and displays the original and current values. The values are the same after the call to UpdateBatch.
For detailed information about how to update data, including what to do when data on the server changes while your Recordset is disconnected, see Updating and Persisting Data.
The Form_Unload subroutine is important for several reasons. First, because this is a sample application, Form_Unload cleans up the changes that were made to the database before the application exits. Second, the code shows how a command can be executed directly from an open Connection object by using the Execute method. Finally, it shows an example of executing a non-row–returning query (an UPDATE query) against the data source.